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The temporal dynamics and effects of burning, mowing, and N fertilization on microbial biomass (MBM) in tallgrass prairie were studied in a field experiment established in 1986. Microbial C (MC) and microbial N (MN), determined by the fumigation‐incubation procedure during the growing seasons of 1989 through 1991, averaged 217 mg C kg−1 and 32.6 mg N kg−1, respectively, for the 0‐ to 30‐cm depth. Accumulation of litter and greater production of roots near the surface resulted in stratification of MBM. Seasonally, MBM was higher in early spring, decreased with the initiation of plant growth, and then recovered by late summer or early fall. Decreases of MN between March and July coincided with plant N uptake. The increase of MC and decrease of MN during the 3 yr of the study were related to increased plant production. Burning had a short‐term and variable effect on MC. Burning tended to reduce MC during dry years and increase it in normal to wet years. Mowing and raking decreased MC and MN, probably because of reduced root biomass and removal of standing vegetation. Nitrogen addition resulted in higher MN and tended to reduce MC, possibly by modifying the composition of the microbial population. Microbial biomass seems to play a critical role in conserving N in the tallgrass prairie ecosystem.

The use of no‐tillage has notably increased in the Pampas region of Argentina during the last 10 yr. Two tillage experiments with contrasting previous agricultural use, degraded and non‐degraded soils, were evaluated in the southeast of Buenos Aires province, Argentina. The objectives were to: (i) quantify the effects of tillage and N fertilization on quantity and vertical distribution of C and N in the soil organic matter (SOM) and particulate organic matter (POM) fractions as well as potentially mineralizable N (PMN), and (ii) evaluate these fractions as indicators of soil quality. Tillage systems were conventional tillage (CT), minimum tillage (MT), and no‐tillage (NT) (main plots), and N fertilization rates were 0, 120, and 150 kg ha−1 (subplots). Total organic C (TOC), total N (TN), POM‐C, POM‐N, and PMN were measured at 0‐ to 7.5‐ and 7.5‐ to 15‐cm soil depth. In Exp. I (degraded soil) TOC was greater under NT (27 g kg−1) than under CT (24 g kg−1) in the 0‐N treatments. No differences in TOC and TN were found in Exp. II at 0 to 7.5 cm (non‐degraded soil). Carbon in POM and POM‐N were greater under NT in the fractions of 212 to 2000 and 53 to 212 μm at 0 to 7.5 cm, but they were similar or greater under CT at 7.5‐ to 15‐cm depth in Exp. I. Stratification of TOC, TN, and POM were observed under NT in Exp. I. Potentially mineralizable N was greater under NT (62 mg kg−1) in Exp. I, however, no differences in PMN were observed in Exp. II. Carbon in POM 212 to 2000 μm and PMN were the more sensitive indicators of tillage effects, mainly in Exp. I.

Terrestrial responses to increasing atmospheric CO2 are important to the global carbon budget. Increased plant production under elevated CO2 is expected to increase soil C which may induce N limitations. The objectives of this study were to determine the effects of increased CO2 on 1) the amount of carbon and nitrogen stored in soil organic matter and microbial biomass and 2) soil microbial activity. A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO2 concentrations in open-top chambers in the field from 1989 to 1992 and compared to unchambered ambient CO2 during the entire growing season. During 1990 and 1991, N fertilizer was included as a treatment. The soil microbial response to CO2 was measured during 1991 and 1992. Soil organic C and N were not significantly affected by enriched atmospheric CO2. The response of microbial biomass to CO2 enrichment was dependent upon soil water conditions. In 1991, a dry year, CO2 enrichment significantly increased microbial biomass C and N. In 1992, a wet year, microbial biomass C and N were unaffected by the CO2 treatments. Added N increased microbial C and N under CO2 enrichment. Microbial activity was consistently greater under CO2 enrichment because of better soil water conditions. Added N stimulated microbial activity under CO2 enrichment. Increased microbial N with CO2 enrichment may indicate plant production could be limited by N availability. The soil system also could compensate for the limited N by increasing the labile pool to support increased plant production with elevated atmospheric CO2. Longer-term studies are needed to determine how tallgrass prairie will respond to increased C input.