Effects of Drought Manipulation on Soil Nitrogen Cycling: A Meta‐Analysis

Abstract

Abstract Many regions on Earth are expected to become drier with climate change, which may impact nitrogen (N) cycling rates and availability. We used a meta‐analytical approach on the results of field experiments that reduced precipitation and measured N supply (i.e., indices of N mineralization), soil microbial biomass, inorganic N pools (ammonium (NH 4 + ) and nitrate (NO 3 − )), and nitrous oxide (N 2 O) emissions. We hypothesized that N supply and N 2 O emissions would be relatively insensitive to precipitation reduction and that reducing precipitation would increase extractable NH 4 + and NO 3 − concentrations because microbial processes continue, whereas plant N uptake diminishes with drought. In support of this hypothesis, extractable NH 4 + increased by 25% overall with precipitation reduction; NH 4 + also increased significantly with increasing magnitude of precipitation reduction. In contrast, N supply and extractable NO 3 − did not change and N 2 O emissions decreased with reduced precipitation. Across studies microbial biomass appeared unchanged, yet from the diversity of studies, it was clear that proportionally smaller precipitation reductions increased microbial biomass, whereas larger proportional reductions in rainfall reduced microbial biomass; there was a positive intercept ( P = 0.005) and a significant negative slope ( P = 0.0002) for the regression of microbial biomass versus % precipitation reduction (LnR = −0.009 × (% precipitation reduction) + 0.4021). Our analyses imply that relative to other N variables, N supply is less sensitive to reduced precipitation, whereas processes producing N 2 O decline. Drought intensity and duration, through sustained N supply, may control how much N becomes vulnerable to loss via hydrologic and gaseous pathways upon rewetting dry soils.