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Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily--and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming., Understanding soil biogeochemistry is essential to the stewardship of ecosystem services provided by soils, such as soil fertility (for food, fibre and fuel production), water quality, resistance to erosion and [...]

Land-use change from [C.sub.3] vegetation (δ[sup.13]C values, 30%o to -24%o) to [C.sub.4] vegetation (δ[sup.13]C values, -14%o to -11%o) provides a useful quantitative technique for estimating organic C turnover in soil, even when total organic C changes arc negligible. We utilised this technique to estimate C turnover in physically fractionated soil organic matter, particulate organic matter C (POM C > 250 µm fraction and POM C 250-53 µm fraction), and the < 53 µm fraction. There were small changes in total soil organic C (SOC) after 23 years of land-use change from native vegetation (mixed vegetation of Acacia harpophylla and Casuarina cristata) to buffel grass (Cenchrus ciliaris L. cv. Biloela) pasture grown on Vertosol-Dermosol-Sodosol soil types. The SOC values (t/ha) under native vegetation were: 31 ± 3 for the 0- 0.1 m depth, 21 ± 1 for the 0.1-0.2m depth, 15 ± 3 for the 0.2-0.3 m depth, and 16 ± 2 for the 0.3-0.4 m depth; the corresponding SOC values under pasture were 25 ± 2, 19 ± 2, 14 ± 2, and 13 ± l t/ha. The respective δ[sup.13]C values in 0-0.1 m depths of the whole SOC and POM C > 250 µm fraction changed from -25.5 ± 0.1%o and -25.5 ± 0.3%o under native vegetation to -20.1 ± 0.5%o and -19.4 ± 0.2%o under pasture. Similar, although smaller, differences were observed for other depths and SOC fractions. The SOC turnover periods (years) were 31 ± 6 for the 0-0.1 m depth, 60 ± 5 for the 0.1-0.2 m depth, 55 ± 15 for the 0.2-0.3 m depth, and 63 ± 20 for the 0.3-0.4 m depth; the corresponding turnover periods for the POM C > 250 µm fraction were 13 ± 2, 19 ± 5, 14 ± 4, and 12 ± 5 years. The turnover periods of SOC in the POM C 250-53 µm and < 53 µm fractions were similar to, or longer than, for the whole SOC at all depths studied. Thus, the lability of the SOC and SOC pools was in the order: POM C > 250 µm fraction > POM C 53-250 µm fraction = POM C < 53 µm fraction = whole SOC. Additional keywords: [C.sub.3], [C.sub.4], δ[sup.13]C, labile C, particulate organic matter C, soil organic C., Introduction Land-use change from native vegetation to (introduced) pasture has shown inconsistent impact (increase, decrease, or no change) on soil organic C (SOC) stocks (Murty et al. 2002). Harms et [...]

Aluminum toxicity is one of the major soil factors limiting root growth in acidic soils. Because of the increase in organic matter content in the upper few centimeters of soils under no-till systems (NTS), most AI in soil solution may be complexed to dissolved organic C (DOC), thus decreasing its bioavailability. The aim of this study was to evaluate the effects of surface liming on Al speciation in soil solution in Brazilian sites under NTS. Field experiments were performed in two regions with contrasting climates and levels of soil acidity: Rondonopolis, Mato Grosso State, on a Rhodic Haplustox, and Ponta Grossa, Parana State, on a Typic Hapludox. The treatments consisted of a control and three lime rates, surface applied to raise the base saturation to 50, 70, and 90%. Soil solution was obtained at soil water equilibrium (1:1 w/w soil/water ratio). The effects of surface liming on soil chemical attributes and on the composition of the soil solution were dependent on weather conditions, time under NTS, and soil weathering. Most Al in soil solution was complexed to DOC, representing about 70 to 80% of the total AI at pH 5.0. Under pH 5.5, the results were closely correlated with the solubility line for amorphous AI. Organic complexes may control [Al.sup.3+] release into soil solution at pH < 5.5. Results suggest that in areas under NTS for a long period of time, AI toxicity might decrease due to its complexation to high-molecular-weight organic compounds. Abbreviations: DOC, dissolved organic carbon; DOM, dissolved organic matter; NTS, no-till systems; OM, organic matter. doi: 10.2136/sssaj2009.0254

Soil organic matter loss and increased soil compaction have been identified as the factors most likely to directly impact tree growth in managed forests. We compared the soil quality of plantations established on former agricultural lands (n = 20) with plantations established following clear cutting of native forests (n = 20). Half of the plantations had been planted with jack pine (Pinus banksiana Lamb.) and half with white spruce [Picea glauca (Moench) Voss], 9 to 27 yr before the study. The old field plantations had lower (at 0-10 and 10-20 cm) mineral soil macroporosity and higher field capacity than forest plantations, indicating more severe soil compaction. The old field plantations, however, also had higher soil C content, raising the permanent wilting point and canceling compaction effects on the available water holding capacity. An indicator of organic matter quality, namely the potential net mineralization per unit of soil Kjeldahl N, was lower in the old fields. Species also affected soil quality indicators--with lower values of macroporosity and higher values of field capacity observed under white spruce. Despite significant differences in soil conditions, no significant effect (P < 0.05) of plantation origin on tree growth could be found. Old fields can support productive plantations of both species. doi: 10.2136/sssaj2009.0264

Forest soils comprise a large portion of the global terrestrial C pool, and soil organic matter (SOM) is essential to soil function and forest productivity; however, responses of SOM quality to changes in fertility, moisture availability, or management are not well understood. We tested the effects of two common forest management practices, fertilization and competing vegetation control using herbicides, on surface SOM distribution and stability characteristics in three ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) plantations of differing productivity, soil type, and parent material in northern California by using density fractionation and aerobic laboratory incubation of the surface soils. The treatment effects on pine productivity revealed responses dominated by moisture limitation at the less productive sites and by nutrient limitation at the most productive site. The stability of surface S0M increased with increasing site productivity. Fertilization increased N concentrations and decreased C/N ratios in whole soils and light fractions at the less productive sites, and the effects persisted for more than a decade. Furthermore, fertilization increased soil C mineralization from the intermediate-productivity site during incubation. In contrast, fertilization decreased C mineralization from the most productive site, suggesting that fertilization increased SOM stabilization at this site. Controlling understory vegetation with herbicides reduced N availability, as evidenced by reduced light-fraction N at the poorest site and decreased N mineralization during incubation. Our study demonstrates the importance of site characteristics and the use of a combination of indices in determining the effects of forest management practices on SOM characteristics and dynamics. Abbreviations: DF, dense fraction; DIN, dissolved inorganic nitrogen; DOC, dissolved organic carbon; DOM, dissolved organic matter; DON, dissolved organic nitrogen; LF, light fraction; SOM, soil organic matter; SPT, sodium polytungstate; TDN, total dissolved nitrogen. doi: 10.2136/sssaj2009.0062

A sandy prairie remnant in the Lower Wisconsin River Valley, encroachment areas within the prairie, and an adjacent red pine (Pinus resinosa Aiton) plantation were studied to determine the influence of woody cover on C dynamics. Field transects, aerial imagery, and a geographic information system were used to quantify, encroachment from 1979 to 2002. A linear encroachment model predicted 100% encroachment of the 6.0-ha prairie in 50 yr. Four field plots in each of pine, prairie, aud encroachment areas were sampled and soils collected (0-18, 18-38, and 38-75 cm) in 2004 and 2008. Total ecosystem C was greater in pine (126.6 Mg C [ha.sup.-1]) and encroachment areas (71.8 Mg C [ha.sup.-1]) than prairie (48.3 big C [ha.sup.-1]). In the 0 to 38 cm, coarse particulate organic matter (POM) (4.1, 6.3, and 7.5 Mg C [ha.sup.-1]) and the POM C/N ratio (15.1, 16.2, and 20.2) increased with woody eucroachment (prairie, encroachment areas, and pine, respectively). Changes in POM suggest more organic inputs and slower decomposition, but due to minimal protection of C within aggregates, increased total soil C was not observed with woody advancement (46.4-47.2 Mg C [ha.sup.-1]). Microbial biomass (0-38 cm) was greatest in encroachment areas, followed by prairie, and then pine (108, 84, and 51 kg N [ha.sup.-1], respectively), probably a result of more favorable microdimate and substrate at the ecotone boundary. Potential N mineralization (0.6, 2.8, and 4.8 kg N [ha.sup.-1] [d.sup.-1]), extractable N[H.sub.4.sup.+] (28, 33, and 57 kg [ha.sup.-1]), and Bray-1 P (380, 402, and 541 kg [ha.sup.-1]) (0-38 cm) increased with woody cover, and increased nutrient availability could lead to a greater aboveground C sink through increased tree growth. Abbreviations: cPOM, coarse particulate organic matter; DBH, diameter at breast height; DON, dissolved organic nitrogen; fPOM, fine particulate organic matter; MBC, microbial biomass carbon; POM, particulate organic matter; scSOM, silt- and clay-associated soil organic matter; SOC, soil organic carbon; SOM, soil organic matter; TEC, total ecosystem carbon. doi: 10.2136/sssaj2009.0223

