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.

Conversion of former agricultural land to grassland and forest ecosystems is a suggested option for mitigation of increased atmospheric CO 2 . A Sharpsburg prairie loess soil (fine, smectitic, mesic Typic Argiudoll) provided treatments to study the impact of long-term land use on soil organic carbon (SOC) content and composition for a 130-year-old cropped, pasture and forest comparison. The forest and pasture land use significantly retained more SOC, 46% and 25%, respectively, compared with cropped land use, and forest land use increased soil C content by 29% compared with the pasture. Organic C retained in the soils was a function of the soil N content (r 5 0.98, Po0.001) and the soil carbohydrate (CH) concentration (r 5 0.96, Po0.001). Statistical analyses found that soil aggregation processes increased as organic C content increased in the forest and pasture soils, but not in the cropped soil. SOC was composed of similar percentages of CHs (49%, 42% and 51%), amino acids (22%, 15% and 18%), lipids (2.3%, 2.3% and 2.9%) and unidentified C (21%, 29% and 27%), but differed for phenolic acids (PAs) (5.7%, 11.6% and 1.0%) for the pasture, forest and cropped soils, respectively. The results suggested that the majority of the surface soil C sequestered in the long-term pasture and forest soils was identified as C of plant origin through the use of CH and PA biomarkers, although the increase in amino sugar concentration of microbial origin indicates a greater increase in microbial inputs in the three subsoils. The practice of permanent pastures and afforestation of agricultural land showed long-term potential for potential mitigation of atmospheric CO 2 .

Soil management is one of the most important factors influencing the structure of soils. The interaction of management (including tillage and crop rotation history) with soil biochemistry, soil aggregation, and soil humus composition was determined in a native prairie and a producer field situation in 1997. A comparison of a native prairie and an adjacent conventional corn (Zea mays L.)-soybean [Glycine max (L.) Merr. 1 rotation on the same soil type found that the Webster soil (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) after soybean (C 3 plant) was lower in monosaccharide content and protein content as determined by ion chromatography, and lower in phenolic acid content than the Webster soil after corn (C 4 plant) or in native prairie as determined by gas chromatography. A wet, nested sieve aggregate stability measurement determined that the prairie soil had a higher mean aggregate size (1.85 mm) when compared with the soil in the presence of decomposing corn (1.0 mm) or soybean (0.34 mm) residues. Mean aggregate size was found to be correlated with soil monosaccharide content (r = 0.75), total soil protein content (r = 0.995***), total soil phenolic acid content (r = 0.997***), and alkaline extractable humic substance content (r = 0.98**). Alkaline extractable humic substances were correlated with the phenolic acid content of the humic substances (r = 0.996***). The results suggest that the decrease in soil stability after soybean growth was due to a decrease in the content of soil humic substances caused by the substantially lower phenolic acids content (humic acid precursors) in the soybean residue.

Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO 2), nitrous oxide (N 2 O), and methane (CH 4). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg 21 soil), open/ forb (6 g organic C kg 21 soil), and sacaton (18 g organic C kg 21 soil) from July 2002-September 2003 in southeastern Arizona. Carbon dioxide and N 2 O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO 2 production, as efflux averaged 235.6 6 39.5 mg CO 2 m 22 h 21 over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO 2 emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 6 13.4 (mesquite), 2.1 6 4.4 (open), and 3.9 6 5.2 mg N 2 O m 22 h 21 (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N 2 O emissions by 47% in the mesquite, but N 2 O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH 4 consumption at all sites was close to zero, but following monsoon moisture input, the CH 4 sink averaged 26.1 6 6.3 mg CH 4 m 22 h 21 through April 2003. Laboratory incubations showed potentials for CH 4 oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH 4 oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO 2 and N 2 O emissions while promoting CH 4 oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.