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Differences in soil nutrients beneath different tree species are often attributed to the impacts of species-level patterns of nutrient uptake and litter chemistry. However, in naturally established forests it is difficult to isolate tree species' influence on soil development from differences in underlying soil properties that can affect tree species establishment. To discern the impacts of tree species on soil properties, we investigated how Norway spruce (Picea abies (L.) H. Karst.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marshall) influence the distribution of carbon, nitrogen, and calcium in a 67-year-old common garden. We expected these species would produce foliar litter with contrasting chemistry, resulting in corresponding variation in organic matter (OM) turnover and nutrient accumulation in soils. Instead, we found that forest floor mean residence time correlated negatively with earthworm density and did not correlate with any measurement of litter chemistry. Red oak exhibited the fastest OM turnover and highest earthworm densities and Norway spruce showed the greatest OM accumulation and fewest earthworms. These findings suggest that future changes in earthworm invasion and forest tree species composition may have strong implications for ecosystem nutrient cycling and retention. Resume: Le fait que les nutriments different dans le sol sous differentes especes d'arbre est souvent attribue aux impacts du caractere specifique de l'absorption des nutriments et des processus chimiques dans la litiere. Cependant, dans les forets qui se sont etablies naturellement, il est difficile d'isoler l'influence d'une espece d'arbre sur le developpement du sol a partir des differences dans les proprietes du sol sous-jacent qui peuvent influencer l'etablissement d'une espece d'arbre. Pour discerner les impacts de l'espece d'arbre sur les proprietes du sol, nous avons etudie comment l'epicea commun (Picea abies (L.) H. Karst.), le chene rouge (Quercus rubra L.) et l'erable a sucre (Acer saccharum Marshall) influencent la repartition du carbone, de l'azote et du calcium dans une meme plantation agee de 67 ans. Nous nous attendions a ce que ces especes produisent une litiere de feuilles dont les caracteristiques chimiques seraient differentes, entrainant une variation correspondante dans le renouvellement de la matiere organique (MO) et l'accumulation des nutriments dans le sol. Au contraire, nous avons trouve que le temps de residence moyen de la couverture morte etait negativement correle avec la densite des vers de terre et n'etait correle avec aucune des mesures des caracteristiques chimiques de la litiere. La densite des vers de terre etait la plus elevee et le renouvellement de la matiere organique etait le plus rapide sous le chene rouge tandis que c'est sous l'epicea commun qu'il y avait le moins de vers de terre et que l'accumulation de MO etait la plus forte. Ces resultats indiquent que des changements futurs dans l'invasion des vers de terre et la composition en especes d'arbre forestier pourraient avoir d'importantes consequences sur la retention et le recyclage des nutriments dans l'ecosysteme. [Traduit par la Redaction], Introduction Forest tree species composition across the northeastern US is expected to change. Species-specific pests and pathogens, such as the hemlock woolly adelgid [Adeldes tsugae (Annand)], beech bark disease, and [...]

Under semiarid Mediterranean climatic conditions, scarce attention has been given to the utilization of permanent cover crops (PCCs) in vineyard agroecosystems to increase soil organic C (SOC) and to improve the soil quality. We evaluated the feasibility of two permanent cover crops (resident vegetation, RV, and 'Aurora gold' blue rescue [Festuca longifolia Thuill.] vegetation, FV) to increase SOC and to improve the soil quality with respect to conventional tillage in a semiarid vineyard. We studied the influence of these management practices on the labile organic matter fractions (particulate organic C [POC], potentially mineralizable N [PMN], and water-soluble C [WSC]), on the water aggregate stability (WAS) and on the stratification ratios (SRs) of these soil attributes. The field experiment was conducted on a fine-loamy, mixed, thermic Typic Haploxerept soil in a vineyard located in the La Rioja region (northeast Spain). Soil samples were collected from four soil layers (at depths of 0-2.5, 2.5-5, 5-15, and 15-25 cm) during June 2008, 4 yr after cover crop establishment. The SOC content, POC, WSC, PMN, and WAS were measured. At a depth of 0 to 2.5 cm, the cover crops increased SOC, POC, WSC, PMN, and WAS. The C sequestration rates for RV and FV were 1.34 [+ or -] 0.31 and 1.52 [+ or -] 0.70 Mg [ha.sup.-1] [yr.sup.-1] respectively. Regression analyses indicated positive strong correlations among all parameters monitored. The SRs of the SOC, POC, and PMN were >2 in all depth ratios calculated and therefore were efficient indicators of the soil quality improvement caused by the PCCs. In a semiarid Mediterranean vineyard, a PCC has proven to be an effective strategy to enhance the SOC and to improve the soil quality in the short term. doi: 10.2136/sssaj2010.0081

Sewage sludge from wastewater treatment contains organic matter and plant nutrients that can play an important role in agricultural production and the maintenance of soil fertility. The present study has aimed to evaluate the degree of humification following sewage sludge application of soil organic matter by laser-induced fluorescence and humic acids using ultraviolet-visible fluorescence, and including comparison with Fourier-transform infrared spectroscopy and elemental analysis. Sewage sludge applications to the soil caused a decrease in the degree of humification of the soil organic matter and humic acids for both a Typic Eutrorthox (clayey) soil and a Typic Haplorthox (sandy) soil of around 14 and 27%, respectively. This effect is probably due to incorporation of newly formed humic substances from the sewage sludge into the characteristics of less humified material, and to the indigenous soil humic substances. The minor alterations observed in the clay soil probably occurred due to both the greater mineral association, which better stabilized the indigenous soil organic matter, and the higher microbial activity in this soil, which accelerated sewage sludge mineralization. Sewage sludge applications increased the C content for the clay and sandy soils by 7.4 and 15.4 g [kg.sup.-1], respectively, suggesting a positive effect on these two soils. Abbreviations: FTIR, Fourier-transform infrared; HA, humic acid; LIE laser-induced fluorescence; SOM, soil organic matter; TOC, total organic carbon. doi:10.2136/sssaj2008.0176

We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.