Flay-two soils under black spruce [Picea mariana (Mill.) Britton et al.]-dominated forest communities were examined and assessed for their organic C (OC) stores in relation to soil characteristics. Study sites were located on a variety of parent materials, landscape positions, and drainage conditions. Results indicate that soils at most sites were weakly developed, commonly with organic (O) horizons ranging from 3 to 39 cm ([greater than or equal to] 100 cm occasionally). Organic C stores tended to increase as drainage changed from somewhat excessive and well to very poorly drained (average to 1 m: 12.6-50.9 kg OC [m.sup.-2], respectively). The lowest OC store for an individual site was 7.1 kg OC [m.sup.-2] in a well-drained soil on an outwash plain and the highest was 109 kg C [m.sup.-2] in a very poorly drained soil. Surface organic horizons contained 13 to 100% of the total pedon OC stores. In Gelisols, permafrost sequestered an average of 9, 19, and 39% of SOC stores for the somewhat poorly, poorly, and very poorly drained soils, respectively. The presence of permafrost in poorly drained sites increased average OC stores from 27.8 to 50.1 kg OC [m.sup.-2] over those without permafrost. Soil bulk density, cation exchange capacity, and extractable acidity assessed in relation to OC stores of genetic horizons illustrate the significant impact of OC on soil properties. In previous ecological studies in Alaska, OC was determined for only the surface horizons; our data suggest that such shallow sampling may underestimate total OC stores by an average of 26% and up to 68%. Abbreviations: CEC, cation exchange capacity; OC, organic carbon; OM, organic matter; SOC, soil organic carbon; SOM, soil organic matter. doi: 10.2136/sssaj2009.0152

High-resolution soil organic C (SOC) maps are a major prerequisite for many environmental studies dealing with C stocks and fluxes. Especially in hilly terrain, where SOC variability is most pronounced, high-quality data are rare and costly to obtain. In this study, factors and processes influencing the spatial distribution of SOC in three soil layers ( Abbreviations: CA, catchment area; C-plan, plan curvature; C-prof, profile curvature; DEM, digital elevation modal; MEF, model-efficiency coefficient; OK, ordinary kriging; RE, relative elevation; RI, relative improvement; RK, regression kriging; RUSLE; Revised Universal Soil Loss Equation; [R.sub.17], 17-m input raster; [R.sub.25], 25-m input raster; [R.sub.50], 50-m input raster; SEDEM, Sediment Delivery Model; SOC, soil organic carbon; SPI, stream power index; WaTEM, Water and Tillage Erosion Model; WI, wetness index. doi: 10.2136/sssaj2009.0325

Estimates of soil organic C (SOC) storage and their variability at various spatial scales are essential to better understand the global C cycle, estimate C sink capacity, identify effective C sequestration strategies, and quantify the amount of SOC sequestered during a specific period of time. This study used a geographically weighted regression (GWR) approach to predict the SOC pool at a regional scale. The GWR considers varying relationships between the SOC pod and environmental variables across the study area. The range of the variogram of SOC observations was used to define a search radius in the GWR. Terrain attributes, climate data, land use data, bedrock geology, and normalized difference vegetation index data were used to predict the SOC pool for seven states in the midwestern United States. The prediction accuracy of this SOC pool map was compared with the multiple linear regression (MLR) and regression kriging (RK) approaches. Higher contrast and wider variability (1.73-39,3 kg [m.sup.- 2]) of the SOC pool were predicted with lower global prediction errors (mean estimation error = -0.11 kg [m.sup.-2], RMSE = 6.40 kg [m.sup.-2]) in GWR compared with the other approaches. A relative improvement of 22% over MLR and 2% over RK was observed in SOC prediction. The total SOC pool to the 0.5-m depth was estimated to be 6.22 Pg. The results suggest that the GWR approach is a promising tool for regional-scale SOC prediction. Abbreviations: GWR, geographically weighted regression; MAEE, mean absolute estimation error; MEE, mean estimation error; MLR, multiple linear regression; MLRA, Major Land Resource Area; RK, regression kriging; SOC, soil organic carbon. doi: 10.2136/sssaj2009.0158

Soil, particularly in forests, is replete with spatial variation with respect to soil C. The present standard chemical method for soil analysis by dry combustion (DC) is destructive, and comprehensive sampling is labor intensive and time consuming. These, among other factors, are contributing to the development of new methods for soil analysis. These include a near- and mid-infrared (NIR and MIR) spectroscopy, laser-induced breakdown pectroscopy (LIBS), and inelastic neutron scattering (INS). These technologies overcome many of the state-of-the-art DC method's shortcomings and offer advances that it cannot. While NIR and MIR measure C bonds, the other two new methods, like DC, are more specific in measuring C and other elements based on chemical, atomic, and nuclear reactions. In addition to their fundamentally different physical principles, these approaches vastly differ in the volumes they typically sample: LIBS, [10.suip.-9] [m.sup.3]; DC, [10.sup.-7] [m.sup.3]; NIR and MIR, [10.sup.-6] [m.sup.3]; and INS, about 0.3 [m.sup.3]. Thus, extra care is needed when comparing the findings from any two of these methods. Also, the high heterogeneity of the soil matrix, the nonuniformity of C distribution, and the presence of coarse fragments, particularly in forested ecosystems, further compound the difficulties in making direct comparisons. We investigated the implications of these differences when correlating any two of these methods and reviewed the processes of comparing a volumetric measurement against a point measurement. We also conducted a detailed comparison of the INS method with the standard DC test. We found that the total (soil organic matter and roots) measured by the INS correlated better than its components with the DC analyses([r.sup.2] = 0.97, P = [10.sup.-7]). The samples for DC analysis were taken from excavations of 40- by 40- by 40-cm plots, in 5- and 10-cm layers. Abbreviations: DC, dry combustion; INS, inelastic neutron scattering; LIBS, laser-induced breakdown spectroscopy. doi: 10.2136/sssaj2009.0205

We quantified the organic matter content of organic and mineral horizons in 1932, 1984, and in 2005/6 at 54 sites in a mixed hardwood-softwood stand and in northern hardwood (NH), pine-dominated (PW), and spruce-fir (SF) forests of the Adirondack Mountains, NY to determine if there were measurable changes in soil organic matter (SOM) pools over the ~75-yr interval. Further, the different land-use histories of these sites provided an opportunity to evaluate the influence of land-use history on forest SOM since the early 1930s. Overall, there were no significant differences in combined organic + mineral horizon (whole-profile) SOM amounts over the interval. There was, however, a significant increase in whole-profile SOM content between 1932 and 2005/6 at 16 sites that had a history of agriculture or fire which amounted to an increase in C content of approximately 0.5 Mg C [ha.sup.-1] [yr.sup.-1]. This indicated that at least some Adirondack forest soils were accumulating C during the 20th century. Abbreviations: LOI, loss-on-ignition; MSD, minimum significant difference; NC, Newcomb; NH, northern hardwood; OM, organic matter; PW, pine-dominated; SE spruce-fir; SOM, soil organic matter. doi:10.2136/sssaj2009.0132N

Temperatures in high-latitude ecosystems are increasing faster than the average rate of global warming, which may lead to a positive feedback for climate change by increasing the respiration rates of soil organic C. If a positive feedback is confirmed, soil C will represent a source of greenhouse gases that is not currently considered in international protocols to regulate C emissions. We present new estimates of the stocks of soil organic C in Alaska, calculated by linking spatial and field data developed by the USDA NRCS. The spatial data are from the State Soil Geographic database (STATSGO), and the field and laboratory data are from the National Soil Characterization Database, also known as the pedon database. The new estimates range from 32 to 53 Pg of soil organic C for Alaska, formed by linking the spatial and field data using the attributes of Soil Taxonomy. For modelers, we recommend an estimation method based on taxonomic subgroups with interpolation for missing areas, which yields an estimate of 48 Pg. This is a substantial increase over a magnitude of 13 Pg estimated from only the STATSGO data as originally distributed in 1994, but the increase reflects different estimation methods and is not a measure of the change in C on the landscape. Pedon samples were collected between 1952 and 2002, so the results do not represent a single point in time. The linked databases provide an improved basis for modeling the impacts of climate change on net ecosystem exchange. Abbreviations: IPCC, Intergovernmental Panel on Climate Change; SOC, soil organic carbon; STATSGO, State Soil Geographic database. doi:10.2136/ssaj2008.0404

Cold (room temperature) and hot water (70-80[degrees]C) extraction methods have been used to assess the availability of organic matter and N in soil; however, the influence of temperature on their extractability is largely unknown. Twenty-one mineral soils from New Zealand and eastern Canada were incubated for 16 h in water (1:6 soil/water ratio) at temperatures ranging from 20 to 80[degrees]C. After centrifugation and removal of the supernatant solution, the soil residue was extracted with 2 tool [L.sup.-1] KCl to recover N[H.sub.4]-N released during incubation. In all cases, water-extractable organic C (WEOC) and N (WEON) increased exponentially with temperature. The temperature dependence of organic matter solubility differed considerably among the soils (estimated increase in WEOC and WEON: 1.1-5.3% [degrees][C.sup.-1]). These differences were not explained by management history or soil physicochemical properties, although there was evidence that organic matter extractability in some clay soils was less sensitive to temperature. Ammonium N was released in significant amounts at all temperatures. Peak N[H.sub.4]-N release typically occurred at 50[degrees]C, where N[H.sub.4]-N comprised an average of 49% of the total recovered N (WEON plus N[H.sub.4]-N). Factors that may account for N[H.sub.4]-N production during the 16-h incubation include mineralization at the lower temperatures and abiotic processes (e.g., release of clay-fixed N[H.sub.4] and thermal degradation of organic N) at higher temperatures. The possibility that mineralization made a significant contribution to N[H.sub.4]-N production at temperatures as high as 50[degrees]C warrants further investigation. Abbreviations: WEOC, water-extractable organic carbon; WEOM, water-extractable organic matter; WEON, water-extractable organic nitrogen. doi:10.2136/sssaj2008.0347

