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.

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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4The impact on soil health of long-term no-tillage (NT) and cover cropping (CC) practices, alone 5 and in combination, was measured and compared with standard tillage (ST) with and without 6 cover crops (NO) in irrigated row crops after 15 years of management in the San Joaquin Valley 7 (SJV) CA, USA. Soil aggregation, rates of water infiltration, content of carbon, nitrogen, water 8 extractable organic carbon (WEOC) and organic nitrogen (WEON), residue cover, and 9 biological activity were all increased by NT and CC practices relative to STNO. However, 10 effects varied by depth with NT increasing soil bulk density by 12% in the 0 -15 cm depth and 11 10% in the 15 -30 cm depth. Higher levels of WEOC were found in the CC surface (0 -5cm) 12 depth in both spring and fall samplings in 2014. Surface layer (0 -15 cm) WEON was higher in 13 the CC systems for both samplings. Tillage did not affect WEON in the spring, but WEON was 14 increased in the NT surface soil layer in the fall. Sampling depth, CC, and tillage affected 1-15 day soil respiration and a soil health index assessment, however the effects were seasonal, with 16 higher levels found in the fall sampling than in the spring. Both respiration and the soil health 17 index were increased by CC with higher levels found in the 0 -5 cm depth than in the 5 -15 and 18 15 -30 cm depths. Results indicated that adoption of NT and CC in arid, irrigated cropping 19 systems could benefit soil health by improving chemical, physical, and biological indicators of 20 soil functions while maintaining similar crop yields as the ST system. 21 . 22 23 Keywords 24

The measurement of soil carbon dioxide respiration is a means to gauge biological soil fertility. Test methods for respiration employed in the laboratory vary somewhat, and to date the equipment and labor required have somewhat limited more widespread adoption of such methodologies. The purpose of this research is to compare the results of measured soil CO 2 respiration using three methods: (1) titration method; (2) infrared gas analysis (IRGA); and (3) the Solvita gel system for soil CO 2 analysis. We acquired 36 soil samples from across the USA for comparison, which ranged in pH from 4.5 to 8.5, organic C from 0.8 to 4.6% and the clay content from 6 to 62%. All three methods were highly correlated with each other after 24-h of incubation (titration and Solvita r 2 = 0.82, respirometer and Solvita r 2 = 0.79 and titration versus respirometer r 2 = 0.95). The 24-h (1-day) CO 2 release from all three methods was also highly correlated to both basal soil respiration (7-28 days) and cumulative 28-day CO 2 respiration. An additional 24 soil samples were acquired and added to the original 36, for a total of 60 soil samples. These samples were used for calibration of the Solvita gel digital color reader results using CO 2 -titration results and regression analysis. Regression analysis resulted in the equation y = 20.6 * (Solvita number) -16.5 with an r 2 of 0.83. The data suggest that the Solvita gel system for soil CO 2 analysis could be a simple and easily used method to quantify soil microbial activity. Applications may also exist for the gel system for in situ measurements in surface gas chambers. Once standardized soil sampling and laboratory analysis protocols are established, the Solvita method could be easily adapted to commercial soil testing labs as an index of soil microbial activity.

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Traditionally, soil-testing laboratories have used a variety of methods to determine soil organic matter, yet they lack a practical method to predict potential N mineralization/immobilization from soil organic matter. Soils with high micro-bial activity may experience N immobilization (or reduced net N mineralization), and this issue remains unresolved in how to predict these conditions of net mineralization or net immobilization. Prediction may become possible with the use of a more sensitive method to determine soil C:N ratios stemming from the water-extractable C and N pools that can be readily adapted by both commercial and university soil testing labs. Soil microbial activity is highly related to soil organic C and N, as well as to water-extractable organic C (WEOC) and water-extractable organic N (WEON). The relationship between soil respiration and WEOC and WEON is stronger than between respiration and soil organic C (SOC) and total organic N (TON). We explored the relationship between soil organic C:N and water-extractable organic C:N, as well as their relationship to soil microbial activity as measured by the flush of CO₂ following rewetting of dried soil. In 50 different soils, the relationship between soil microbial activity and water-extractable organic C:N was much stronger than for soil organic C: N. We concluded that the water-extractable organic C:N was a more sensitive measurement of the soil substrate which drives soil microbial activity. We also suggest that a water-extractable organic C:N level > 20 be used as a practical threshold to separate those soils that may have immobilized N with high microbial activity

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