Different separation procedures are suggested for studying the stability and functionality of soil organic matter (OM). Density fractionation procedures using high-molarity, water-based salt solutions to separate organic particles may cause losses or transfers of C between particle and soluble OM fractions during separation, which may be a result of solution processes. The objective of this study was to separate coarse organic particles (> 0.315 ram) from air-dried surface soil samples to avoid such solution processes as far as possible. Air-dried surface soil samples ( 0.315 ram) from air-dried surface soil samples. Because aqueous solutions are not used, the electrostatic attraction procedure to separate coarse organic particles avoids C losses and transfers associated with solution-dependent techniques. Therefore, this method can be used as a pretreatment for subsequent density- or solubility-based soil OM fractionation procedures. Abbreviations: coarse [OP.sub.el], coarse organic particles separated by electrostatic attraction; LF, light fraction; OC, organic carbon; OM, organic matter; [OP.sub.water], organic particles separated from samples after electrostatic separation of coarse organic particles by mixing with water; POM, particulate organic matter; SEM, scanning electron microscope; SOC, soil organic carbon. doi: 10.2136/sssaj2009.0046

Laboratory sorption experiments were conducted to determine the extent to which soil organic matter derived from guayule (Parthenium argentatum A. Gray) residues can affect the sorption of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine]. Soils where guayule had been grown for 0 to 38 mo were collected for sorption experiments from beneath actively growing plants. An additional treatment was collected before planting cotton (Gassypium hirsutum L.) in a field 1 yr after the final harvest of guayule that had been grown for 36 mo was also included. Continuous guayule cultivation resulted in a modest increase in soil organic C from 0.29% ([+ or -] 0.01%) to 0.76% ([+ or -] 0.03%) after 38 mo of production. Pendimethalin sorption to the soil increased with increasing guayule organic deposition as measured by time under cultivation. Soils where guayule was grown for 38 mo had an organic C distribution coefficient, [K.sub.OC], 23 times greater than the control soil where guayule had not been grown (389,400 vs. 16,900 L [kg.sup.-1]). In the treatment before planting cotton, however, the combination of time and cultivation resulted in a reduction of [K.sub.OC] from 389,400 to 21,500 L [kg.sup.-1]. The use of pendimethalin in a guayule-cotton rotation requires sufficient time and primary tillage before planting cotton. In addition, the use of pendimethalin during regrowth of guayule from the stump following harvest may require higher application rates to control weeds. Adsorption of pendimethalin to soil using batch equilibrium indicated that soil beneath actively growing guayule has a higher sorption capacity for pendimethalin than soil without guayule. Abbreviations: [K.sub.D], distribution coefficient; KOC, organic carbon distribution coefficient; SPE, solid phase extraction. doi: 10.2136/sssaj2008.0339

Accurate measurement of soil total N (TN) content in agricultural fields is important to guide reasonable application of nitrogenous fertilizer. Estimation of soil TN content with limited in situ data at an acceptable level of accuracy is important because laboratory measurement of N is a time- and labor-consuming procedure. This study was conducted to evaluate cokriging of soil TN with predicted soil organic matter (SOM) content as auxiliary data. The SOM content was predicted by cokriging with a digital number (DN) of Band 1 of Landsat Enhanced Thematic Mapper (ETM) imagery. Soil TN content was estimated by using 88 soil samples for prediction and 43 soil samples for validation in a study area of 367 [km.sup.2] in Haining City, China. Field-measured soil TN content ranged from 0.47 to 2.48 g [kg.sup.-1], with a mean of 1.25 g [kg.sup.-1]. Soil TN content of all 131 soil samples including samples for prediction and validation was highly correlated with measured (r = 0.81,p < 0.01) and predicted (r = 0.81,p < 0.01) SOM content in paddy fields. Then, the predicted SOM content was used as auxiliary variable for the prediction of soil TN content. By using the 43 samples for validation, we had a mean error (ME) of 0.03 g [kg.sup.-1] and a root mean square error (RMSE) of 0.31 g [kg.sup.-1] for kriging, and a mean error of 0.00 g [kg.sup.-1] and a root mean square error of 0.25 g [kg.sup.-1] for cokriging, respectively. Our results indicate cokriging with predicted SOM content data was superior to kriging. In addition, predicted data of the auxiliary variable have the potential to be useful for cokriging when the predicted auxiliary data have high prediction accuracy. Abbreviations: DN, digital number; ETM, (Landsat) Enhanced Thematic Mapper; GPS, global positioning system; ME, mean error; RMSE, root mean square error; SOM, soil organic matter; TN, total nitrogen.

Soil organic carbon (OC) content is often estimated by determining organic matter (OM) content via mass loss on ignition (LOI) and assuming that there are 580 g OC [kg.sup.-1] OM. We utilized thermogravimetry (TG), oven-determined mass LOI, and the Walkley-Black (WB) wet oxidation method to evaluate the relationship between OC and OM contents of carbonatic and organic soils from Florida and Puerto Rico and Spodosols from Florida. These soils have contrasting components and formed under different ecological settings. The OC content of OM, based on TG-determined OM mass LOI (TG OM) and WB OC, was higher for Histosols and Spodosols than for carbonatic soils. Regression coefficients between WB OC and TG OM were high for all soils. The factor to convert OM to OC, based on TG OM and WB OC, was 0.56 [+ or -] 0.04 ([+ or -] 0.04 is the 95% confidence interval) ([R.sup.2] = 0.93) for carbonatic soils, 0.67 [+ or -] 0.06 ([R.sup.2] = 0.94) for Spodosols, and 0.69 [+ or -] 0.04 ([R.sup.2] = 0.99) for Histosols. Results show that accuracy in estimating OC from OM content may be improved if the conversion equation is developed from soils within limited compositional and/or biogeochemical ranges. As such, there is no universal conversion factor between OM and OC for all soils. Loss on ignition is not a reliable OM measure if there are mass losses below 200[degrees]C that are mistakenly attributed to OM combustion, as in the case of reversible dehydration for Histosols of this study. However, TG enables detection of and accounting for such mass losses. Isothermal TG also confirmed that reversible mass loss between 105 and 200[degrees]C was energy--rather than time dependent. Abbreviations: CI, confidence interval; LOI, Loss on ignition; OC, Organic carbon; OM, Organic matter; PR, Puerto Rico; TG, Thermogravimetry; TOC, Total organic carbon; WB, Walkley and Black.