Turfgrass is a major vegetation type in the urban and suburban environment. Management practices such as species selection, irrigation, and mowing may affect C input and storage in these systems. Research was conducted to determine the rate of soil organic C (SOC) changes, soil C sequestration, and SOC decomposition of fine fescue (Festuca spp.) (rainfed and irrigated), Kentucky bluegrass (Poa pratensis L.) (irrigated), and creeping bentgrass (Agrostis palustris Huds.) (irrigated) using C isotope techniques. We found that 4 yr alter establishment, about 17 to 24% of SOC at 0 to 10 cm and 1 to 13% from l0 to 20 cm was derived from turfgrass. Irrigated fine fescue added the most SOC (3.35 Mg C [ha.sup.-1] [yr.sup.-1]) to the 0- to 20-cm soil profile but also had the highest rate of SOC decomposition (2.61 Mg C [ha.sup.-1] [yr.sup.-1]). The corresponding additions and decomposition rates for unirrigated fine fescue, Kentucky bluegrass, and creeping bentgrass in the top 20-cm soil profile were 1.39 and 0.87, 2.05 and 1.73, and 2.28 and 1.50 Mg C [ha.sup.-1] [yr.sup.-1], respectively,. Irrigation increased both SOC input and decomposition. We found that all turfgrasses exhibited significant C sequestration (0.32-0.78 Mg [ha.sup.-1] [yr.sup.-1]) during the first 4 yr after turf establishment. The net C sequestration rate was higher, however, for irrigated fine fescue and creeping bentgrass than for Kentucky bluegrass. To evaluate total C balance, additional work is needed to evaluate the total C budget and fluxes of the other greenhouse gases in turfgrass systems. Abbreviations: SOC, soil organic carbon; SON, soil organic nitrogen; [[delta].sup.13]C, carbon isotope ratio. doi:10.2136/sssaj2009.0075

Conservation management of degraded land has the potential to build soil fertility, restore soil functions, and mitigate greenhouse gas emissions as a consequence of surface soil organic matter accumulation. Literature from the southeastern United States was reviewed and synthesized to: (i) quantitatively evaluate the magnitude and rate of soil organic C (SOC) sequestration with conservation agricultural management; (ii) evaluate how conservation management affects surface SOC accumulation and its implications on ecosystem services; and (iii) recommend practical soil sampling strategies based on spatial and temporal issues to improve the detection of statistically significant SOC sequestration. Soil organic C sequestration was 0.45 [+ or -] 0.04 Mg C [ha.sup.-1] [yr.sup.-1] (mean [+ or -] standard error, n = 147, 20 [+ or -] 1 cm depth, 11 [+ or -] 1 yr) with conservation tillage compared with conventional tillage cropland. Establishment of perennial pastures sequestered 0.84 [+ or -] 0.11 Mg C [ha.sup.-1] [yr.sup.-1] (n = 35, 25 [+ or -] cm depth, 17 [+ or -] 1 yr). Stratification of SOC with depth was common under conservation agricultural management and appears to be integrally linked to abatement of soil erosion, improvement in water quality, and SOC sequestration. Sampling of conservation management systems should ideally occur repeatedly with time in controlled and replicated experiments, but there is also an urgent need for chronosequence and paired-field surveys of SOC on working farms in the region to validate and expand the scope of inference of experimental results. Landowners in the southeastern United States have great potential to restore soil fertility and mitigate greenhouse gas emissions with the adoption of and improvement in conservation agricultural systems (e.g., continuous no-till, high-residue crop rotations, high organic matter inputs). Abbreviations: NT, no-till; SOC, soil organic carbon. doi:10.2136/sssaj2009.0079

There is uncertainty about manure P availability for crops and need for P fertilization when manure is applied. This study evaluated effects of liquid swine (Sus scrofa domesticus) manure and fertilizer P on early plant growth and P uptake, grain yield, and grain P removal for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Sixteen trials were conducted in Iowa from 2000 to 2002. Replicated treatments were the combinations of three manure rates (0, 29, and 57 kg total P [ha.sup.-1] on average across sites) and four P fertilizer rates (0 to 30 kg P [ha.sup.-1]). Initial soil Bray-[P.sub.1] (15-cm depth) was 9 to 89 mg P [kg.sup.-1]. Fertilizer rates applied across all plots were 168 kg N [ha.sup.-1] for corn and 56 to 120 kg K [ha.sup.-1] for both crops according to soil-test K. Measurements were early plant dry weight (DW) and P uptake (V5-V6 stage), grain yield, and grain P removal. Increases in plant DW, P uptake, and grain P removal were unrelated to soil-test P (STP), and were more fiequent to manure than to fertilizer. Yield increases (P [less than or equal to] 0.05) were observed at six sites with Bray-[P.sub.1] [less than or equal to] 20 mg P [kg.sup.-1], with response only to manure at one site and to both manure and fertilizer at five sites but to fertilizer only when manure was not applied. The study provided no evidence of lower crop-P availability from liquid swine manure than fertilizer P for corn or soybean in these Iowa fields. Abbreviations: DW, dry weight; OM, organic matter; STE soil-test P.

The Wien effect, that is, the dependence of the electrical conductivity of dilute suspensions of soil particles on electrical field strength, was explored in the intermediate field-strengths range of 0.9 x [10.sup.6] to 5 x [10.sup.6] V [m.sup.-l], at higher resolution than in previous studies. This enabled the detection of a local minimum in the electrical conductivity-field strength relationship in which the declining phase is termed the negative Wien effect. Suspensions of clay-size soil particles of three soil types, with various mono-, di-, and trivalent cations, were tested. A negative Wien effect was observed with homoionic soil particles saturated with divalent cations, with electrodialyzed soil particles, and with suspensions of black soil particles that contained organic matter. Two quantifiers of the declining and increasing slopes: (i) polarizability (counter ion polarization and re-adsorption); (ii) ion-stripping intensity--on the respective sides of the local minimum were used for characterizing the interaction between the soil particles and the counter ions. The higher mean ion-stripping intensities found for the [Na.sup.+] suspensions--twice those for the [K.sup.+] suspensions--reflect the easier stripping off of the Na cations and the stronger adsorption of the K cations. The mean ion-stripping intensities of all divalent cations for the three soils were lower than those determined for the monovalent ones, which reflects the tighter binding of the divalent cations. The [Ca.sup.2+] and [Zn.sup.2+] ions were stripped off most easily from the surfaces of soil particles, and tighter binding was found for [Pb.sup.2+] to brown soil and black soil, and for [Cd.sup.2+] to yellow-brown soil. In general, lower polarizabilities were correlated with higher ion-stripping intensities (e.g., for [Ca.sup.2+] and [Zn.sup.2+]). However, the exceptions (e.g., the polarizabilities of [Cu.sup.2+] and [Pb.sup.2+] shed more light on the interaction between the soil particles and the cations. The proposed method of characterization is superior to other methods for characterizing soil particle-counter ion interactions because it characterizes adsorption directly and not via exchange measurement, and it is less laborious than, for example, exchange isotherm measurements. Abbreviations: DL, double layer; EC, electrical conductivity; i.d.m., induced dipole moment; OM, organic matter.

Corn (Zea mays L.) stover is an asset to recycle essential plant nutrients and buffer soil against natural and human perturbations. Its indiscriminate removal for industrial uses may thus adversely impact soil fertility and productivity. Research data on the impacts of stover removal are needed to establish threshold levels of stover management. Thus, this study documented the 4-yr impacts of a systematic removal of stover on selected soil fertility indicators and structural stability across three contrasting soils in Ohio under no-till (NT) management, including a Rayne silt loam, Celina silt loam, and Hoytville clay loam. Stover was removed at rates of 0, 25, 50, 75, and 100% after harvest for 4 yr. Complete stover removal reduced the total N pool by, on average, 0.82 Mg [ha.sup.-1] in the silt loams but had no effect in the clay loam. It reduced available P by 40% and exchangeable [Ca.sup.+2] and [Mg.sup.+2] and the cation exchange capacity by 10% on the sloping silt loam. Exchangeable [K.sup.+] decreased by 15% on the silt loams for stover treatments [greater than or equal to] 75% removal and by 25% under complete removal on the clay loam in the 0- to 10-cm depth. Stover removal at rates [greater than or equal to] 25% reduced soil macroaggregates (>4.75 mm) by 40% in nearly level soils while 100% removal reduced them by 60% on the sloping soil. Available P and [K.sup.+] were predictors ([R.sup.2] = 0.77) of grain yield while soil organic C (SOC) and total N pools were predictors ([R.sup.2] = 0.45) of stover yield on the sloping silt loam. Based on the data (e.g., SOC pools) from this and previous studies, we determined that only about 25% of stover might be available for removal, and that stover removal has the most adverse impacts on sloping and erosion-prone soils. Abbreviations: CEC, cation exchange capacity; CSL, Celina silt loam; EC, electrical conductivity; HCL, Hoytville clay loam; MWD, mean weight diameter; NT, no-till; SOC, soil organic carbon; SOM; soil organic matter; RSL, Rayne silt loam; WSA, water-stable aggregates.

Tillage has been associated with soil organic matter (SOM) decline. A case study of two adjacent farms was conducted in eastern South Dakota. One farm used no-till (NT) and the other used chisel tillage (CT). We hypothesized that soil under NT, compared with tillage, would have both greater quantity and greater quality of SOM and that this improved SOM condition would result in increased water stable aggregation (WSA). A rotary sieve was used to sort dry field aggregates into six size groups: 19 ram. Water stable aggregation, soil organic C (SOC), N, glomalin, and basidiomycete fungi were measured. Fine particulate soil organic matter (fPOM, 0.5-0.053 mm) and coarse particulate organic matter (2.0-0.5 mm) were isolated by sieving. Quantitative solid-state [sup.13]C nuclear magnetic resonance was used to determine C type in humic acid, humin, and whole soil. The fPOM/SOM ratio was greatest in Abbreviations: AM, arbuscular mycorrhizal; cPOM, coarse particulate organic matter; CT, chisel tillage; DPMAS, direct-polarization magic-angle spinning; EC, electrical conductivity; fPOM, fine particulate organic matter; HA, humic acid; IRTG, immunoreactive total glomalin; NMR, nuclear magnetic resonance; NT, no-till; POM, particulate organic matter; SOC, soil organic carbon; SOM, soil organic matter; TPOM, total particulate organic matter; WSA, water stable aggregation.

