Anthropogenic sources of energy production and land use change may lead to increases in precipitation amount in the northeastern United States. This could alter soil microbial community structure, nitrogen cycling and, ultimately, above-ground and below-ground carbon sequestration. To test the effect of altered precipitation amount on soil microbial community structure and nitrogen cycling, precipitation amount was manipulated for two years in an oak-pine forest of the New Jersey Pinelands. Twelve rainout shelters were constructed in order to divert rainfall into tanks. Rainfall was applied to plots weekly. Treatments included no rainfall, 100% or 200% of actual rainfall. The microbial community was characterized using the phospholipid fatty acid analysis. I measured microbial production of plant-available nitrogen (nitrogen mineralization + amino acid production) using initial and in situ incubation cores.

Results/Conclusions

Soil microbial community structure was influenced by precipitation treatments. After only three months of treatments, gram⁺ and gram^- bacteria biomass were positively correlated with soil moisture (biomass vs. soil moisture: r² = .49, p = .003; r² = .32, p=.02 respectively). However, actinomycete and fungal biomass remained fairly stable across the range of soil moisture (r² = .14, p = .15 respectively). Production of plant-available nitrogen was strongly and positively correlated with soil moisture during summer only. This data suggests that changes in precipitation amount will predominantly impact soil bacteria, which could lead to higher rates of soil carbon cycling. The effect of altered precipitation amount on nitrogen cycling will be most pronounced during summer, when microbes are most active and highly water-limited.