Accurately measuring soil organic matter content (SOM) in paddy fields is important because SOM is one of the key soil properties controlling nutrient budgets in agricultural production systems. Estimation of this soil property at an acceptable level of accuracy is important; especially in the case when SOM exhibits strong spatial dependence and its measurement is a time- and labor-consuming procedure. This study was conducted to evaluate and compare spatial estimation by kriging and cokriging with remotely sensed data to predict SOM using limited available data for a 367-[km.sup.2] study area in Haining City, Zhejiang Province, China. Measured SOM ranged from 5.7 to 40.4 g [kg.sup.-1], with a mean of 19.5 g [kg.sup.-1]. Correlation analysis between the SOM content of 131 soil samples and the corresponding digital number (DN) of six bands (Band 1-5 and Band 7) of Landsat Enhanced Thematic Mapper (ETM) imagery showed that correlation between SOM and DN of Band I was the highest (r = -0.587). We used the DN of Band 1 as auxiliary data for the SOM prediction, and used descriptive statistics and the kriging standard deviation (STD) to compare the reliabilities of the predictions. We also used cross-validation to validate the SOM prediction. Results indicate that cokriging with remotely sensed data was superior to kriging in the case of limited available data and the moderately strong linear relationship between remotely sensed data and SOM content. Remotely sensed data such as Landsat ETM imagery have the potential as useful auxiliary variables for improving the precision and reliability of SOM prediction. Abbreviations: CV, coefficient of variation; DN, digital number; ETM, Landsat Enhanced Thematic Mapper; NDVI, normalized difference vegetation index; SE, standard error of the estimate; SOM, soil organic matter content; STD, standard deviation; TM, Landsat Thematic Mapper.

Application of organic manure (OM) to arable lands improves soil tilth. The objectives of this study were to: (i) simulate the effect of OM application rates (0, 168, 336, and 672 kg total N [ha.sup.-1]) and types (chicken [Gallus domesticus] and dairy manures) on soil organic C (SOC) and C[O.sub.2] emissions from a Hawaiian highly weathered tropical soil; and (ii) correlate SOC, C[O.sub.2] emissions, and two major soil properties: bulk density ([p.sub.b]) and saturated hydraulic conductivity ([K.sub.sat]). Measurements of SOC and [p.sub.b] were conducted on samples collected from the top 10 cm of soil tilled before and after manure application, cultivated with sweet corn ('Super Sweet 10,' Zea mays L. ssp. mays), and drip irrigated for two consecutive growing seasons. The [K.sub.sat] values were calculated from infiltration data measured with a tension infiltrometer. The Rothamsted C turnover model was used to simulate SOC and C[O.sub.2] emissions. The simulated and measured SOC agreed reasonably during model calibration ([r.sup.2] = 0.93) and validation ([r.sup.2] = 0.90). Results revealed that SOC, C[O.sub.2] emissions, and [K.sub.sat] increased while [p.sub.b] decreased with increasing OM rates. There was no significant effect of OM type. There was a highly significant (P < 0.01) correlation between the measured and simulated SOC and between the measured SOC and the simulated C[O.sub.2] emissions. The [K.sub.sat] values significantly correlated (P < 0.05) with the measured and simulated SOC and the simulated C[O.sub.2] emissions. A significant inverse relationship between [p.sub.b] and [K.sub.sat] was observed. We concluded that, in addition to improving soil aggregation, decreasing [p.sub.b], and increasing [K.sub.sat], OM application to this tropical soil increases SOC pools that contribute to atmospheric C[O.sub.2] following tillage and other agricultural practices. Abbreviations: BIO, microbial biomass; DPM, decomposable plant material; HUM, humified organic matter; IOM, inert organic matter; OM, organic manure; RothC, Rothamsted carbon model; RPM, resistant plant material; SOC, soil organic carbon; SOM, soil organic matter.

This study examined the influence of organic residue quality and N fertilizer on aggregate-associated soil organic matter (SOM) in maize (Zea mays L.) cropping systems of southern Ghana. Six residue treatments of differing quality [Crotalaria juncea L., Leucaena leucacephala (Lam.) de Wit, maize stover, sawdust, cattle manure, and a control with no residues added] were applied at 4 Mg C [ha.sup.-1] [yr.sup.-1] both with and without fertilizer N additions (120 kg N [ha.sup.-1] [season.sup.-1]). Soils (0-15 cm) were sampled 3 yr after study implementation and wet sieved into four aggregate size classes (8000-2000, 2000-250,250-53, and 250 [micro]m), microaggregates within macroaggregates (53-250 [micro]m), and macroaggregate-occluded silt and clay ( Abbreviations: SOM, soil organic matter.

The bioavailability of pesticides in soils is affected by the addition of crop-residue-derived char, which alters the sorption and desorption characteristics of the soils. Sorption, desorption, and biodegradation experiments were performed using atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-l,3,5-triazine-2,4-diamine] as the target compound in two soils (Hartsells and Grady) with and without a wheat (Triticum aestivum L.) char, and the char alone. The sorption isotherms for both soils were highly linear, whereas the isotherms for 1% char-amended soils showed an increased sorption capacity and isotherm nonlinearity. The amount of atrazine sorbed by the char alone was 800 to 3800 times greater than sorbed by the soils. Successive desorption experiments showed that char-amended soils contained higher nondesorbable fractions of atrazine than soils without char amendment. Pseudomonas sp. strain ADP capable of rapidly mineralizing atrazine was used to evaluate atrazine bioavailability. The presence of char reduced [sup.14]C[O.sub.2] production by 11 and 20% in the char-amended soils and char, respectively. The two-step successive desorption experiments appeared to be able to predict the bioavailabilty of atrazine in char-amended soils. This study shows that the presence of char in soils significantly influences the sorption and desorption processes and, ultimately, the bioavailability of atrazine. Abbreviations: LSC, liquid scintillation counting; SOM, soil organic matter.

