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The mineralogy of bulk and rhizosphere soils was compared to assess the effect of roots on mineral weathering in a Typic Dystrochrept supporting Norway spruce (Picea abies [L.] Karst) and oak (Quercus sessiliflora Smith). In an experimental forest site (Breuil-Chenue, France), systematic soil sampling was performed in forty pits. The soil adhering to the roots was considered as rhizosphere soil. The remaining material was regarded as bulk soil. The mineralogy of the clay-sized particles of both fractions was determined by x-ray diffraction (XRD). Amorphous solid phases were estimated by extracting Fe and Al with hot Na-citrate and dithionite-citrate-bicarbonate. Total chemical analyzes were performed on the clay-sized particles via Ba[Cl.sub.2]-saturation and an iso-zirconium weathering balance was calculated. The XRD demonstrated an increase of illite-like minerals in the rhizosphere of both species and the selective extractions revealed a decrease in the amorphous phases. The total chemical analyzes showed that the rhizosphere clay-sized fraction contained significantly more Si and K (for oak only) and less Fe and Al than the bulk soil. By way of the iso-zirconium weathering balance, these losses of Fe and AI in the rhizosphere were estimated at several tens of kilograms by hectare for the surface horizon (0-3 cm). This study demonstrates that, despite the short duration of the contact between the active part of a root and the solid mineral phase, the intensity of the processes occurring in the rhizosphere significantly increases mineral weathering. Abbreviations: B, bulk soil; BS, base saturation; subscript c, hot Na-citate treatment; CEC, cation exchange capacity; subscript d, dithionite-citrate-bicarbonate treatment; EA, exchangeable acidity; EG, ethylene glycol treatment; HIS, hydroxy-interlayered smectite; HIV, hydroxy-interlayered vermiculite; ICP-AES, inductively coupled plasma spectrometry-atomic emission spectrometry; ICP-MS, inductively coupled plasma spectrometry- mass spectrometry; R, rhizosphere; subscript [tot], total; XRD, x-ray diffraction.

Rhizosphere conditions are of great importance fur nutrient uptake and thus plant growth. Nutrient availability in the rhizosphere may differ substantially from that of the bulk soil and specific sampling of the rhizosphere soil is crucial to the understanding of plant growth, particularly where roots are restricted in growth due to high bulk density of the soil. The purpose of this study was to develop suitable methods for investigating the chemical composition of soil around pores with and without roots in soils with high bulk density. Two different methods were undertaken: one approach was to perform sequential extractions with [H.sub.2]O, 0.01, 0.1, and 1 M N[H.sub.4]N[O.sub.3] on soil samples taken at different distances from the pore wall to determine differences in element availability. Another approach was to analyze resin-impregnated, undisturbed soil samples from the same site for elemental composition of pure wall material using a scanning electron microscope (SEM) equipped with an x-ray micro analyzer. In general, the sequential extraction detected few statistically significant differences in nutrient availability between pores containing routs compared with pores without roots. The SEM analysis showed lower amounts of easily weatherable minerals around pores containing roots. Both methods are suitable for investigating differences between rhizosphere anti bulk soil characteristics.

Traditional methods of measuring tree root biomass are labor intensive and destructive in nature. We studied the utility of ground-penetrating radar (GPR) to measure tree root biomass in situ within a replicated, intensive culture forestry experiment planted with loblolly pine (Pinus taeda L.). The study site was located in Decatur County, Georgia, in an area of the Troup and Lucy (loamy, kaolinitic, thermic Grossarenic Kandiudults and Arenic Kandiudults, respectively) soils. With the aid of a digital signal processing GPR, estimates of root biomass to a depth of 30 cm were correlated to harvested root samples using soil cores. Significant effects of fertilizer application on signal attenuation were observed and corrected. The correlation coefficient between actual root biomass in soil cores and GPR estimates with corrections for fertilizer application were highly significant (r = 0.86, n = 60, p < 0.0001). Where site conditions are favorable to radar investigation, GPR can be a powerful cost-effective tool to measure root biomass. Verification with some destructive harvesting is required since universal calibrations for root biomass are unlikely, even across similar soil types. Use of GPR can drastically reduce the number of soil cores needed to assess tree root biomass and biomass distribution. The quality and quantity of information resulting from a detailed GPR survey, combined with soil cores on a subset of plots, can be used to rapidly estimate root biomass and provide a valuable assessment of lateral root biomass distribution and quantity.

A two-dimensional model that combines root-system development and water extraction by roots is proposed to simulate the dynamic interaction between root growth and soil-water flow. Both of hydrotropismand gravitropism were considered as the controlling factors of root growth in the proposed root-system development model. The finite-element method was employed to compute the soil-water flow caused by water extraction, evaporation, and irrigation. We succeeded in simulating the plagiogravitropic elongations of lateral roots under a plane condition, and the asymmetric architecture of root system under a slope condition by the proposed model in which the root hydrotropism is considered. On the other hand, we cannot simulate such morphological characteristics of a root system by the use of the conventional model in which a random elongation factor is employed, and root hydrotropism is not considered. The results support the importance of hydrotropism in root-system development and the availability of the proposed model in which the hydrotropism is considered.

