Soil enzyme responses to varying rainfall regimes in Chihuahuan Desert soils

Extracellular enzyme activities (EEAs) are indicators of both soil microbial activity and nutrient availability for plants. However, it is unclear how EEAs change over the growing season in desert grasslands. We examined whether EEAs changed in response to the size and frequency of rain events during the summer monsoon in the northern Chihuahuan Desert, and if the response varied between plant and interspace associated soils. Potential EEAs were measured within a rainfall manipulation experiment at the Sevilleta National Wildlife Refuge in central New Mexico, USA. Rainfall treatments included either three 10-mm events or one 30-mm rain event per month throughout the three month summer monsoon (July-September). EEAs were measured immediately before and within hours after experimental rain events under plants and in unvegetated interspaces. Throughout the season hydrolase activities were higher under vegetation than in interspace soils. Potential activities of hydrolytic enzymes were similar for the two rainfall regimes. Activities increased following early season rain, showed little response to mid-season rain, and decreased following late-season rain. Although enzyme activities did not differ between rainfall treatments, ratios between enzymes varied, indicating different nutrient limitations imposed by rain event size and frequency. Larger nitrogen and phosphorus limitations occurred in interspace soils that experienced large, frequent rain events. Many factors, including location relative to plants, seasonality, and rainfall size and frequency, influenced enzyme activities and nutrient availability in these Chihuahuan Desert soils throughout the monsoon season.