Soil bulk density ([p.sub.b]) is required to estimate, evaluate, and calculate many physical soil properties and processes and is essential to convert data from weight-based to volume- and area-related data. One of the dominating factors changing [p.sub.b] is the soil's organic matter (SOM) concentration that alters the soil's compressibility; [p.sub.b] is an important soil structure attribute. Currently, no parameter for characterizing soil compactness giving directly comparable values for all soils is available. Therefore, our aim was to develop a general approach to calculate the effect of SOM concentration on [p.sub.b] that would be universally valid for soils different in their genesis, compaction, and type of land use. To describe the effect of SOM on [p.sub.b] mathematically, we used a nonlinear regression model that was parameterized and validated using published data from experiments where SOM concentration was the main [p.sub.b]--affecting factor (long-term fertilization and proctor experiments, wetlands, reclaimed soils, and volcanic soils). To obtain a standardized parameter describing the present compaction status of a site, we introduced the standardized bulk density [sp.sub.b]. Mathematically, [sp.sub.b] is the intercept parameter of the used nonlinear regression model, and ranged between 0.7 and 2.1 Mg [m.sup.-3] and was very simple to estimate. Another distinct advantage of this novel concept is that only one representative pair of [p.sub.b] and SOM has to be known to calculate [sp.sub.b] as well as the bulk densities corresponding to other SOM concentrations measured on the site. This concept might also be helpful for identifying similar universal approaches to standardize the effect of other [p.sub.b] affecting parameters (e.g., texture, soil depth, tillage regime), however, reassessed from the SOM effect. Abbreviations: OM, organic matter; [p.sub.b], bulk density; RMSE, root-mean-square error; RC, relative compaction; SD, standard deviation; SE, standard error; SOC, soil organic carbon; SOM, soil organic matter; [sp.sub.b], standardized bulk density.

The magnitude, duration, and return intervals of surface water flooding and dewatering of the landward extent of the hydric soil indicators muck (LM), histic epipedon (LHE), and Histosol (LH) were quantitatively defined, providing a better understanding of the hydrologic conditions maintaining these hydric soil indicators. Land surface elevations were determined for the LM, LHE, and LH at 16 lakes with long-term (30-60-yr) modeled or gauged hydrologic data. The probability of flooding and dewatering of the elevations of the LM, LHE, and LH were determined from frequency analysis of hydrologic data from each lake. The resulting hydrologic signatures for the LM, LHE, and LH are composed of magnitude and return interval of 1, 30, 90, 183, 274, and 365-d duration flooding and dewatering events. As an example, the LM, LHE, and LH were flooded for 30 continuous days with average annual probabilities of 42, 65, and 77%, respectively. As a second example, the LM, LHE, and LH were dewatered for 365 continuous days with average annual probabilities of 49, 24, and 16%, respectively. Probabilities of flooding and dewatering for the LM, LHE, and LH are presented for 1, 30, 90, 183, 274, and 365-d durations. Mean hydrologic signatures reduce variability and may be considered representative of each soil characteristic. Quantitatively defining the hydrology associated with the presence of the LM, LHE, and LH as well as other soil characteristics is essential for environmental protection, assessment of hydrologic impacts, wetlands restoration, wetlands creation, and other environmental management applications. Abbreviations: DO, dissolved oxygen; LH, landward extent of Histosol; LHE, landward extent of histic epipedon; LM, landward extent of muck; NPP, net primary production; SOM, soil organic matter.

Condensed organic matter (OM) such as nonhydrolyzable C (NHC) is important in C cycling and the sorption and fate of hydrophobic organic contaminants in soils and sediments, but its structural components are rarely known. Nonhydrolyzable C in three contaminated soils and 22 bulk and size-fractionated sediments from the Pearl River delta and estuary in China were measured following treatment with an HCl-HF-trifluoroacetic acid method. It was characterized using elemental analysis, radiocarbon accelerated mass spectroscopy, and Raman microspectrometry. The results show that NHC is an important or even dominating component of the organic C (OC) in the investigated soils and sediments. The NHC contents in this study are highly correlated with the OC contents, with a slope of 0.647 for the 25 soil and sediment samples, which is higher than previously reported NHC-OC correlations in the soils. The radiocarbon analysis demonstrates the importance of ancient OM in the NHC samples. The NHC fractions are chemically and structurally different from the biopolymer and humic substances in the soils and sediments, and originated from different thermally matured bitumen or kerogens, polymethylene C (algaenan, cutan, cutin, cuticle, and lipid), aged terrigenous OM (lignin and humin), and black carbon. Abbreviations: BC, black carbon; HA, humic acid; NHC, nonhydrolyzable carbon; NMR, nuclear magnetic resonance; NR, North River; OC, organic carbon; OM, organic matter; PRD, Pearl River Delta; SOM, soil and sediment organic matter; TFA, trifluoroacetic acid; WR, West River.

While P sorption in mineral soils has been extensively studied, P sorption behavior in organic-rich soils is less known. This study was conducted to determine the relationships between Langmuir P sorption maxima ([S.sub.max]) and selected physicochemical properties of soils, with particular emphasis on organic matter (OM) content. The [S.sub.max] values were determined for 72 soil samples from the North Carolina Coastal Plain, along with pH, clay and OM contents, oxalate-extractable P ([P.sub.ox]), Al ([Al.sub.ox]), and Fe ([Fe.sub.ox]), and Mehlich 3 extractable P ([P.sub.M3]), Al ([Al.sub.M3]), and Fe ([Fe.sub.M3]). Path analysis was used to examine direct and indirect effects of soil properties on [S.sub.max]. In the oxalate path analysis, the direct effects of clay, [Al.sub.ox], and [Fe.sub.ox] on [S.sub.max] were significant in the order [Al.sub.ox] > clay > [Fe.sub.ox] (P < 0.05). The [S.sub.max] was highly influenced by the indirect effect of [Al.sub.ox] and [Fe.sub.ox] through OM content. A two-piece segmented linear relationship existed between [S.sub.max] and OM and the regression slope in soils with OM [less than or equal to] 49 g [kg.sup.-1] was 10-fold greater than that for soils with OM > 49 g [kg.sup.-1]. This finding suggested that noncrystalline or organically bound Al and Fe in the soils with OM > 49 g [kg.sup.-1] is less effective for P sorption than in the soils with lower OM content. In the Mehlich 3 path analysis, the direct effects of clay, OM, and [Al.sub.M3] on [S.sub.max] were significant in the order [Al.sub.M3] > OM > clay (P < 0.05) while the direct effect of [Fe.sub.M3] on [S.sub.max] was not significant. Oxalate may be better suited than Mehlich 3 as an extractant for predicting P sorption capacity in the Coastal Plain soils. Abbreviations: OM, organic matter; PM3, AlM3, and FeM3, Mehlich-3 extractable phosphorus, aluminum, and iron; Pox, Alox, and Feox, oxalate-extractable phosphorus, aluminum, and iron; [S.sub.max], Langmuir phosphorus sorption maxima.

