Bajada Summer 2015 Soil Nutrients

In dryland soils, spatiotemporal variation in surface soils (0-10 cm) plays an important role in the function of the “critical zone” that extends from canopy to groundwater. Understanding connections between soil microbes and biogeochemical cycling in surface soils requires repeated multivariate measurements of nutrients, microbial abundance, and microbial function. We examined these processes in resource islands and interspaces over a two-month period at a Chihuahuan Desert bajada shrubland site. We collected soil in Prosopis glandulosa (honey mesquite), Larrea tridentata (creosote bush), and unvegetated (interspace) areas to measure soil nutrient concentrations, microbial biomass, and potential exoenzyme activity. We monitored the dynamics of these belowground processes as soil conditions dried and then rewetted due to rainfall. Most measured variables, including inorganic nutrients, microbial biomass, and soil enzyme activities were greater under shrubs during both wet and dry periods, with the highest magnitudes under mesquite followed by creosote and then interspace. Two exceptions were soil organic carbon, which was slightly higher in interspace areas, and nitrate, which was highly variable and did not show resource island patterns. Temporally, rainfall pulses were associated with substantial changes in soil nutrient concentrations, though resource island patterns remained consistent during all phases of the soil moisture pulse. Microbial biomass was more consistent, decreasing only when soils were driest. Potential enzyme activities were even more consistent, and did not decline in dry periods, potentially helping to stimulate observed pulses in CO2 efflux following rain events observed at a co-located eddy flux tower. These results indicate a critical zone with organic matter cycling patterns consistently elevated in shrub resource islands, high decomposition potential that limits soil organic carbon accumulation across the landscape, and nitrate fluxes that are decoupled from the organic matter pathways.