Several research groups have evaluated the Illinois soil N test (ISNT) as a tool for identifying the N responsiveness of corn (Zea mays L.). These studies have shown mixed results. Research in New York improved the ISNT accuracy by adjusting the critical ISNT value (0-20-cm depth) for organic matter estimated by loss-on-ignition (LOI), resulting in an ISNT x LOI curve above which corn tended to be nonresponsive. We conducted 34 N rate trials (2005-2007) to further evaluate the accuracy of the ISNT x LOI curve in predicting the N responsiveness of corn in rotation with grass-legume sod or soybean [Glycine max (L.) Merr.]. First-year corn did not respond to sidedress N (16 trials), yet the ISNT x LOI curve predicted that sidedress N was needed for six sites. For second-, third-, or fourth-year corn after sod and corn after soybean, the ISNT x LOI curve accurately predicted N responsiveness for 15 of 18 sites (eight responsive and 10 nonresponsive sites). The ISNT values of first- and second-year corn were closely correlated ([ISNT.sub.year2] = -12.8 + 1.1 [ISNT.sub.year1], [r.sup.2] = 0.96, P < 0.001). The results of laboratory and field trials on ISNT dynamics from spring sod plowdown through sidedress showed an ISNT peak 1 to 4 wk after plowdown, indicating that samples taken at or after sidedress do not reflect sod N credits. We conclude that the ISNT x LOI curve accurately identified nonresponsive sites as long as the sample was not collected within 5 wk of sod plowdown and that sod N credits need to be taken into account for first-year corn. Abbreviations: DM, dry matter; EONR, economically optimum nitrogen rate; ISNT, Illinois soil nitrogen test; LOI, loss-on-ignition; PSNT, presidedress nitrate test.

Abundance of fungi and bacteria in long-term no-till (NT) and intensively tilled (IT) soils in the Northern Great Plains were measured using phospholipid fatty acid analysis (PLFA) to determine if a shift in the relative abundance of fungi and bacteria occurs as the result of conversion to NT. Four tillage trials located in four different soil zones were sampled in spring of 2005 and 2006 before the crop was seeded to evaluate the long-term effect of tillage on the microbial community. With the exception of one site-year, total, bacterial, and fungal PLFA were greater in NT than IT soils at the soil surface (0- to 5-cm depth) (p < 0.05). Increases ranged from 8 to 202% for total biomass, 26 to 58% for bacterial biomass, and 0 to 120% for fungal biomass. At one site (Ellerslie) all biomass measurements were greater in IT than NT in 2005 and bacterial biomass was also greater under IT in 2006. The influence of tillage on microbial biomass was less pronounced with depth. Fungal dominance is commonly assumed under NT; however, our results demonstrate that although biomass of both fungi and bacteria increase in NT, the abundance of fungi vs. bacteria was not consistently greater under NT in the soils studied. Further research is needed to determine if fungi may be able to exert a more functionally dominant role in NT soils without an increase in relative abundance. Abbreviations: IT, intensive-tillage; NT, no-tillage; PLFA, phospholipid fatty acid analysis; SOM, soil organic matter.

Little is known about soil redistribution by tillage and its impact on the properties of soil profiles with thin soil layers on hillslopes. In this study, consecutive tillage by hoeing was performed 5 and 15 times on an Orthic Regosol within a steep hillslope landscape of the Sichuan Basin, China, to simulate the impact of short-and long-term tillage on soil profile properties at different slope positions. After intense tillage, significant changes in [sup.137]Cs inventories of the soil occurred in the summit and toeslope positions. The soil profile (originally [less than or equal to] 0.3 m in depth) disappeared on the slope summit due to intense soil downslope translocation, and the soil profile was truncated in upper slope positions in the proximity of the summit, while a thickened soil profile was present in toeslope positions, where the original soil profile was buried. The summit was characterized by an exposure of bedrock corresponding to areas of 4.77 and 10.0% of the experimental plot for 5 and 15 tillage operations, respectively. Soil organic C (SOC) and P were completely depleted with the disappearance of soil profiles at the summit position, while a substantial increase in SOC and extractable P inventories of the post-tillage soil profile was found in the toeslope position. Intense tillage caused remarkable soil redistribution within the landscape of the steep slope, resulting in net soil loss and gain close to the upslope and downslope boundaries of the field, respectively. Tillage results in an intense transformation of soil profiles and the variation in soil properties in steeply sloping fields. Abbreviations: SOC, soil organic carbon.

The cation exchange capacity (CEC) of soils depends on the amount and composition not only of clay minerals but also of soil organic matter (SOM). While the CEC of soil clay minerals has been intensively studied, little is known about the CEC of organic matter (OM) fractions, in particular those obtained with newly developed SOM extraction techniques. The objective of this study was to develop and test a method to quantitatively determine the CEC of extracted OM fractions and to relate CEC(OM) to OM functional groups possibly responsible for sorption of cations. Water- and pyrophosphate-soluble OM fractions were sequentially extracted from differently managed arable soils of two well-known long-term field experiments. The CEC of a freeze-dried pyrophosphate-soluble OM fraction [OM(PY)] (0.03 g) mixed with quartz sand (4.97 g) was determined by applying the standard percolation method. The chemical composition of OM(PY) was analyzed by Fourier-transform infrared (FTIR) spectroscopy. For all plots except those fertilized with farmyard manure, the sorption properties of the OM(PY) fraction were found to be site specific and to reflect soil and crop rotation effects. The relative contribution of the CEC of OM(PY) to the CEC of the soil (0.8-11.6%) is dependent on soil C content and extractability. For all plots, however, the relative contents of carboxylic functional groups in OM(PY) determined with FTIR spectroscopy was found to be linearly related to the CEC of the OM(PY), similar to pure organic substances. This relationship indicates the usefulness of CEC determination on OM(PY) fractions. The results suggest that the relative contents ofcarboxylic functional groups in OM(PY) reflect long-term effects of femlization and crop rotation on the sorption properties of the SOM. Abbreviations: BL, Bad Lauchstadt; CEC, cation exchange capacity; FTIR, Fourier-transform infrared; FYM, farmyard manure; HL, Halle; LFE, long-term field experiment; OM, organic matter; OMC, organo-mineral complexes; OM(PY), pyrophosphate-soluble organic matter fraction; R1, 4-yr crop rotation at Bad Lauchstadt; R2, 8-yr crop rotation at Bad Lauchstadt; SOC, soil organic carbon; SOM, soil organic manet.

The speciation of Cu in soils and surface waters is largely influenced by complexation reactions with natural organic matter (NOM). In this study, ion selective electrode data for the binding of [Cu.sup.2+] to a forest peat soil were collected as a function of equilibration time, pH (2.4-6.6), and total Cu(II) concentration (1-54g Cu [kg.sup.-1] dry soil). As a first step, a one-site Langmuir isotherm was successfully fitted to the Cu adsorption data for the complete concentration range at pH 4.6. In a second step, structural information extracted from extended x-ray absorption fine structure (EXAFS) spectroscopy, showing that Cu(II) forms five-membered rings with possible combinations of amine, carboxyl, and carbonyl functional groups in NOM, were used as input for chemical speciation calculations (using the chemical equilibrium model MINTEQA2). In agreement with the EXAFS results, a model consisting of one RN[H.sub.2], forming monodentate complexes ([Cu.sup.2+] + RN[H.sub.2] [left and right arrow] R[H.sub.2]N[Cu.sup.2+]; log stability constant [K.sub.RH2NCu.sup.2+] = 9.2; -log acid dissociation constant [p[K.sub.a]] = 9.0 for RN[H.sub.3.sup.+]), and two adjacent RCO[O.sup.-] groups, forming bidentate complexes ([Cu.sup.2+] + 2RCO[O.sup.-] [left and right arrow] Cu[(OOCR).sub.2]; log stability constant [[beta].sub.(RCOO)2Cu ]= 4.7; p[K.sub.a] = 4.5 for RCOOH), gave the best fit to the experimental data. Determined stability constants for Cu(II)-amine and Cu(II)-carboxyl complexes were in good agreement with well-defined Cu complexes with amino acids and carboxyls, respectively. Abbreviations: EXAFS, extended x-ray absorption fine structure; ISE, ion selective electrode; NOM, natural organic matter; RCOOH, carboxyl; RN[H.sub.2], amine; SOM, soil organic matter; XANES, x-ray absorption near edge structure.