The binding and cementing potentials of clay and organic matter (OM) and the weakening effect of wetting are important in the stability of soil aggregates. They can therefore influence aggregate breakdown (AB) and splash detachment (SD), which are initial steps in the erosion process. We investigated the interactive effects of clay and OM on AB and SD under various wetting rates (WR) and antecedent moisture contents (AMC) using six agricultural soils from Trinidad. The selection criteria for the six soils were based on three levels of clay; low (45%) and two levels of OM; low ([less than or equal to] 3%), and high (>3%). Samples were prewetted with mist at slow (7.5 mm [h.sup.-1]) and fast (75 mm [h.sup.-1]) WR to AMC of 0.5 of field capacity (FC) and FC and exposed to intense simulated rainfall of 120 mm [h.sup.-1]. The sensitivity of a sample to disruption under varying wetting conditions and intense rainfall depended on the level of combination of clay and OM in the sample. Increase in clay beyond the medium level without raising OM to high level strengthened disruptive forces and increased the proportion of microaggregates. The AB and SD of the medium clay-high organic matter (McHom) soils were significantly lower than their high clay-low organic matter (HcLom) counterparts irrespective of WR and AMC. This implies that a threshold clay content exists beyond which an accompanying increase in OM is required to mitigate detachment mechanisms and erosion under intense rainfall. Abbreviations: AB, aggregate breakdown; AMC, antecedent moisture content; ANOVA, analysis of variance; CEC, cation-exchange capacity; ESP, exchangeable sodium percentage; FC, field capacity; HcLom, high clay-low organic matter; HcHom, high clay-high organic matter; KE, kinetic energy; LcLom, low clay-low organic matter; LcHom, low clay-high organic matter; McLom, medium clay-low organic matter; McHom, medium clay-high organic matter; OM, organic matter; SD, splash detachment; WR, wetting rate.

Fire transforms soil organic matter (SOM) to recalcitrant forms of C. The degree to which SOM is altered is dependent on fire severity. This study investigated changes in SOM composition and mineralization by controlling the fire severity of laboratory burns on reconstructed soil profiles (O, A1 [0-1 cm], and A2 [1-2 cm] horizons). Burning simulated low-, moderate-, and high-severity fires. Organic and mineral soils were incubated for 180 d and C[O.sub.2] production was measured with soda lime traps. Soils were analyzed for SOM composition pre- and post-incubation using an alkaline extraction. Higher severity burning resulted in lower O horizon decomposition rates on a C basis. Fulvic acid C in the control and low-severity O horizons was reduced by 13% by incubation, which was negatively correlated with cumulative C mineralized (r = -0.564). After incubation, the SOM composition of the burned O horizons remained different from the control. Higher severity burning caused mineral soil to initially have higher C mineralization rates, which disappeared by the end of the incubation. Fulvic acid concentration was reduced by 61 and 38% during incubation of the A1 and A2 horizons, respectively, returning the SOM composition to control levels. The sum of alkaline insoluble forms (e.g., humin) of SOM accounted for 74 and 61% of total soil C and N, respectively, which was not significantly different between treatments. The fire severities examined show that the major impacts to SOM occurred to the O horizon and any fire-related changes to the mineral soil were not persistent. Abbreviations: BC, black carbon; FA, fulvic acid; HA, humic acid; HO, other hydrophobic materials; LOI, loss-on-ignition; NH, nonhumic; IS, insoluble; SOM, soil organic matter.

Despite the broad consensus that humic substances (HS) can act as electron shuttles in bioreduction processes occurring in aquatic and terrestrial environments, no published studies have presented direct evidence that electron shuttling by HS plays a role in the mechanisms of Fe(III) bioreduction in soils. We conducted experiments in which two HS with very different chemical properties, a soil humic acid and a mixture of peat humic acid with aquatic organic matter, were added to an Ultisol and incubated under anoxic conditions to evaluate their effect on Fe(III) bioreduction by the indigenous microorganisms in the soil. Our experiments were designed to have the same initial reducing capacity available from the added HS but to have very different initial total carboxyl concentrations to examine the role of Fe(II) complexation by HS in the bioreduction process. The addition of HS significantly increased soluble Fe(II) production relative to that observed in control experiments without added HS. Moreover, after an initial period of incubation, soluble Fe(II) production was accelerated by the presence of a higher initial carboxyl content. These trends support the hypothesis that, at the initial stages of in Fe(III) bioreduction, electron shuttling dominates, whereas later Fe(II) complexation dominates. Our results apparently are the first to demonstrate directly that added HS can indeed enhance the bioreduction of Fe(III) minerals in soil by at least two different mechanisms. Abbreviations: AQDS, anthraquinone-2,6-disulfonate; CRC, chemical reducing capacity; ESHA, Elliot soil humic acid; HS, humic substances; ICP, inductively coupled plasma; MRC, microbial reducing capacity; PPHA, Pahokee peat humic acid; SRNOM, Suwannee River natural organic matter.

