Anthropogenic activity is changing climate and increasing nitrogen deposition globally. The effects of these changing resource amounts and regimes on productivity and microbial respiration is especially important to understand in drylands which are drivers of interannual variation in carbon flux. Given that significant biomass in dryland systems is belowground, we investigated 1) above and belowground plant productivity responses and 2) microbial respiration temperature response from three long-term experiments: one which manipulated the monsoon rainfall regime, one which manipulated nitrogen addition, and one which manipulated nighttime temperature and spring rainfall regime. For the plant responses, we compared the magnitude of response to the ambient SPEI, a relative measure of hot/dry or cool/moist conditions, to evaluate hypotheses related to hierarchical resource limitations of water being most limiting and N becoming limiting only when water is plentiful. For the microbial respiration, we asked whether acclimatization to different mean and variability of temperature and soil moisture affected response to novel temperatures.
Results/Conclusions
For plant responses, we found up to two-fold differences in total biomass across years, and increased rainfall increased productivity whereas N addition decreased allocation belowground. We found that resource addition marginally increased total biomass in relatively dry but not more mesic years, but that allocation belowground was relatively unresponsive across the observed range of SPEI. These results did not support our hypothesis that N was limiting only when water was plentiful and suggests that long-term N deposition may dampen response to this resource. For microbial respiration, we found that antecedent soil moisture was the best model of temperature response of incubated soils compared to either antecedent mean or range of soil temperatures, but there was overall no difference in respiration by antecedent soil moisture at any novel incubated temperature. We found no evidence of a compensatory response from soils originating from higher mean temperatures in contrast to studies that compared soils across wide geographic ranges where mean and range were highly correlated. Overall, we found that manipulated temperature, soil moisture, and N addition did not affect the belowground dynamics as predicted and suggest that dryland systems are more resistant to these changes than they are sometimes considered to be.