Soil Microbial Biomass Dynamics and Net Nitrogen Mineralization in Northern Hardwood Ecosystems

Soil temperature, water potential, and substrate (C) availability are the primary constraints on microbial activity within soil and display substantial seasonal variation within northern deciduous forests. Following autumn litterfall, soil C availability is relatively high, and N should be assimilated by soil microorganisms to maintain or form new biomass. Conversely, N should be mineralized from microbial biomass during midsummer when C availability is relatively low and soil temperatures are high. Because N availability is directly controlled by microbial activity, we hypothesized that microbial biomass and net N mineralization are inversely related on a seasonal basis. To test this hypothesis, we studied the temporal relationship between microbial biomass (C and N) and rates of net N mineralization in two different northern hardwood ecosystems, one dominated by sugar maple (Acer saccharum Marsh.) and basswood (Tilia americana L.) and the other by sugar maple and red oak (Quercus rubra L.). In situ buried bags were used to estimate net N mineralization and nitrification at monthly intervals for 1 yr. Microbial C and N contents of the incubated soil were determined using the chloroform fumigation-incubation method. Net N mineralization displayed marked seasonal variability, ranging from 35 to 115 mg N m⁻² d⁻¹ during the growing season. In contrast, microbial biomass C and N were relatively constant throughout the year, averaging 112 g C m⁻² and 17 g N m⁻². Neither microbial biomass (C or N) nor the change in microbial biomass between sampling dates were significantly inversely correlated with mean daily rates of net N mineralization. As such, our data do not support the idea that N availability is controlled by large seasonal fluctuations in soil microbial biomass. Rather, our results suggest that N availability is primarily controlled by changes in the turnover rate of microbial biomass such that a relatively constant pool is maintained through time. In addition, mean annual rates of net N mineralization and nitrification did not differ significantly from those previously measured by others in the same stands, suggesting that annual rates may be relatively consistent in climatically similar years.