Interactive effects of precipitation manipulation and nitrogen addition on soil properties in California grassland and shrubland

Soil microbial communities and pools of carbon (C) and nitrogen (N) play an important role in ecosystem responses to precipitation variability and N deposition. In southern California, ecosystem vulnerability to these environmental change drivers may differ for grassland versus shrubland vegetation types. We hypothesized that (1) these vegetation types would differ in their responses to precipitation and N manipulation; (2) reduced precipitation (“drought treatment”) would have a negative effect on soil microbial abundance and alter microbial community composition, (3) these changes would be associated with reductions in soil C and N pools, (4) N addition would increase microbial abundance as well as soil C and N pools, and (5) combined drought and N deposition would have offsetting effects on soil properties. We tested these hypotheses at the Loma Ridge Global Change Experiment in southern California. Across vegetation types, we found that microbial biomass based on phospholipid fatty acids declined with drought and N addition. Microbial composition differed more strongly by vegetation type than with environmental change treatments. Added precipitation had little effect on microbial biomass but reduced labile C and N pools; these reductions were mitigated by N addition. Drought reduced labile forms of soil C and N, whereas N addition increased labile soil C pools and all soil N pools. Negative effects of drought and N addition were additive for microbial biomass, which could inhibit soil C cycling if both of these environmental changes occur together. Drought interacted with N addition to significantly increase the most labile N pool under the drought + N treatment, which suggests a build-up of available N under these conditions. These results imply that multiple environmental changes may combine non-additively to affect below-ground microorganisms and soil C and N pools, which may have important consequences for ecosystem services such as productivity, biodiversity, and soil quality in Mediterranean climate regimes of North America.