Decreasing P buffer capacity of the soil surface in no-tilled soils has been attributed to enrichment of both P and OM (OM). As soil surface enrichment with P and OM usually occurs simultaneously in no-tilled soils, it is not clearly established whether the increase of soil OM affects the capacity of soils to retain P. A study was conducted to assess whether the variations in soil OM (total and particulate) at soil surface affects P retention capacity. Thirty five soils from the Rolling Pampa (17 tilled and 18 no-tilled) were selected. All soils had medium to low levels of available P ([P.sub.Brayl] < 20 mg [kg.sup.-1]). Soil samples were taken from the top 5 cm, and several soil characteristics determined: available and total P, P sorption index, OM, particulate OM (POM) (>53 [micro]m), particle size distribution, and pH. Tilled and no-tilled soils did not differ in soil texture, pH, total and available P content. On the contrary, no-tilled soils had more OM (+14%) and POM (+56%) than tilled soils. The capacity of soils to retain added P was not different in both groups of soils. Phosphorus sorption index was not related to soil content of total OM or POM. Variation in P sorption index was only related to soil clay content ([r.sup.2] = 0.44). This study provides evidence contradicting the long held assumption that the reported decrease in P sorption at the surface of no-tilled soils was caused in part by OM enrichment. Abbreviations: OM, organic matter, POM, particulate OM, PSI, phosphorus sorption index.

Eliminating summer fallow or establishing perennial grass elicited measurable gains in soil organic C (SOC) after just 6 yr in a crop rotation study on an Aridic Boroll in southern Alberta. This study was resampled after 12 yr to determine if SOC increases were continuing with time and to evaluate the impact of alternative crop rotation and fertilizer treatments on SOC. The crop rotation treatments included in this study were fallow-wheat (FW), fallow-wheat-wheat (FWW), fallow-flax-wheat (FXW), legume-wheat (LW), continuous wheat (W) and continuous grass (G). The gain in SOC due to the elimination of fallow was 1.5 Mg C [ha.sup.-1] after 12 yr, no greater than that observed after 6 yr. Soil organic C was the same for all rotations that included fallow (FW, FWW, FXW). Fertilizer treatments that had the greatest benefit on grain yields of annual crops also tended to increase SOC, although differences were barely detectable. The gain in SOC of unfertilized grass compared to the FW rotation was no higher after 12 yr than after 6 yr (3 Mg C [ha.sup.-1]). Under fertilized grass, in contrast, SOC continued to increase at a rate of approximately 0.5 Mg C [ha.sup.-1] [yr.sup.-1]. Accumulation of light fraction C accounted for most of the gains in SOC that occurred with elimination of fallow or establishment of grass. These findings suggest that much of the SOC gain due to adoption of C-conserving practices in soils like those of this study may occur early, within the first decade, and consist primarily of decomposable soil fractions. If confirmed, this means that C sequestration in these soils may be comparatively shortqived and vulnerable to future loss. Abbreviations: FW, fallow-wheat; FWW, fallow-wheat-wheat; FXW, fallow-flax-wheat; G, continuous grass; LW, legume-wheat; SOC, soil organic carbon; W, continuous wheat.

Rice (Oryza sativa L.), the main staple food in Asia, is typically produced on submerged anaerobic soils, which generally have slower decomposition of soil organic matter (SOM) than aerobic soils. We sampled four long-term experiments in the Philippines, with two or three rice crops grown each year with continuous or near-continuous soil submergence, to determine the effect of fertilizer management on long-term changes in soil C and N and on C and N balances. Soils were an Aquandic Epiaquoll, an Entic Pellustert, and a Typic Pelludert; soil pH ranged from 5.9 m 6.7. After 17 to 21 yr of continuous rice cultivation, the concentration of total soil organic C (SOC) and total soil N ([N.sub.T]) in the topsoil (0-20 cm) were greater with N-P-K fertilization than without fertilization. During 15 yr of additional continuous rice cropping, topsoil SOC and [N.sub.T] were consistently maintained or increased regardless of N-P-K fertilizer regime. Topsoil SOC increased up to 10% in an experiment with three rice crops per year and removal of all aboveground plant biomass after each crop. Subsoil SOC and [N.sub.T] (20-80 cm) were not affected by fertilization. The N balances indicated that biological [N.sub.2] fixation averaged 19 to 44 kg N [ha.sub.-1] [crop.sub.-1] across the four experiments. Anaerobic N mineralization (ANM) in the topsoil was maintained during 15 yr of continuous rice cropping with N-P-K fertilization in all four experiments. The results suggest that continuous cultivation of irrigated rice with balanced fertilization on submerged soils maintained or slightly increased SOM and maintained soil N-supplying capacity. Abbreviations: ANM, anaerobic N mineralization; BIARC, Bicol Integrated Agricultural Research Center; BNE biological dinitrogen fixation; LTCCE, long-term continuous cropping experiment; LTFE, long-term fertility experiment; [N.sub.T] total soil nitrogen; PhilRice, Philippine Rice Research Institute; POC, permanganate-oxidizable carbon; SOC, soil organic carbon; SOM, soil organic matter.

Irrigation in semiarid regions can alter soil C sequestration processes compared with those of native soils. To better understand the effect of these altered processes, we studied the C sequestration and dynamics of two soils from major irrigated agricultural regions of California (the San Joaquin Valley and the Imperial Valley). Soils were sampled from selected native and cultivated fields to represent a span of almost a century of irrigated farming. Field soil samples were analyzed for total soil C and soil inorganic C as carbonate (SIC). Soil organic C (SOC) was then calculated from the measured data. Results showed that SOC stock increased above that stored in the native soil after five decades of irrigated farming. The SIC stock showed opposing trends within the top meter of the two studied soils: a decrease was measured after 55 yr in the San Joaquin Valley soil, while SIC in the Imperial Valley soil increased after 85 yr of irrigated agriculture and appears to represent a significant form of sequestered soil C. Our results indicate that long-term irrigated farming can significantly increase SOC due primarily to SOC added below the 10-cm soil depth, while significant increases in SIC may be partially due to the conversion of increased soil C[O.sub.2] to carbonates under a regime of Colorado River irrigation water. Thus, when considering C sequestration in irrigated agriculture in semiarid regions, it is important to determine levels of both SOC and SIC. Abbreviations: IMP, Imperial Valley; OM, organic matter; SIC, soil inorganic carbon; SJV, San Joaquin Valley; SOC, soil organic carbon; SOM, soil organic matter.

Soil organic C (SOC) pools under long-term management practices provide information on C sequestration pathways, soil quality maintenance, and crop productivity. Farmyard manure (FYM), paddy straw (PS), and green manure (GM) along with inorganic fertilizers were used in a 19-yr-old rice (Oryza sativa L.)--wheat (Triticum aestivum L.) cropping system in subtropical India to evaluate their impact on SOC stock, its different pools--total organic C ([C.sub.tot]); oxidizable organic C ([C.sub.oc]) and its four fractions of very labile ([C.sub.frac1]), labile ([C.sub.frac2]), less labile ([C.sub.frac3]), and nonlabile C ([C.sub.frac4]); microbial biomass C ([C.sub.mic]); and mineralizable C ([C.sub.min]). Cropping with only N-P-K fertilization just maintained SOC content, while N-P-K plus organics increased SOC by 24.3% over the control, their relative efficacy being FYM > PS > GM. A minimum of 3.56 Mg C ha-1 yr-1 was required to be added as organic amendmes to compensate for SOC loss from cropping. The passive ([C.sub.frac3]+[C.sub.frac4]) and [C.sub.min] constituted about 39 and 11.5%, respectively, of [C.sub.tot]. Organics contributed toward the passive pool in the order FYM > PS > GM. Most of the pools were significantly (P = 0.005) correlated with each other. Yield and sustainable yield index were strongly related with [C.sub.frac1], [C.sub.oc], [C.sub.mic], and [C.sub.min]. Results suggest [C.sub.frac1] as a useful indicator for assessing soil health, and balanced fertilization with FYM as suitable management for sustaining crop productivity of the rice--wheat system. Abbreviations: BSR, basal soil respiration; [C.sub.frac1], very labile carbon; [C.sub.frac2], labile carbon; [C.sub.frac3], less labile carbon; [C.sub.frac4], nonlabile carbon; [C.sub.mic], microbial biomass carbon; [C.sub.min], mineralizable carbon; [C.sub.oc], oxidizable organic carbon; [C.sub.tot], total organic carbon; FYM, farmyard manure; GM, green manure; MQ, microbial quotient; PS, paddy straw; qC[O.sub.2], respiratory quotient; SOC, soil organic carbon; SYI, sustainable yield index.

Efficacy of soil clay minerals for the sorption of nonionic organic compounds may be reduced by soil organic matter (SOM) association with clays. Previously, we quantified the fractional availability ([f.sub.a]) of soil clay surfaces for sorption of para-nitrocyanobenzene (p-NCB). Removal of SOM from a soil increased p-NCB sorption, demonstrating that SOM reduced the availability of clay surfaces for sorption of p-NCB. In this study, we extended our investigation of SOM--smectite interactions by measuring the availability of smectite surfaces in four soils to four nitroaromatic compounds (NACs) including p-NCB, 2,4-dinitrotoluene (2,4-DNT), 1,4-dinitrobenzene (1,4-DNB), and 1,3,5-trinitrobenzene (1,3,5TNB). Increased NAC sorption was consistently observed after SOM was removed from whole soils, indicating that SOM blocks the availability of soil smectite surfaces to NACs. Estimates for the [f.sub.a] of smectite surfaces in the four soils ranged from 1 to 0.3. These [f.sub.a] values were strongly negatively correlated with the ratio of SOM/smectite contents. Also, [f.sub.a] values differed systematically with NAC probe molecules, with SOM blocking fewer clay mineral sites from more strongly sorbing NACs. This suggests that smectite surface availability in soils may be an operationally defined quantity whose value depends on the probe molecule. Values of [f.sub.a] approaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound. Abbreviations: 1,3,5-TNB, 1,3,5-trinitrobenzene; 1,4-DNB, 1,4-dinitrobenzene; 2,4-DNT, 2,4-dinitrotoluene; NAC, nitroaromatic compound; NOC, nonionic organic compound; p-NCB, paranitrocyanobenzene; SOM, soil organic matter.

