Response of soil microbial communities and the production of plant-available nitrogen to a two-year rainfall manipulation in the New Jersey Pinelands

Projected changes in precipitation patterns in the northeastern U.S. may alter soil moisture dynamics and cause a shift in the structure and function of soil microbial communities. We studied the potential for such changes by manipulating annual precipitation amount in an oak pine forest of the New jersey Pinelands. During a two-year field study we tested the effects of a complete rain exclusion, as well as a doubling of rainfall, on soil microbial biomass, community composition (phospholipid fatty acid analysis) and the production of plant-available nitrogen (nitrogen mineralization + amino-acid production). We found that neither microbial biomass nor community composition was affected by the experimental manipulations. Despite having studied the organic horizon, the relatively high sand content appeared to influence this response by limiting the extent to which soil moisture increased in response to elevated rainfall. Furthermore, a strong correlation between soil moisture and the physiological status of Gram-negative bacteria suggested that soil microbes in the New jersey Pinelands are well adapted to soil drying. We observed a sustained accumulation of ammonium in drought plots that was more than four times the value of all other treatments after one year. The relationship between soil moisture and nitrogen mineralization changed with season, suggesting that the effect of changing rainfall patterns on nitrogen cycling will depend upon microbial physiological demand and substrate diffusion. Based on available estimates of foliar N concentration in the New jersey Pinelands, we conclude that neither the accumulation of ammonium in drought plots, nor the changes in nitrogen mineralization rates in response to high and low soil moisture will affect plant nitrogen demand. However, if the ammonium pool in dry soil had been mobilized by precipitation, a shift towards a higher bacteria:fungi ratio and therefore higher nitrogen mineralization rates may have occurred. (C) 2010 Elsevier Ltd. All rights reserved.