No abstract

Quantifying changes in soil microbial biomass and mineralizable C and N is important in understanding the dynamics of the active soil C and N pools. Our objectives were to quantify long-term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and N (SMBN), and mineralizable C and N in continuous sorghum [Sorghum bicolor(L.) Moench] and sorghum-wheat (Triticum aestivum L.)/soybean [Glycine max (L.) Merr.] sequences under conventional tillage (CT) and no tillage (NT) with and without N fertilization. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in south-central Texas was sampled after planting in April, during flowering in June, and following sorghum harvest in August. More crop residue C input was retained as SOC and SMBC under NT than under CT. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest at a depth of 0 to 50 mm under NT. Mineralizable C and SMBC averaged 18% greater in rotation than in monoculture, probably due to greater C input via crop roots and residues in rotation and a shorter fallow. Mineralizable N with N fertilization was 36% greater in continuous sorghum but not different in rotated sorghum. Mineralizable C and SMBC increased an average of 5%, but mineralizable N decreased 41% from planting to flowering, probably due to rhizodeposition. From planting to post-harvest, mineralizable C and SMBC increased 9% but mineralizable N decreased 15% due to crop residue addition. Soil N availability was reduced by plant additions in the short term but enhanced in the long term.

Herbicides applied to soils potentially affect soil microbial activity. Quantity and frequency of glyphosate application have escalated with the advent of glyphosate-tolerant crops. The objective of this study was to determine the effect of increasing glyphosate application rate on soil microbial biomass and activity. The soil used was Weswood silt loam. The isopropylamine salt of glyphosate was added at rates of 47, 94, 140, and 234 µg ai g−1 soil based on an assumed 2-mm glyphosate–soil interaction depth. Glyphosate significantly stimulated soil microbial activity as measured by C and N mineralization but did not affect soil microbial biomass. Cumulative C mineralization, as well as mineralization rate, increased with increasing glyphosate rate. Strong linear relationships between mineralized C and N and the amount of C and N added as glyphosate (r 2 = 0.995, 0.996) and slopes approximating one indicated that glyphosate was the direct cause of the enhanced microbial activity. An increase in C mineralization rate occurred the first day following glyphosate addition and continued for 14 d. Glyphosate appeared to be directly and rapidly degraded by microbes, even at high application rates, without adversely affecting microbial activity.

Grasslands and savannas worldwide are experiencing increases in woody plant abundance. In the subtropical Rio Grande Plains of southern Texas and northern Mexico, this change in physiognomy typically results in soil C and N accumulation. The extent to which this accumulation is the result of increased C and N inputs vs. decreased losses is not known. To address this issue, we compared soil C and N pools, soil respiration, soil microbial biomass, and potential C and N mineralization and nitrification rates in remnant grassland communities and adjacent woody plant communities known to have developed on grassland within the past 100 years. Mean soil organic C (SOC) and total N pools in the upper 20 cm of the profile were 2× larger in wooded communities (3382 and 273 g/m2 for C and N, respectively) than in remnant grasslands (1737 and 150 g/m2). The larger pool sizes in the wooded communities supported higher annual soil respiration (SR; 745 vs. 611 g C·m−2·yr−1 for woodlands and grasslands, respectively) and greater soil microbial biomass C (444 vs. 311 mg C/kg soil), potential rates of N mineralization (0.9 vs. 0.6 mg N·kg−1·d−1) and nitrification (0.9 vs. 0.4 mg N·kg−1·d−1). However, despite higher SR rates, mean residence time of near‐surface SOC in wooded communities (11 years) exceeded that of remnant grassland communities (6 years). The fact that increased fluxes of soil C and N were accompanied by increases in SOC and N pools and total SOC mean residence time suggests that shifts from grass to woody plant dominance have increased both labile and recalcitrant pools of SOC and total N, the latter to a greater extent than the former. Given the widespread increase in woody plant abundance in drylands in recent history, the observed net increase in soil C storage that potentially accompanies this change could have global implications for C and N cycling and the climate system.

Crop management strategies that alter the timing, placement, quantity, and quality of crop residue input can affect the size, turnover, and vertical distribution of the active and passive pools of soil organic matter (SOM). Our objectives were to quantify long‐term changes in soil organic, soil microbial biomass (SMB), and mineralizable C and N in continuous wheat (Triticum aestivum L.), continuous wheat/soybean [Glycine max (L.) Merr.], and wheat/soybean‐sorghum [Sorghum bicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with and without N fertilizer. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas was collected from a 9‐yr field study. Soil microbial biomass C (SMBC) and N (SMBN) were determined with the chloroform fumigation‐incubation method and mineralizable C and N were determined from 10‐d aerobic incubations at 25°C. More crop residue C input was retained as soil organic C (SOC), SMBC, and mineralizable C under NT than under CT. Soil organic C, SMBC, and mineralizable C at a depth of 0 to 50 mm were 33 to 125% greater under NT than under CT. Increasing cropping intensity increased SOC up to 22%, SMBC up to 31%, and mineralizable C up to 27% under NT. Differences in crop management systems significantly altered SMB and the associated mineralizable N level, which supplies crops with mineral N. High clay content soils of central Texas can be effectively managed to increase the active and passive pools of SOM using minimal fallow with NT.