Recently, it has been suggested that soil organic C (SOC) does not always respond linearly to increasing C input, thereby limiting the rate and efficiency of C stabilization in soils. Therefore, we postulated that when a soil is exposed to a broad range of C inputs through a range of manure treatments, it will show C saturation behavior and different SOC pools will saturate at different rates. To test this, different SOC pools were isolated by physical fractionation techniques from a long-term agricultural experiment in Lethbridge, Canada. In this experiment, manure has been applied since 1973 at rates of 0, 60, 120, and 180 Mg [ha.sup.-1] [yr.sup.-l] (wet weight). In the total mineral soil as well as the small macroaggregates (250-2000 [micro]m), microaggregates (53-250 [micro]m), and the silt plus clay fraction (2000 [micro]m) were the only water-stable aggregate fraction that increased in C content across all manure input levels. Further physical separation of macroaggregates into subpools by microaggregate isolation showed that coarse (>250 [micro]m) particulate organic matter (POM) was the fraction that accounted most for the increase in C content of the large macroaggregates. Furthermore, the turnover of large macroaggregates increased with increasing manure applications, as indicated by decreased formation and stabilization of intramicroaggregate POM within the large macroaggregates. We conclude that as C input increases, the mineral fraction of a soil saturates and consequently additional C input will only accumulate in labile soil C pools that have a relatively faster turnover. Abbreviations: cPOM, coarse particulate organic matter; fine iPOM, fine intramicroaggregate particulate organic matter; LF, light fraction; POM, particulate organic matter; SOC, soil organic carbon; SOM, soil organic matter.

Ultraviolet spectrophotometry was used to investigate the effects, 30 yr after planting, of tree species substitution on the aromatic C content and related properties of dissolved organic carbon (DOC). Precautions were taken to correct measurements for the absorbance of N[O.sub.3] and dissolved Fe. In litter leachates, a significant reduction in the aromatic content of DOC was found in the Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation but not in the beech (Fagus sylvatica L.) plantation. The disturbance of short-term C dynamics thus revealed agreed well with field observations. Significant differences in aromatic content were also found in capillary soil solutions from the two planted stands. Overall, these modifications, produced by the substitution of trees 30 yr previously, mostly concerned the beech plantation. Soil processes, and probably adsorption, played a central role in controlling the quality of DOC in this soil and appeared to be influenced by the species planted. In low-capillary solutions, located in larger pores, changes to the aromatic content were only detectable in the surface soil of the beech plantation. We found a more pronounced effect of tree substitution in high-capillary solutions filling soil micropores, where the aromatic content of DOC might be tightly controlled by soil organic matter. It is difficult to say, however, whether the planting of Douglas-fir had actually accelerated soil recovery, or whether there will be future changes to attain a completely new equilibrium. Abbreviations: DOC, dissolved organic carbon; SUVA, specific ultraviolet absorbance; UV, ultraviolet.

Eucalypt plantations have been increasingly fertilized with N applied postplanting in Uruguay, but information on the benefits of the practice is scarce. The objective of this study was to identify the N availability indices (NAI) able to differentiate responsive from unresponsive sites when N was applied to eucalypts 6 or 12 mo after planting (MAP). The NAI were based on soil and plant analyses. Volume yield response to N was evaluated in 20 experiments conducted in plantations of Eucalyptus globulus Labille and E. grandis Hill ex Maiden. The NAI were related to volume response expressions by using linear and quadratic-plateau models, and the model with the highest [R.sup.2] was selected. Leaf N concentration (LNC) was the NAI most strongly related to N response in both E. grandis fertilized at 6 MAP (seven sites) and E. glabulus fertilized at 12 MAP (six sites) and the estimated critical levels were 34.6 and 20.9 g [kg.sup.-1], respectively. None of the NAI could be selected in the E. globulus sites fertilized at 6 MAP, but soil mineralizable N (NMIN) was clearly related to N response when all E. glabulus sites were pooled (12 sites), resulting in a critical level of 109 mg N [kg.sup.-l]. Although the [R.sup.2] of the models describing relationships between the selected NAI and N response varied from 0.52 to 0.94, most NAI separated responsive from unresponsive sites. The results suggest that LNC and NMIN can be used as tools to improve prediction of the early eucalypt volume response to N in Uruguay. Abbreviations: AVI, absolute volume increase; CONTV, control volume; LNC, leaf nitrogen concentration; MAP, months after planting; MAXV, maximum volume; NM, nitrogen availability indices; NMIN, soil mineralizable nitrogen; RV, relative volume; SOM, soil organic matter.

We examined the effects of 10 yr of reduced tillage (RT) management, compared with conventional tillage (CT), on the chemical composition of soil organic matter (SOM) using a combination of physical fractionation and pyrolysis-field ionization mass spectroscopy (Py-FIMS). Surface soil samples (0-10 cm), which were collected from two adjacent CT and RT managed fields, were separated into particulate organic matter (POM) fractions and sand, silt, and clay size separates. Higher amounts of C (77%) and N (64%) were observed in all size separates of the RT soil and, in particular, the proportion of intramicroaggregate POM was two times as large as in the CT soil. The Py-FIMS spectra showed that larger proportions of free fatty acids, sterols, and N-containing compounds were present in the RT soil's sand fraction than in the CT soil. Similar observations were also made for both free and intramicroaggregate POM fractions. In the silt and clay size fractions, no substantial differences in SOM composition were measured between tillage treatments, except that the RT soil's clay fraction had a higher proportion of N-containing compounds and a lower proportion of alkylaromatics. Ratios of carbohydrate hexose to pentose subunits and of lignin monomers + phenols to lignin dimers confirmed a less decomposed character of the SOM in the silt and clay size fractions of the RT soil. These observations can be logically linked to the reduced mixing of crop residues and manure in the soil profile and higher surface soil microbial biomass under RT. Abbreviations: CT, conventional tillage; fPOM, free particulateorganic matter; iPOM, i ntramicroaggregate particulate organic matter; LE light fraction; NMR, nuclear magnetic resonance; OC, organic carbon; OM, organic matter; Py-FIMS, pyrolysis field ionization mass spectroscopy; RT, reduced tillage; SOC, soil organic carbon; SOM, soil organic matter; TII, total ion intensity.

The significance of soil water-repellent properties has been discussed with respect to water dynamics and distribution; however, there are indications of their importance also for C stabilization processes. Water-repellent aggregates, for example, have been shown to protect soil organic matter (SOM) due to their stability against water slaking. Soil wettability can act as a key factor for SOM decomposition as it controls the microbial availability of water and nutrients. The main objective of this study was therefore to investigate the impact of wettability on C release by linking the wetting properties in terms of the contact angle to soil respiration parameters. For this, the wetting properties of two topsoil samples (an Orthic Luvisol and a Dystric Cambisol) were altered by the addition of particles that were hydrophobized by treatment with dichlorodimethylsilane (DCDMS). Additionally, aggregates were created to assess whether artificial aggregation also can contribute to SOM protection. Environmental scanning electron microscopy revealed a locally confined distribution of water for the DCDMS-treated material, compared with untreated soil where water was uniformly distributed. Measurements indicated an increasing contact angle with increasing amount of DCDMS-treated particles in the mixtures. With increasing contact angle, C release decreased, suggesting that wettability-induced changes in water distribution can significantly affect the decomposition of SOM. Respiration from artificial aggregates, however, was not reduced compared with the corresponding homogeneous material. We conclude that wettability is an important factor for SOM decomposition as it governs the spatial distribution and availability of water necessary for microbial activity. Abbreviations: DCDMS, dichlorodimethylsilane; ESEM, environmental scanning electron microscopy; SOC, soil organic carbon; SOM, soil organic matter.

Very little is known about dissolved organic matter (DOM) originating from fine roots in forest soils in comparison to DOM originating from leaf litter. To compare the fate of root- vs. leaf-derived dissolved organic carbon (DOC), we added [sup.14]C-labeled root litter at depths of 10 cm, a 'shallow root treatment,' and 40 cm, a 'deep root treatment,' and leaf litter to 50-cm column soil microcosms. We analyzed the solution leached from the columns during a 47-d simulated snowmelt treatment, trapped respired [sup.14]C[O.sub.2], and measured translocation of [sup.14]C within the columns. In general, total C losses, as a percentage of [sup.14]C added, were substantial and highest for the leaf treatment (8.2% leached as DOC, 13.4% translocated, and 14.8% respired), intermediate for the shallow root treatment (2.3, 5.2, and 3.9%, respectively) and lowest for the deep root treatment (2.4, 1.9, and 2.9%, respectively). The C lost to DOC leaching was similar, however, for the deep and shallow root treatments. As a percentage of total losses (i.e., the sum of DOC leaching, respiration, and translocation), [sup.14]C lost as DOC leaching was significantly higher in the deep root treatment than other treatments. These observations suggest that leaf-derived DOC may contribute to the formation of an A horizon and even to accumulation of soil organic carbon (SOC) in the B horizon during soil development, either by adsorption or microbial biomass incorporation. The [sup.14]C data further showed that root-derived DOC, especially from root litter at greater depths, may help explain both the presence of SOC at depth and a portion of the DOC draining from soil profiles. Abbreviations: DIC, dissolved inorganic carbon; DOC, dissolved organic carbon; DOM, dissolved organic matter; SOC, soil organic carbon; SUVA, specific ultraviolet absorbance.

