An increase in precipitation amount and intensity, which are projected for the northeastern U.S., may lead to changes in the terrestrial nitrogen cycle. We studied the potential for such changes in the Pinelands ecosystem of southern New Jersey. During a two-year field study we measured the response of inorganic nitrogen pools, potential net nitrogen mineralization rates and the bacteria:fungi ratio to an experimental doubling of ambient rainfall and a complete rain exclusion (Experiment 1). In a second two-year field study (Experiment 2), we analyzed the potential effects of elevated winter precipitation amount on the bacteria:fungi ratio and nitrogen cycling.
Results/Conclusions
During Experiment 1, we found a negative relationship between soil moisture and nitrogen mineralization in early spring (r = -0.48, p = 0.006) and a positive relationship between soil moisture and nitrogen mineralization during mid summer (r = 0.39, p = 0.04). Ammonium pools in drought plots increased throughout the study and were over four times the values of all other treatments after one year. During Experiment 2, elevated winter rainfall had no effect on nitrogen mineralization rates. However, at the end of a relatively cool winter with a defined freezing period, ammonium pools were three times higher in ambient plots in relation to plots to which water was applied (p < 0.05). Following a more mild winter with only minimal freezing, there was no effect of watering on ammonium pools. Neither experiment had any effect on the bacteria:fungi ratio.
The response of nitrogen mineralization in Experiment 1 was consistent with previous studies that demonstrate a link between nitrogen mineralization and soil temperature, substrate diffusion and microbial physiological demand. The ammonium buildup in both studies is best explained by reduced plant uptake, which was likely the resulted of reduced solute diffusion in dry soil (Experiment 1) and freeze-damaged roots (Experiment 2). These changes in the nitrogen cycle are unlikely to affect plant nitrogen demand in the New Jersey Pinelands. However, ammonium is known to be toxic to many fungi, and could therefore potentially feedback on the nitrogen cycle by altering the bacteria:fungi ratio of soil. We suggest that periods of extended drought or soil freezing followed rainfall may represent an important time for studying the response of microbial community composition and nitrogen cycling.
