Microbial Biomass Dynamics in Tallgrass Prairie Fernando O. Garcia and Charles W. Rice* ABSTRACT 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-', respectively, for the 0- to 30-cnt 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. M ICROBIAL BIOMASS is considered a transformation agent of soil organic materials and a labile reser- voir of nutrients such as C, N, P, and S (Jenkinson and Ladd, 1981). Because of its role in the N cycle, MBM has been thoroughly studied in agroecosystems (Carter and Rennie, 1984; McGill et al., 1986). The size of the MBM is regulated by substrate and water availability, protection capacity of the soil, and temperature (van Veen et al., 1984; McGill et al., 1986). Its role in con- F.O. Garcia, Departamento Agronomia, E.E.A. INTA, C.C. 276, (7620) Balcarce, Argentina; and C.W. Rice, Dep. of Agronomy, Kansas State Univ., Throckmorton Hall, Manhattan, KS 66506-5501. Received 6 Aug. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 58:816-823 (1994). serving mineral nutrients and transforming organic nutri- ents into plant-available forms could be critical in the tallgrass prairie ecosystem in which external additions or losses of nutrients, such as N, are minimal (Wood- mansee, 1978). Clark (1977) indicated that N require- ments in the shortgrass prairie ecosystem are met by internal translocation by plants and mineralization of soil organic N by MBM. In tallgrass prairie, which is a major ecosystem in the U.S.A., fire is a common disturbance. Burning increases the photosynthetic capacity of postburn plant growth (Knapp and Seastedt, 1986) and results in changes in soil temperature, water, and nutrient status. Ojima (1987) reported that annual burning resulted in lower soil organic matter but higher plant productivity compared with no burning. This apparent contradiction may be explained by (i) synchronization of nutrient release with plant up- take and microbial activity; (ii) extension of the growing season because of earlier soil warming; (iii) changes in the rate of ecosystem processes that allow for recovery of volatilized nutrients (i.e., N2 fixation); (iv) changes in the utilization of available nutrients; or (v) a combina- tion of all these reasons (Knapp and Seastedt, 1986; Ojima, 1987). Although MC and MN were also reduced by long-term annual burning (>40 yr), they were not affected after only 1 to 2 yr of burning (Ojima, 1987). Grazing is an important element in most tallgrass prairie ecosystems and interacts with burning in de- termining the structure and composition of vegetation (Anderson, 1990; Hobbs et al., 1991). Grazing tends to reduce root growth and rhizome carbohydrate reserves (Rains et al., 1975; Turner et al., 1993). The decrease of belowground C inputs could result in a reduced C/N ratio of belowground plant biomass, reduced micro- Abbreviations: MC, microbial C; MN, microbial N; MBM, microbial biomass; LSD, least significant difference.