This study examined the amount of soil organic carbon (SOC) incorporated by cryoturbation into the active layer and near-surface permafrost of Turbels from northern Alaska. An analysis of 21 pedons revealed that an average of 55% of the SOC density of the active layer and near-surface permafrost could be attributed to redistribution from cryoturbation. Cryoturbation occurs most strongly under conditions of poor drainage, where the parent materials are enriched in silt, and where frost boils are present. Based on published radiocarbon dates of buried SOC, cryoturbation was particularly important during periods of the mid-Holocene when the arctic underwent warming. These results suggest that continued warming of the arctic could accelerate cryoturbation and enable the soil to store more SOC than at present, thereby mitigating some of the loss of C[O.sub.2] to the atmosphere from increased soil respiration. Abbreviations: SOC, soil organic carbon.

Greater knowledge of the dynamics of organic matter in different locations in the soil matrix can provide valuable information for implementing tillage practices that may favor C sequestration and improve soil quality. The objective of this study was to investigate the effect of tillage practices on the dynamics and turnover of organic matter located outside (free light fraction [FLF]) and inside (occluded light fraction [OLF]) aggregates, and in intimate association with soil minerals (heavy fraction [HF]). Composite soil samples from 11-yr-old corn (Zea mays L.) plots under no-till (NT) and conventional tillage (CT) practices, and from >60-yr-old tobacco (Nicotiana tabacum L.)--rye (Secale cereale L.) plots under CT were fractionated into these fractions, and changes in the amounts of total soil organic C (SOC), corn-derived ([C.sub.4]) C, and native ([C.sub.3]) C and the turnover of native C as a consequence of tillage were estimated. Adoption of NT increased (P < 0.05) standing SOC and [C.sub.4]-C stocks in whole soil and all its density fractions in the 0- to 5-cm depth, but it did not alter these in the plow layer (0-20-cm soil depth), indicating that NT primarily redistributed C within the profile without necessarily increasing SOC storage. Relative to CT, NT slowed the turnover of native C by 1.5 times and marginally (P = 0.1) increased the sequestration of [C.sub.4]-C by 23% in the plow layer in OLF only; however, [C.sub.3]- and [C.sub.4]-C of OLF accounted for only small (6-10%) proportions of the soil's [C.sub.3]- and [C.sub.4]-C. Therefore, NT did not result in increased SOC sequestration in the plow layer. Abbreviations: CT, conventional tillage; [C.sub.3]-C, native C; [C.sub.4]-C, corn-derived C; FLF, free light fraction; HF, heavy fraction; LE light fraction; NT, no-till; OLE occluded light fraction; SOC, soil organic carbon; SOM, soil organic matter.

Soil structure can be an important factor affecting soil organic carbon (SOC), but it is also a dynamic property affected by texture, mineralogy, land use, soil fauna, and also SOC. Assuming that structure affects SOC mostly by occlusion of particulate organic matter (POM) within aggregates, it was hypothesized that structure exerts a major control on SOC retention in soils of the Brazilian Cerrado region. Water-stable aggregates (WSA) were obtained from the 1-m depth of three different-textured, uncultivated soils. The mean weight diameter (MWD) of WSA was used as a structural indicator, and SOC concentrations were determined in intact WSA and their respective sand fractions (estimating occluded POM). Clay + silt content in bulk soils was correlated with MWD in all depths but more strongly in the top 10 cm. Although equally correlated with clay + silt contents, SOC concentrations were well correlated with MWD only in the 0- to 5-cm layer. Sand-flee SOC concentrations in WSA fractions were proportional to sand content, indicating that the SOC dilution effect reported in particle size fractionations occurs naturally in the soil fabric. Occlusion of POM within aggregates was proportional to clay + silt contents, but this did not result in larger total POM pools, and the weak correlations obtained did not warrant predictive models. Aggregates produced by macrofauna comprised a minor but significant part of the soil and were mostly SOC enriched. We concluded that the structural control on SOC retention is less significant than the textural and mineralogical controls, since aggregation depends on those properties and is not as strongly correlated with SOC concentration and POM occlusion. Abbreviations: MWD, mean weight diameter; POM, particulate organic matter; SOC, soil organic carbon; WSA, water-stable aggregates.

Simulation models integrate our knowledge of soil organic C (SOC) dynamics and are useful tools for evaluating impacts of crop management on soil C sequestration; yet, they require local calibration. Our objectives were to calibrate the Environmental Policy Integrated Climate (EPIC) model, and evaluate its performance for simulating SOC fractions as affected by soil landscape and management. An automated parameter optimization procedure was used to calibrate the model for a site-specific experiment in the Coastal Plain of central Alabama. The ability of EPIC to predict corn (Zea mays L.) and cotton (Gossypium hirsutum L.) yields and SOC dynamics on different soil landscape positions (summit, sideslope, and drainageway) during the initial period of conservation tillage adoption (5 yr) was evaluated using regression and mean squared deviations. Simulated yield explained 88% of measured yield variation, with the greatest disagreement on the sideslope position and the greatest agreement in the drainageway. Simulations explained approximately 1, 34, and 40% of the total variation in microbial biomass C (MBC), particulate organic C (POC), and total organic C (TOC), respectively. The lowest errors in TOC simulations (0-20 cm) were found on the sideslope and summit. We conclude that the automated parameterization was generally successful, although further work is needed to refine the MBC and POC fractions, and to improve EPIC predictions of SOC dynamics with depth. Overall, EPIC was sensitive to spatial differences in C fractions that resulted from differing soil landscape positions. The model needs additional refinement for accurate simulations of field-scale SOC dynamics affected by short-term management decisions. Abbreviations: CT, conventional tillage; CTm, conventional tillage plus manure; EPIC, Environmental Policy Integrated Climate; FHP, fraction of humus in the passive pool; HI, harvest index; MBC, microbial biomass carbon; MSD, mean squared deviation; NT, no-till; NTm, no-till plus manure; PARM 20, microbial decay rate; PARM 51, microbial activity in the top layer; POC, particulate organic carbon; SOC, soil organic carbon; SOM, soil organic matter; TOC, total organic carbon; WA, biomass/energy ratio.

Soil organic carbon (SOC) retention is a function of climate, vegetation, drainage, and management interactions, but also of intrinsic soil properties such as texture, mineralogy, and structure. To assess these edaphic controls, three soils of the Brazilian savanna (Cerrado) under similar climate, vegetation, and slope but of contrasting texture were sampled to 1-m depth and characterized for textural, chemical, and mineralogical properties, and SOC concentration (in bulk samples and clay, silt, and sand fractions). The basic assumption was that SOC particle size determines its retention mechanism: colloidal forms are retained by sorption, while particulate organic matter (>20 [micro]m) can occur outside or inside aggregates. It was hypothesized that SOC retention is controlled simultaneously by soil texture, mineralogy, and depth. The three soils are clayey, loamy, and sandy Haplustox, all kaolinitic with minor contents of Fe and Al oxides, vermiculite, and illite. The SOC concentrations in particle size fractions were inversely related to the content of the respective fraction in soil (SOC dilution effect), thus SOC partition is better assessed by determination of SOC pools in each particle size on a total soil mass basis rather than on a size-fraction concentration basis. The positive linear relations between SOC and clay + silt concentrations in bulk soil were explained mostly by greater clay-sized SOC pools, which could be modeled as a function of clay content (related to specific surface area) and depth. Quantitative clay mineralogy showed that bulk SOC and clay-sized SOC pools were well correlated with Fe oxides in topsoil and amorphous AI oxides in subsoil, but this mineralogical control is secondary to the textural control, since it depends on clay content. Abbreviations: BET, Brunauer-Emmet-Teller; CBD, citrate-bicarbonate-dithionite; POM, particulate organic matter; SOC, soil organic carbon; SSA, specific surface area.

Soil aggregation is an important process of C sequestration and hence a useful strategy to mitigate the increase in concentration of atmospheric C[O.sub.2]. We studied water stability of soil aggregates (WSA) and soil organic carbon (SOC) associated with aggregates and primary particles in surface (0-10 cm) and subsurface (10-20 cm) layers of cultivated (khet, irrigated lowland, and bari, rainfed upland) and forest lands (dense Shorea forest, degraded forest and shrub land, pine-Shorea forest, Shorea-pine-Schima forest, and Schima-Castanopsis forest) in a mountain watershed of Nepal. Macroaggregates (>2 mm) were abundant in forest soils (41-70%) while microaggregates ( Abbreviations: BD, bulk density; CEC, cation exchange capacity; DF, degraded forest and shrub land; DS, dense Shorea forest; PS, pine-Shorea forest; SC, Schima-Castanopsis forest; SNK, Student-Newman-Kuels test; SOC, soil organic carbon; SOM, soil organic matter; SPS, Shorea-pine-Schima forest; WSA, water stability of soil aggregates.

Agricultural biomass is a potential renewable biofuel that may partially replace nonrenewable fossil fuels. Corn stover is rich in cellulose and hemicellulose, both of which can be converted to sugars and fermented to ethanol. This fermentation process results in a high-lignin fermentation byproduct (HLFB) that could be converted to energy products or used as a soil amendment. We had two objectives: (i) to determine whether HLFB (0.1, 1.0, or 10 kg [m.sup.-2]) could improve soil properties in two soils with contrasting levels of soil organic carbon (SOC); and (ii) to assess the impact of HLFB on crop growth. These goals were addressed with separate experiments. In the soil experiment, two soils were amended with HLFB or ground corn (Zea mays L.) stover and then incubated in pots for 118 d. Flux of C[O.sub.2] was monitored and soil properties were measured after incubation. In the plant experiment, corn and soybean [Glycine max (L.) Merr.] were grown in pots, without amendment or amended with 1.0 kg [m.sup.-2] corn stover or 1.0 kg [m.sup.-2] HLFB. The soil experiment indicated that the addition of 10 kg [m.sup.-2] HLFB increased C[O.sub.2] emission, humic acid concentration, and water-stable aggregates, and decreased bulk density ([D.sub.b]). No adverse impacts on crop growth were measured when HLFB was applied at a rate of 1.0 kg [m.sup.-2]. Much of the HLFB may be used by the energy industry, but perhaps a percentage could be returned to the field to reduce the impact of corn stover removal on soil C. Abbreviations: HLFB, high-lignin fermentation byproduct; SOC, soil organic carbon.

