Substrate composition is one of the most important factors in¯uencing the decomposition of plant residues in soils. The interaction of organic residue biochemistry with residue decomposition rates, soil aggregation and soil humus composition was determined in a laboratory experiment. Addition of seven dierent organic residues (2% w/w alfalfa, oat, canola, clover, soybean, corn and prairie grasses) to a Webster soil resulted in a rapid, transient increase in aggregate mean weight diameters (MWD) when incubated for 9 d with residues with low phenolic acid content (alfalfa, canola and clover) and was inversely correlated with soil carbohydrate content r À0X63). More pronounced improvement in aggregate size was noted upon increased incubation to 84 d with organic residues higher in phenolic acid content (corn, prairie grasses, oat and soybean) and was related to soil phenolic acid r 0X65 and soil carbohydrate content r 0X70). Total plant residue phenolic acid content was related to MWD measured after incubation for 84 d by a quadratic response and plateau function r 0X96 and the MWD quadratically increased with an increase in vanillin-vanillic acid concentrations in the plant residues r 0X997). Soil organic C after 84 d was related to the MWD r 0X82 and the residue's vanillin-vanillic acid content r 0X86), suggesting that C remaining in the soil following decomposition maybe related to the speci®c phenolic acid content. The results suggest that transient aggregate stability initiated by microbial decomposition of the carbohydrate and amino acid content of the residue, is then strengthened by the interaction with phenolic acids such as vanillin or vanillic acid released by microbial decomposition from residues structural components. Published by Elsevier Science Ltd.

The presence of auxins in soil may have an ecological impact affecting plant growth and development. A rapid and simple colorimetric method was used to assess California soils for their potential to produce auxins upon the addition of L-tryptophan (L-TRP). The auxin content measured by colorimetry was expressed as indole-3-acetic acid (IAA)-equivalents. A substrate (L-TRP) concentration of 5.3 g kg 1, glucose concentration of 6.7 g kg -1, no nitrogen, pH 7.0, 40°C, shaking (aeration) and 48 h incubation time were selected as standardized conditions to assay for auxin biosynthesis in soil. IAA was confirmed as a major microbial metabolite derived from L-TRP in soil by use of high performance liquid chromatography (HPLC). Under standardized conditions, L-TRP-derived auxins in 19 soils varied greatly ranging from 18.2 to 303.2 mg IAA equivalents (auxins) kg -I soil. This study suggests that the phenotypic character of the soil microbiota has more of an influence on auxin production than the soil physicochemical properties (e.g., pH, organic C content, CEC, etc.).

No abstract

Understanding the speciation of the multioxidation states of selenium is vital to predicting the mineralization, mobilization, and toxicity of the trace element in natural systems. A sequential extraction scheme (SES) was developed for identification of Se oxidation states that first employed 0.1 M (pH 7.0) K 2 HPO 4 -KH 2 PO 4 (P-buffer) to release soluble selenate (Se +VI ) and selenide (Se -II ) and ligandexchangeable selenite (Se +IV ). The second step involved oxidation of organic materials with 0.1 M K 2 S 2 O 8 (90°C) to release Se -II and Se +IV associated or occluded with organic matter. The final step used HNO 3 (90°C) to solubilize insoluble Se remaining in the sample. The solubilized Se compounds were speciated by a selective hydride generation atomic absorption spectrophotometry technique. Accuracy of the developed SES method (96-103% recovery) was verified by use of prepared Se compounds of known speciation, NIST standard reference materials, and existing seleniferous soils. The average precision (relative standard deviation) for the P-buffer extraction ranged from 5.5 to 7.7% (n ) 12); the precision of the persulfate extraction ranged from 2.6 to 8.4% (n ) 12); and the precision of the nitric acid extraction ranged from 2.8 to 7.4% (n ) 12) for three soils extracted at four different time periods.The method was applied to analyze Se species in seleniferous plant, soil, and sediment samples.

Slow water infiltration in some California soils results in considerable irrigation water loss through increased runoff and evaporation. This 25‐mo study was conducted to evaluate the effects of different organic amendments on soil physical parameters and water infiltration rates on an irrigated soil. Incorporation of three loadings (25 Mg ha−1 each) of poultry manure, sewage sludge, barley straw (Hordeum vulgare L.), and alfalfa (Medicago sativa L.) to an Arlington soil (coarseloamy, mixed, thermic Haplic Durixeralf) for 2 yr increased soil respiration rates (139‐290%), soil aggregate stability (22‐59%), organic C content (13‐84%), soil saccharide content (25‐41%), soil moisture content (3‐25%), and decreased soil bulk density (7‐11%). The change in soil physical properties resulted in significantly increased cumulative water infiltration rates (18‐25%) in the organic‐amended plots as compared with the unamended plots. Although additions of poultry manure and sewage sludge contributed to higher soil organic matter compared with straw and alfalfa, the straw amendment was statistically more effective in increasing soil aggregate stability, total saccharide content, infiltration rates, and soil respiration rates and in decreasing bulk density in the tillage zone. The increase in cumulative infiltration rates measured with the first organic addition (April 1987–January 1988) were significantly correlated with increased soil aggregation (P ≤ 0.01). Cumulative infiltration rates during the second (February 1988–September 1988) and third (October 1988–May 1989) organic incorporation were significantly correlated with decreased bulk density (P ≤ 0.01), but not with aggregate stability. Multiple linear regression analyses indicated that water infiltration rates in the organic‐amended soils were initially increased by stimulation of microbial activity, which increased the stability of soil aggregates. Cumulative infiltration rates were further increased by a decrease in soil bulk density with additional organic treatments to the tillage zone.