The nonlinear behavior of water repellency (WR) with respect to soil water content ([[theta].sub.g]) may be described by curve parameters derived from two widespread methodologies such as the water drop penetration time (WDPT) and molarity of an ethanol droplet (MED) tests. While the former measures WR persistence in terms of the infiltration time of a water drop sitting on the soil surface, the latter quantifies the repellency degree as contact angle or 90[degrees] surface tension, [[gamma].sub.90o]. From a hydrologic point of view, the WDPT test may be considered more closely related to the natural infiltration process of the soil, but the infiltration times become prohibitively large (on the order of hours) in highly repellent soils; thus, the MED test is more convenient. This study was conducted to determine whether the spatial interpolation of time-consuming WDPT measurements may be complemented with less time-demanding WR parameters derived from the MED test. Measurements from saturation to oven dryness using both MED and WDPT were performed on 140 topsoil samples from a humid evergreen forest watershed of the Garajonay National Park (Canary Islands). We found useful links between curve parameters derived from the MED and WDPT tests. The area below the WR-[[theta].sub.g] curve measured with the WDPT test ([S.sub.WDPT]) was found to be correlated with that measured with the MED test ([S.sub.MED]) and with the [[theta].sub.g] at which repellency is triggered ([[theta].sub.g-min]). By contrast, previously published relations that established a logarithmic relationship between 1/WDPT and [[gamma].sub.90o] were valid only in the dry soil state, but must be ruled out if soil water content is taken into account. We took advantage of the existence of such parameter correlations to show how cokriging geostatistical tools may be applied to improve the spatial interpolation of [S.sub.WDPT] with [S.sub.MED] or [[theta].sub.g-min] as subsidiary variables. In general, cokriging was superior to kriging in terms of mean variance minimization with little extra labor cost. A minimal-effort spatial sampling strategy may be designed taking into account these considerations. Abbreviations: IRDI, integrative repellency dynamic index; MED, molarity of an ethanol droplet; SOM, soil organic matter; WDPT, water drop penetration time; WR, water repellency.

Land use change is one of the anthropogenic interventions that may induce substantial modifications to both the quantity and quality of soil organic matter (SOM). Soils from three cultivated areas (two types of rainfed upland and one irrigated lowland) and two forests (subtropical managed and temperate unmanaged) in a midhill watershed of Nepal were studied to assess the effect of land use change on SOM quality. Bulk SOM was analyzed using solid-state [sup.13]C nuclear magnetic resonance (NMR) and fractionated by acid hydrolysis into pools with different chemical composition and, thus, different tendencies to decomposition. The NMR analysis showed a clear prevalence of O-alkyl C over alkyl C and aromatic C, with major differences between both soils and two depth intervals of the same soil (0-20 and 20-40 cm). Acid hydrolysis revealed that the labile C pool is larger in the 0- to 20-than the 20- to 40-cm soil depth of each land use. Labile C was larger in the temperate unmanaged forest and the irrigated cultivated land than in the other soils at both soil depths. Of the cultivated rainfed soils, the recently reclaimed soil contained less labile C than the historically cultivated soil, while the labile C in the irrigated soil exceeded that in both rainfed soils. The results suggest that changes in land use from forest to cultivation and the irrigation of the cultivated land may significantly affect not only the quantity but also the quality of the SOM and, more importantly, its chemical recalcitrance to degradation. As a consequence, any future decisions regarding land management in the fragile mountain areas of Nepal should be carefully considered with respect to the nature of the SOM. Abbreviations: Bari 1, anciently reclaimed rainfed cropland; Bari 2, recently reclaimed rain fed cropland; CEC, cation exchange capacity; CP, crossed polarization; Khet, irrigated rice field; LP, labile pool; MAS, magic angle spinning; NMR, nuclear magnetic resonance; SC, temperate forest of Schima wallichii and Castanopsis indica; SOC, soil organic carbon; SOM, soil organic matter; St, managed dense forest of Shorea robusta.

Increasing cropping intensity in the central Great Plains of the United States has the potential to increase organic carbon (OC) stored in the soil and lead to improved soil physical properties. A cropping systems study was started in 1990 at the Central Great Plains Research Station near Akron, CO. In 2005 soil samples were taken in six 95-mm increments to a depth of 370 mm to measure OC, water stable macroaggregates (water stable aggregates > 250 [micro]m), bulk density ([[rho].sub.b]), total porosity ([[psi].sub.total]), water storage porosity ([[psi].sub.ws]), and saturated hydraulic conductivity ([k.sub.sat]). Samples were collected from permanent grass plots {45% smooth brome [Bromus inermis (Leyss.)], 40% pubescent wheat grass [Agropyrons trichophorum (Link) Richt.], and 15% alfalfa [Medicago sativa (L.)]}, plots in a wheat {[Triticum aestivum (L.)]-corn [Zea mays (L.)]-millet [Panicum miliaceum (L.)]} rotation, and plots in a wheat-fallow rotation. Increased cropping intensity significantly increased OC, water stable macroaggregates, and [k.sub.sat], but had no significant effect on [[rho].sub.b], [[psi].sub.total], or [[psi].sub.ws]. Permanent grass increased OC compared with the annually cropped rotations, particularly deeper in the soil. Plots in permanent grass had greater [k.sub.sat] and this may indicate greater pore continuity and stability under saturated conditions. Organic carbon and water stable macroaggregates were poorly correlated. Water stable macroaggregates was negatively correlated with [[rho].sub.b] and positively correlated with [k.sub.sat]. Increasing soil OC may not immediately lead to changes in soil aggregation in a semiarid climate. Increased time and biological activity may be needed to convert the crop residues into organic compounds that stabilize aggregates and soil pore systems. Abbreviations: CI, cropping intensity; [k.sub.sat], saturated hydraulic conductivity; OC, organic carbon; WCM, wheat-corn-millet; WF, wheat-fallow; [[theta].sub.g33], gravimetric water content; [[theta].sub.v1500], wilting point water content; [[rho].sub.b], bulk density; [[psi].sub.total], total porosity; [[psi].sub.ws], water storage porosity

Physically separated soil organic matter (SOM) fractions may take different functions in soil N dynamics. We studied the effect of long-term organic matter (OM) management and different soil biological activity on the incorporation of N added with organic and mineral amendments into aggregate fractions and size density fractions. We applied [sup.15]N-labeled sheep feces, urine, and mineral fertilizer to microplots installed in plots of conventional (CONMIN) and bio-organic (BIOORG) cropping systems. Soil sampled 112 d after amendment was separated into macro-, microaggregates, and microstructures. Aggregates were then fractionated into free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and the mineral-associated organic matter fraction (MF). Of total soil N, 67% was contained in macroaggregates. Size density fractionation of aggregates revealed that about 60% of soil N was stored in MF while LF and iPOM contained together Abbreviations: AF, aggregate fraction; Amd, amendment; BIOORG, bio-organic cropping system; CONMIN, conventional cropping system; CS, cropping system; DM, dry matter; HF, heavy fraction; iPOM, intra-aggregate particulate organic matter; LF, light fraction; MF, mineral-associated organic matter fraction; MineralN, mineral fertilizer N ([sup.15]N[H.sub.4][sup.15]N[O.sub.3]); Ndflc, N derived from the labeled component of the amendment, OM, organic matter; POM, particulate organic matter; SDF, size density fraction; SlurryF, sheep slurry (unlabeled urine + [sup.15]N-labeled feces); SlurryU, sheep slurry ([sup.15] N-labeled urine + unlabeled feces); SOM, soil organic matter; ON, unfertilized control.