A model that couples the diffusion of [O.sub.2] to plant roots at the microscale to diffusion of [O.sub.2] through the soil at the macroscale is derived. The solution is complex, but if the length scales ([Z.sub.r] and [Z.sub.m]; see APPENDIX for list of symbols) are equated a simple analytical expression can be obtained. This model is used to investigate relationship between a critical air-filled porosity ([[theta].sub.m]) and the other parameters; viz temperature (T), [O.sub.2] concentration in the bulk soil (C*), [O.sub.2] concentration at the root surface ([C.sub.r]), root length density (L), the ratio of root radius (a) to the water film radius (R), microbial respiration ([M.sub.o]), and length scales ([Z.sub.m] and [Z.sub.r]) related to the depth to which microbial and plant respiration are active in the model using sensitivity analysis. The model shows that [[theta].sub.m] is not very sensitive to the [O.sub.2] concentration at the root surface ([C.sub.r]), or the ratio of root radius (a)/ water film radius (R), but is sensitive to all the other parameters in some part of their range. The results indicate that indices used to define soil aeration; [O.sub.2] diffusion rate (ODR) or [O.sub.2] flux, [O.sub.2] concentration, or air-filled porosity, which have been previously used, are related and a single critical value for these is unlikely. If a constant critical value exists for one of these indexes it cannot exist for the other two. It is also shown that it is highly unlikely that a universal critical parameter related to soil aeration exists for any of these parameters. It is concluded that more parameters than ODR, [O.sub.2] concentration, or air-filled porosity need to be measured if progress in soil aeration research is to be made.

Quantitative description of root-water uptake under combined salinity and water stress is needed to optimize crop yields and water management in arid and semiarid regions. This study was conducted to develop a simple macroscopic root-water uptake model for nonuniform transient soil water content and salinity conditions in the root zone. This new model and previous models were tested against detailed experimental data obtained with Alfalfa (Medicago sativa L.) grown in the greenhouse in packed sandy loam (Typic Haplaquent) columns. Soil water content, pressure head, and osmotic head distributions in the root zone were varied by means of the amounts, application intervals, and salinities of the irrigation water. Experimental data under separate and combined stresses were used to test the various models using mean values of soil solution osmotic and pressure heads. The simple additive reduction function provided the worst agreement with the experimental data, while for most cases the multiplicative reduction functions could not adequately account for both water and salinity stress conditions. The newly proposed linear reduction function is neither additive nor multiplicative, but was assumed that both the intersect and slope of the reduction function increased with salinity. This model provided excellent agreement with the experimental data, particularly at higher soil solution salinities. The new reduction function could be used with any other nonlinear salinity reduction function.

The effects of differences in root geometry and root length densities in water uptake are evaluated. These root parameters were used in two- dimensional modeling of horizontal water transport from soil to branched segments of root axes of cowpea and pearl millet. It was found thatunder soil water deficits, the dense and clumped local root geometry of millet became less efficient in water uptake compared to the cowpea's less dense and more evenly distributed root geometry.

Quantification of root or root-induced changes in soil organic C (SOC), water-soluble organic C (WSOC), and microbial biomass C (MBC) is important for understanding processes of soil C storage. A greenhouse study was conducted on a Bainsville loamy sand (Typic Hapludoll) to evaluate root or root-induced quantitative and compositional changes in various SOC pools by growing corn (Zea mays L.), a [C.sub.4] plant, on a historically [C.sub.3] soil. Significant shifts in [sup.13]C of SOC pools, most noticeably in WSOC and MBC were observed. During the course of a growing season, the proportion of [C.sub.4]-derived C varied from 0 to 12.3% of whole SOC, from 0 to 30.7% of WSOC, and from 0 to 52% of MBC, indicating a major contribution of root or root-induced C to various soil C components, especially WSOC and MBC. The amount of [C.sub.4]-derived C in the entire soil estimated by the [sup.13]C natural abundance ([delta][sup.13]C) was remarkably consistent with the amount of [C.sub.4]-C retained in the soil microbial biomass, WSOC, and corn roots, suggesting that measurements of [sup.13]C of the entire soil following the shift of [C.sub.3] to [C.sub.4] plants can be used as an indirect measure of root or root-induced C pools during the growing period.

Although roots are an important source of soil organic matter (SOM) and are thought to be the major constituent of the particulate organic matter (POM) fraction, few studies have documented the fate of belowground C inputs in situ. The main purpose of this experiment was to determine the fate of root-derived C vs. shoot-derived C and to identify factors contributing to any differences in the retention of aboveground vs. belowground C inputs. We labeled hairy vetch (Vicia villosa Roth subsp, villosa) in situ with [sup.13][CO.sub.2] and followed both root- and shoot-derived C in total soil organic C (SOC) and labile C pools for the first growing season following hairy vetch incorporation. At the end of the growing season, nearly one-half of the root-derived C was still present in the soil, whereas only 13% of shoot-derived C remained. A greater proportion of root-derived C was found as occluded POM and associated with the clay and silt fraction. Greater root-derived C also was retained as chloroform-extractable microbial biomass. We suggest that three different mechanisms contributed to the increased retention of root-derived C: (i) the greater biochemical recalcitrance of root litter, (ii) increased physical protection of root-derived POM within aggregates, and (iii) the continuous nature of root C inputs from exudates and fine root turnover. We conclude that shoot residues are broken down rapidly and serve as the source of N for the following cash crop, whereas the root litter is probably largely responsible for the short-term soil structural improvements associated with the use of green manures. Furthermore, on the basis of these findings, we hypothesize that the greater retention of root-derived C in the first 6 mo of decomposition will increase the persistence of this C in SOM in the long term.