Soil quality assessment could become more standardized with the development of a simple, rapid, and reliable method for quantifying potential soil biological activity. We evaluated the flush of CO2 following rewetting of dried soil under standard laboratory conditions as a method to estimate an active organic matter fraction. The flush of CO2 following rewetting of dried soil (3 d incubation at ≈50% water‐filled pore space and 25°C) was assessed for 20 soil series containing a wide range of organic C (20 ± 13 g kg−1) from Alberta–British Columbia, Maine, Texas, and Georgia. This flush of CO2 explained 97% of the variability in cumulative C mineralization during , 86% of the variability in soil microbial biomass , and 67% of the variability in net N mineralization during Accounting for geographical differences in mean annual temperature and precipitation, which could affect soil organic matter quality, further improved relationships between the flush of CO2 and active, passive, and total C and N pools. Measuring the flush of CO2 following rewetting of dried soil may have value for routine soil testing of biological soil quality because it (i) is an incubation procedure patterned after natural occurrences in most soils, (ii) exhibits strong overall relationships with active organic pools, (iii) shows relatively minor changes in relationships with active organic pools that may be due to climatic variables, (iv) has a simple setup with minimal equipment requirements, and (v) has rapid analysis time.

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.

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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.