The seasonal dynamics of soil organic carbon (SOC) fractions and dry aggregates were investigated to determine whether aggregate turnover rates explain the variable influence of no-till (NT) practices on SOC accrual in Illinois. Soils were collected (0-15 cm) of two 17-yr-old trials where NT and conventional tillage (CT) practices had variable effects on SOC sequestration in the 0- to 30-cm depth. Soil was fractionated into loose particulate organic matter (LPOM), macroaggregate-occluded particulate organic matter (OPOM), and humified fractions (HFs) associated with microaggregates and minerals. Dry aggregate mean weight diameter (DMWD) was used to evaluate structure expressed in the field. Within-season changes in fraction concentrations indicate that C derived from LPOM and plant roots was transferred to OPOM at both sites. At DeKalb (silty clay loam), NT practices had less influence on SOC sequestration than at Monmouth (silt loam). DeKalb HF contents increased under NT while OPOM contents were unchanged. At Monmouth, NT practices increased SOC sequestration by increasing OPOM concentrations. Site-based differences in SOC accrual were related to aggregate turnover rates, which were increased by NT practices at Monmouth but were unaffected by tillage at DeKalb. Trends in DMWD and aggregate turnover suggest that when aggregate turnover is slow, as at DeKalb, OPOM formation is limited. Slow turnover in the heavier soil permitted accumulation of HF because residues had longer contact times with particle surfaces. When aggregate turnover is faster, as in the Monmouth NT soil, there is greater chance for residue incorporation into aggregates and accumulation of OPOM. Rapid aggregate turnover limits opportunities for mineral affiliation and HF accrual. Aggregate dynamics predicted site-based differences in the form and rate of SOC accrual caused by the use of NT practices. Abbreviations: CT, conventional tillage; DMWD, dry aggregate mean weight diameter; HF, humified fraction; LPOM, loose particulate organic matter; NT, no-till; OPOM, occluded particulate organic matter; POM, particulate organic matter; SOC, soil organic carbon.

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.

Whether the structure of Oa and A horizons varies according to animal activity is still a matter of conjecture, especially in amphi, a humus form with mixed features of mull and moder, which has been described in environments with strong seasonal contrasts. The Oa and A horizons of spruce [Picea abies (L.) Karst.] coniferous forests of the Province of Trento (Italy) were sampled in six sites with a total of 134 humus profiles along transect lines, embracing the variety of parent rocks, climate, and vegetation conditions that prevail at the upper montane level in this region. The distribution of humus components (plant debris, roots, animal feces, minerals) was assessed by an optical method and analyzed with correspondence analysis (CA). Moder humus forms were characterized by enchytraeid activity, with concomitant deposition of organic and mineral-organic feces in Oa and A horizons, respectively. Conversely, amphis were characterized by the concomitant deposition of organic and mineral-organic earthworm feces in Oa and A horizons, respectively. We conclude that Oa and A horizons of moders and amphis differed only quantitatively in the content of mineral matter in animal feces. The fine-grained mineral-organic structure that is mostly found in the A horizon of amphis results from the alimentary activity of small epigeic earthworms that mix organic matter with mineral matter, like anecic earthworms do in mulls. Abbreviations: AN, Acid North; AS, Acid South; BN, Basic North; BS, Basic South; CA, correspondence analysis; IN, Intermediate North; IS, Intermediate South.

Under semiarid conditions, soil quality and productivity can be improved by enhancing soil organic matter content by means of alternative management practices. In this study, we evaluated the feasibility of no-till (NT) and cropping intensification as alternative soil practices to increase soil organic C (SOC). At the same time, we studied the influence of these management practices on two SOC fractions (particulate organic matter C, POM-C, and the mineral-associated C, Min-C), in semiarid agroecosystems of the Ebro River valley. Soil samples were collected from five soil layers (0-5-, 5-10-, 10-20-, 20-30-, 30-40-cm depth) during July 2005 at three long-term tillage experiments located at different sites in the Ebro River valley (northeast Spain). Soil bulk density, SOC concentration and content, SOC stratification ratio, POM-C, and Min-C were measured. Higher soil bulk density was observed under NT than under reduced tillage (RT), subsoil tillage (ST), or conventional tillage (CT). At the soil surface (0-5-cm depth), the highest total SOC concentration, POMC, and Min-C were measured under NT, followed by RT, ST, and CT, respectively. In the whole soil profile (0-40 cm), similarly, slightly greater SOC content was measured under NT than under CT with the exception of the Selvanera site, where deep subsoil tillage combined with moldboard plowing accumulated more SOC than NT. In semiarid Mediterranean agroecosystems where CT consists in moldboard plowing, NT is a viable management practice to increase SOC. Abbreviations: AG, Agramunt; PN-CF, cereal-fallow rotation at the Penaflor site; CT, conventional tillage; Min-C, mineral-associated carbon; NT, no-till; PN-CC, continuous cropping system at the Penaflor site; POM, particulate organic matter; POM-C, particulate organic matter carbon; RT, reduced tillage; SOC, soil organic carbon; SOM, soil organic matter; ST, subsoil tillage; SV, Selvanera.

