R. L. Haney

Effect of glyphosate on soil microbial activity and biomass

Abstract 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 (r2 = 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. Nomenclature: Glyphosate; Sorghum bicolor (L.) Moench., sorghum.

Active fractions of organic matter in soils with different texture

Summary-Relationships between soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C and N during a 21 d incubation, and basal soil respiration (BSR) were evaluated on eight soil types from Texas that varied in soil texture (745% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relationships of mineralizable C and N and BSR to SGC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 + 0.4 mg mineralizable C g-r SOC d-r. The amount of mineralizable C and N and BSR per unit of SMBC, however, decreased with increasing clay content, indicating that the soil microbial biomass (SMB) was more active in coarse-textured soils than in fine-textured soils. The average specific respiratory activit was 29 mg mineralizable C g-’ SMBC d-’ with 10% clay and 11 mg mineralizable C g-’ SMBC d- Y wrth 40% clay. The C-to-N ratio of the mineralizable fraction was 10 f 3 and not affected by soil texture. The established relationships between active soil organic matter (SOM) fractions and soil texture could be used in models predicting SOM turnover. Published by Elsevier Science Ltd

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28 ^ 11 mg g 21 (na 24) in a frigid‐dry region (Alberta/British Columbia), 25 ^ 5m g g 21 (na 12) in a frigid‐wet region (Maine), 11 ^ 4m g g 21 (na 117) in a thermic‐dry region (Texas), and 12 ^ 5m g g 21 (na 131) in a thermic‐wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO2‐C g 21 SOC d 21 in the thermic compared with the frigid regions, P , 0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g 21 SOC 24 d 21 , P , 0.001), and greater SMBC (53 vs 21 mg SMBC g 21 SOC, P , 0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO2‐C g 21 SMBC d 21 , P , 0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO2‐C g 21 SOC d 21 in the wet compared with the dry regions, P , 0.01) and lower SMBC (31 vs 43 mg SMBC g 21 SOC, P , 0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions ( < 33 vs 29 mg CO2‐C g 21 SMBC d 21 , P , 0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2‐3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC. Published by Elsevier Science Ltd.

Relationships of chloroform fumigation–incubation to soil organic matter pools

Microbial biomass is part of the active pool of soil organic matter that plays focal roles in decomposition of organic materials, nutrient cycling and biophysical manipulation of soil structure. We compared two commonly used variants of the chloroform fumigation‐incubation method in their relationships with other active, passive and total soil C and N pools in soils from Texas, Georgia, Alberta and British Columbia. The relationship of potential C mineralization with chloroform fumigation‐incubation without subtraction of a control was much stronger (r 2 =0.8120.10 among five data sets with a total of 844 observations) than with subtraction of a control (r 2 =0.3020.22). Similarly, the relationship of soil organic C with chloroform fumigation‐incubation without subtraction of a control was better (r 2 =0.8020.13) than with subtraction of a control (r 2 =0.3820.32). Relationships of net N mineralization, flush of N following fumigation‐incubation, flush of CO2-C during the first day following rewetting of dried soil, particulate organic C and N, mean weight diameter of water-stable aggregation and total porosity with chloroform fumigation‐incubation were also better without subtraction of a control than with subtraction of a control. In analyses of data taken from published reports, chloroform fumigation‐incubation without subtraction of a control was better related with active soil C pools than with subtraction of a control. Chloroform fumigation‐ incubation without subtraction of a control, unlike that with subtraction of a control, should be considered a more robust method to determine microbial biomass under a wide range of environmental conditions. # 1999 Elsevier Science Ltd. All rights reserved.

Assessing biological soil quality with chloroform fumigation-incubation: Why subtract a control?

Microbial biomass, as part of the active pool of soil organic matter, is critical in decomposition of organic materials, nutrient cycling, and formation of soil structure. We evaluated chloroform fumigation-incubation with subtraction of a control (CFI/F–C) and without subtraction of a control (CFI/F) as methods to assess biological soil quality. Relationships between CFI/F and potential C mineralization, particulate organic C, and soil organic C were stronger (r2 = 0.86 ± 0.07, n = 232) than those between CFI/F–C and the same soil C pools (r2 = 0.25 ± 0.09) in soils from Georgia. From published data, relationships of CFI/F with potential C mineralization and soil organic C were stronger than those of chloroform fumigation-extraction and substrate-induced respiration with these soil C pools. Effects of land management on biological soil quality using CFI/F were consistent with those determined using other soil C pools as response variables. However, land management effects on biological soil quality using...

A rapid procedure for estimating nitrogen mineralization in manured soil

Abstract A routine soil testing procedure for soil N mineralization is needed that is rapid and precise. Not accounting for N mineralization can result in the over-application of N, especially in soils with a history of manure application. Our objectives were to compare results from a recently proposed rapid laboratory procedure with: (1) long-term N mineralization under standard laboratory conditions, and (2) actual forage N uptake from soil receiving dairy cattle (Bos taurus) manure in a 2-year field study. The rapid procedure is based on the quantity of CO2-C evolved during 24 h under optimum laboratory conditions following the rewetting of dried soil. Dairy cattle manure was surface applied beginning in 1992 at annual rates of 0, 112, 224, or 448 kg N ha–1 to field plots on a Windthorst fine sandy loam soil (fine, mixed, thermic Udic Paleustalf) near Stephenville, Texas (32°N, 98°W). Results of the one-day CO2 procedure were highly correlated with soil N mineralized from samples collected in March of 1995 (P=0.004) and 1996 (P<0.001) and with forage N uptake (P<0.001) both years of the study. Residual inorganic N in the same soil samples was poorly correlated with soil N mineralization and forage N uptake.

Soil C extracted with water or K2SO4: pH effect on determination of microbial biomass

Routine determination of soil microbial biomass C has shifted during the past decade from chloroform fumigation-incubation to chloroform fumigation-extraction using 0.5 M K2SO4 as extractant. We compared extractable C with water and 0.5 M K2SO4 in eight soils ranging in pH from 5.4 to 8.3. In unfumigated soils with low pH, extractable C was 0.8- to 1.2-fold greater with water than with 0.5 M K2SO4. However, in unfumigated soils with pH > 7.7, extractable C, although not statistically significant, was 11 to 19% less with water than with 0.5 M K2SO4. In fumigated soils, no difference in extractable C between water and 0.5 M K2SO4 was detected among soils with pH   7.7. Our results suggest that 0.5 M K2SO4 (1) may flocculate soil and cause adsorption of solubilized C onto colloids at pH   7.7, thereby differentially affecting the fate of solubilized C depending upon soil pH....