In California grasslands climate models project either increases or decreases in annual precipitation, depending on the general circulation model and emissions scenario used. These changes in total precipitation will alter the frequency and duration of wet and dry spells in ways that are critical for plant and soil processes, but are difficult to predict. Changing precipitation regimes may alter important ecosystem processes, including nutrient and water cycling. We have used a mesocosm experiment to examine how changing precipitation may alter ecosystem function in monospecific stands of the annual grass, Avena barbata. We compared microbial and plant responses to low and high precipitation treatments in a low nutrient soil collected from Southern California (Sedgwick Reserve), and a higher nutrient soil collected from Northern California (Hopland Experimental Station). To mimic natural rainfall variability, low precipitation treatments experienced fewer rain events and longer periods between rain events than the high precipitation treatment. Seasonal responses of soil microbial community composition, nutrient process rates, plant gene expression, leaf and canopy level water and CO₂ fluxes, productivity, and phenology were measured. Soil type had large effects on microbial communities and plant responses to the precipitation treatments.

Results/Conclusions

Soil type was the primary factor separating microbial community composition, which was correlated with nitrification potentials in both soil types. In the nutrient poor soil there was less total RNA in the leaves and a lower level of nitrate reductase mRNA. In addition, changes in the expression of RbcS and GS2 indicated that during the dry periods Avena plants were more water stressed when growing in the higher nutrient soil. These results are supported by plant growth and physiology data. The leaf area index of Avena stands was higher in the high nutrient soil; however, canopy level water and CO₂ fluxes of these dense canopies were more sensitive to the dry periods between watering cycles. Late in the season, canopy level water and CO₂ fluxes in the low precipitation treatments were higher for plants growing in the low nutrient soil compared to the higher nutrient soil. Under high precipitation, water fluxes did not differ between the soil types. Under the low precipitation treatment, Avena flowered earlier when growing in Sedgwick soil but later when growing in Hopland soil.   The results demonstrate that precipitation interacts strongly with soil type in California grasslands to affect the microbial and plant processes controlling ecosystem function.