Although it is well known that treated sewage effluent enhances trace elements and nutrient solubility in soil solution through their complexation with dissolved organic matter (DOM), no information is available yet for B. The main purpose of this study was to evaluate the effect of DOM with B and native soil organic matter (OM) on B adsorption by soils. Batch equilibrium studies were conducted to measure the B adsorption by DOM (pH 7.7) that was selected from a municipal sewage plant. Effluent DOM was found to have a low affinity for the soils and its application resulted in a release of native soil OM into solution. The OM release was enhanced significantly by an increase in soil mass/solution volume ratio and effluent DOM concentration. The B adsorption capacity of DOM (294-333 mg [kg.sup.-1]) was less than that found for different humic acids (583-2235.6 mg [kg.sup.-1]). Nevertheless, the presence of DOM reduced the free-B concentration in solution due to formation of B-DOM complexes. As the total DOM concentration increased, the slope of the isotherms for B adsorption by soil decreased. All the B adsorption isotherms obtained for the different DOM concentrations merged into one isotherm, however, when free-B solution concentration was taken into consideration. The results suggest that the B-DOM complex did not interact with the soil. Abbreviations: DOM, dissolved organic matter; OC, organic carbon; OM, organic matter; SAR, sodium adsorption ratio.

Soil organic matter (SOM) is a heterogeneous mixture of compounds that are derived from a number of sources. Therefore, trying to assess the stage of SOM alteration is challenging. Recently, our research has proposed a number of degradation parameters based on the measurement of SOM biomarkers. These biomarkers, which can be attributed to a specific organism source and the stage of alteration (oxidation) of a compound can provide information regarding the stage and rate of SOM degradation. The previously developed biomarker ratios for assessing SOM degradation have not been tested with degradation parameters derived from other molecular methods. This study investigates the use of solid-state [13.sub.c] nuclear magnetic resonance (NMR) and organic matter biomarker methods for the assessment of SOM degradation in four soils from Western Canada. Total solvent extraction was used to isolate free lipids, base hydrolysis was used to extract bound lipids, and copper (II) oxide (CuO) oxidation was used to liberate lignin monomers and dimers from the soil. The presence of plant sterols and their oxidative degradation products facilitated the analysis of SOM alteration. Cutin and suberin degradation parameters were applied to study the relative amounts of suberin versus cutin inputs into soil as well as the stage ofcutin and suberin degradation. The extent of oxidation for lignin-derived compounds was also tabulated. The trends observed by solid-state [13.sub.c] NMR (alkyl/O-alkyl ratios) were consistent with those found with the biomarker techniques, however, it should be noted that the biomarker methods provide detailed information on specific compounds whereas solid-state [13.sub.c] NMR only provides information on the 'bulk' structure of SOM. Because only a small portion of SOM components are amenable to analysis by biomarker methods, it is advantageous to use these methods in tandem with solid-state [13.sub.c] NMR spectroscopy and other molecular techniques when investigating SOM sources and biogeochemistry. Abbreviations: SOM, soil organic matter, NMR, nuclear magnetic resonance.

Interaction of water and organic matter (OM) in pores affects the capacity for long-term C sequestration and water retention in soils. This study investigated the effects of the amount and distribution of OM in microstructures on water retention in soil microaggregates. The focus of the study was on the hysteretic soil water characteristic (i.e., the difference between the main drying and wetting curves), which has been largely ignored in previous studies on water-OM relations. The main drying and wetting branches of the water retention curve were measured using a water activity meter on water-stable microaggregates (53-250 [micro]m), which were collected from surface soils (0-15 cm) subject to different management practices: tallgrass prairie restoration on a Mollisol, and tillage with and without N fertilization on an Alfisol. The results show that the presence of OM in Abbreviations: ANCOVA, analysis of covariance; OC, organic carbon; OM, organic matter; SSA, specific surface area.

Particulate organic matter (POM), an established soil quality indicator, is too costly for routine testing by analytical labs. Chemical oxidation of labile soil organic matter is less costly and may prove to be an equally effective indicator. The objectives of this study were to test the relationship between POM and chemically labile organic matter (CLOM) and to evaluate the effects of soil management on POM and CLOM. The study was conducted within a long-term crop rotation x fertility treatment study in central Pennsylvania. Crop rotation sequences were continuous corn (Zea mays L.), corn-soybean [Glycine max (L.) Merr.], 4 yr of corn followed by 4 yr of alfalfa hay (Medicago sativa L.), and corn-oat (Avena sativa L.)winter wheat (Triticum aestivum L.)-2 yr of red clover hay (Trifolium pratense L.). Fertilizer treatments were mineral fertilizer, N-based liquid dairy manure, and P-based liquid dairy manure. A significant linear relationship between POM-C and CLOM-C treatment means was observed (re = 0.74). Both POM-C and CLOM-C concentrations were greatest for manure-based fertility treatments and for crop rotations receiving the most frequent applications of manure. Only CLOM-C, however, distinguished between the N-based and P-based manure treatments. Further development of CLOM-C as a soil quality indicator may yield a reliable, cost-effective soil quality management tool. Abbreviations: CA, 4 yr of corn followed by 4 yr of alfalfa; CC, continuous corn; CLOM, chemically labile organic matter; COWH, corn-oat-wheat-2 yr of red clover hay; CS, corn-soybean; HRE, Hunter Rotation Experiment; MF, mineral fertilizer; POM, particulate organic matter; SOM, soil organic matter.

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.

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.

The amount of organic N mineralized as plant available nitrogen (PAN) from soil organic matter (SOM) during a crop growing season has proved difficult to assess using traditional soil testing. It was the objective of this study to evaluate the potential for computer simulation in the estimation PAN from SOM during the growing season. A secondary objective was to evaluate this approach as a way to identify soils where crops would be non-responsive to fertilizer N. Plant available N from SOM was estimated using a simple, first-order kinetic equation. Plant available N from manure was estimated using the computer model, DECOMPOSITION. Plant available N from the previous crop and from preplant fertilizer N were taken from published data. No additional fertilizer N was applied. Corn (Zea rnays L.) yield was significantly related to total PAN (sum of SOM N, manure N, crop residue N, and preplant N) and the response of corn to total PAN was typical of previously published values. Total PAN was not a good predictor of soils where corn was nonresponsive to fertilizer N, while a history of manure applications was a good predictor of nonresponsiveness. Abbreviations: ISNT, Illinois soil nitrogen test; PAN, plant available nitrogen; SOM, soil organic matter.