Linked biochemical and fluvial processes in discontinuous ephemeral streams may support sustained productivity of soils farmed by southwestern Native Americans for [greater than or equal to] 3000 yr. Ephemeral stream channels transport forest floor litter and soil materials from upland hillslopes to alluvial fans. Improved understanding of how ephemeral streams transport and process forest-floor organic materials could improve conservation of ecologically important and productive headwater alluvial fans. We analyzed organic and mineral materials from source to mouth along two ephemeral streams and analyzed suspended sediments from four collection traps in each. Results suggest that decomposition processes differ by reach and frequent, low-energy flows preferentially transport organic detritus as it decomposes. Processing of organic-rich sediments in canyon reaches is dominated by microbial immobilization (low inorganic N and available P, high C/N ratio). Arroyo reaches receive organic materials chiefly from upstream so mineralization plays in increasing role as materials are transported and decomposed downstream without fresh inputs. The frequency of flow decreases in a downstream direction as water infiltrates sandy streambeds. In lower arroyo and fan reaches, inputs of organic-rich sediments are infrequent. Relatively frequent wetting and drying stimulates mineralization of organic materials so concentrations of inorganic N and available P in detritus are relatively high. Results suggest that organic-rich sediments processed through ephemeral streams and deposited on unincised alluvial fans are important in sustaining one of the most productive landscape positions in semiarid regions.

Soil organic matter (SOM) is important to soil function. The objectives of this work were to determine the effect of cropping rotation and soil management on SOM, components of SOM, and water-stable aggregation (WSA) of soil near the surface. Measurements were made on soil collected from the top 50 mm of seven sites representing contrasts between alternative and conventional management. Management included tillage, crop rotation, native grass pasture, and corn (Zea mays L.) stover removal as silage. At each site, approximately 10 kg of soil was collected from each replication. Soil was separated into six aggregate groups using a rotary sieve. Aggregate size ranges for Groups 1 to 6 were: 19 mm. Mean weight diameter was calculated using dry aggregate size distribution. Dry aggregate stability, WSA, soil carbon (SC), SOM, fine particulate organic matter (fPOM), and coarse POM were measured on aggregates from each aggregate group. Components of SOM were not uniformly distributed among aggregate groups. Average SC (seven sites) was significantly greater under alternative (31.0 g [kg.sup.-1]) than conventional (22.3 g [kg.sup.-1]) management. No tillage (NT) increased fPOM/SOM by 19 and 37% compared with tillage following 4 and 10 yr of NT, respectively. A 5-yr diverse rotation increased fPOM/SOM by 36% compared with monoculture. There was a significant, positive relationship ([r.sup.2] = 0.79) between WSA and fPOM/SOM. Diversity of rotation or reduction of tillage increased fPOM and WSA and this may help to curb soil loss by maintaining surface conditions resistant to erosion. Abbreviations: ALT, alternative farming practice; CON, conventional farming practice; cPOM, coarse (2.0-0.5 mm) particulate organic matter; DAS, dry aggregate stability; DASD, dry aggregate size distribution; EF, erodible fraction; fPOM, fine (0.5-0.053 mm) particulate organic matter; LOI, loss on ignition; MWD, mean weight diameter; NT, no tillage; POM, particulate organic matter; SC, soil carbon; SOM, soil organic matter; WSA, water-stable aggregation.

Sorption to micro- and mesoporous mineral phases can stabilize organic matter (OM) against microbial decay in soil. Formation of strong bonds that reduce desorbability is one plausible explanation for that effect. With time after sorption, sorbed OM may undergo changes in configuration or may migrate into intraparticle spaces. We tested the possible effects of residence time of OM sorbed to ferrihydrite and goethite. The minerals were loaded with different amounts of water-soluble OM from an Oa horizon, then stored moist (10% w/w water) for up to 1080 d at 4[degrees]C. We monitored the content of organic C, the desorbability and chemical stability (by extraction with 0.1 M NaOH-0.4 M NaF and treatment with 1 M NaOCl), and, after freeze-drying, the micro- and mesopore volume (by [N.sub.2] and C[O.sub.2] adsorption-desorption). Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to characterize the OM on the mineral surfaces at the beginning and end of the experiment. There was no detectable decrease in sorbed organic C during the experiment; also, the micro- and mesoporosity of the samples remained unchanged. The proportion of desorbable organic C, however, decreased by up to 16%. This was paralleled by more pronounced bands indicative of complexed organic functional groups in the DRIFT spectra. We conclude that with increasing residence time, OM sorbed to porous minerals becomes decreasingly desorbable by the formation of additional chemical bonds to the surface via ligand exchange but not by diffusion into small pores. The decrease in desorbability was accompanied by a decrease in chemical destructibility with NaOCl. The stability of sorbed OM against biological degradation may similarly increase with residence time. Abbreviations: DRIFT, diffuse reflectance infrared Fourier transform; OM, organic matter.

Crop residue decomposition is affected by environmental factors and residue biochemical properties. These factors may contribute to management protocols that enhance soil organic matter accumulation in hot climates, which have characteristically high oxidation rates. The purpose of this experiment was to determine [O.sub.2] effects on soil polyphenol oxidase (PPO), peroxidase (PO), water-soluble polyphenolics (WEP), arginine ammonification (ARG), and water-extractable C (WEC). Cowpea (Vigna unguiculata L.) and sorghum [Sorghum bicolor (L.) Moench] residues in soil were incubated with headspace [O.sub.2] concentrations of 0.5, 10, or 21% (ambient) for up to 110 d. The PPO and PO were negatively related to WEP and WEC, mostly at 10 and 21% (P < 0.05). The WEP and WEC were significantly (P < 0.05) and negatively related to [O.sub.2] concentration. The WEP ranged from 0.5 mg [kg.sup.-1] soil at 0.5% [O.sub.2] to near 0 at 10 and 21% [O.sub.2]. The WEC ranged from about 350 mg C [kg.sup.-1] soil at 0.5% [O.sub.2] to Abbreviations: ARG, arginine ammonification; PO, peroxidase; PPO, polyphenol oxidase; SOC, soil organic carbon; WEC, water-extractable C; WEP, water-extractable polyphenolics.

Soil C dynamics below the plow layer have been little studied, in spite of proven large C stocks and suspected large C stabilization potential. The objective of the present study was to determine C-turnover kinetics throughout the 1-m profile of a cultivated loam soil of the Paris basin, France. The soil [sup.13]C signature was determined to depths of 1.05 m in 32 replicated plots having received from 0 to 10 yr of maize after wheat. Above- and below-ground maize-residue biomass inputs were estimated throughout the 10-yr period. After 10 yr, maize-derived soil organic carbon (SOC) constituted about 10, 5, and 2% of the total SOC at 15-, 50-, and 100-cm depths, respectively. About one-third of recently deposited maize-derived SOM present in the 1-m soil profile was retrieved below the Ap horizon. The ratios of maize-derived soil C to the cumulative maize above- and below-ground inputs over the 10-yr period averaged 17% across the soil profile. This ratio was lower in the Ap horizon (i.e., 13%) than in deeper soil horizons. Circumstantial evidences suggest that the distribution profile of recently deposited maize-derived C was influenced by fine root activities, bioturbation, and dissolved organic carbon (DOC) transport, the latter being substantiated by a high correlation ([r.sup.2] = 0.86) between SOC contents and amorphous Fe + Al contents. In conclusion, our study stresses the need to take into account the full 1-m soil profile in C sequestration studies.

Binding of soil organic matter (OM) at micropore entrances within small mesopores (2-10 nm) has been suggested as a potential mechanism for the stabilization of OM against biodegradation. We hypothesized that the mineral-associated fraction of stable OM [OM resisting treatment with 6% sodium hypochlorite (NaOCl) and subsequently extracted by 10% hydrofluoric acid] is associated with pores

Management practices that alter plant residue production and distribution influence SOC (soil organic carbon) dynamics. The objectives of this study were to investigate the impacts of tillage, cropping system, and N fertilization on SOC and soil N pools through physical fractionation of a central Texas soil after 20 yr. Nitrogen fertilization and no-tillage (NT) significantly increased wheat (Triticum aestivum L.) straw yield. Compared with conventional tillage (CT), SOC under NT in surface (0-5-cm) samples was 38, 69, and 68% greater for continuous wheat (CW), wheat-soybean [Glycine max (L.) Merr.]-sorghum [Sorghum bicolor (L.) Moench] rotation (SWS), and double-cropped wheat-soybean (WS), respectively. The greatest SOC was observed in WS under NT with N fertilization, and the lowest occurred in CW under CT without N. Increased cropping intensity increased SOC compared with monoculture. Nitrogen fertilization only significantly increased SOC sequestration under NT. No-tillage increased SOC concentration in all physical size fractions compared with CT. Increased cropping intensity and N fertilization significantly increased SOC sequestration in most size fractions only under NT. Intraparticulate organic matter C (IPOM-C) was proportionally more affected by tillage than total SOC, indicating that this fraction was more sensitive to management. Carbon concentrations in all size fractions were significantly correlated with each other as well as SOC. Our results indicated that NT associated with enhanced cropping intensity and N fertilization sequestered greater SOC and soil total N.