Climate dictates the effects of increased drought and physical disturbance on dryland soil biogeochemsitry

Background/Question/Methods

Drylands occupy 45% of Earth’s land area and support ~35% of the world’s population. Carbon and nutrient cycling in these vast ecosystems have important regional and global implications, including regulation of the interannual variability and trend in the global terrestrial carbon sink. However, drylands face growing pressure from high levels of human activity and increasingly frequent and severe drought events. Drylands are predicted to be exceptionally vulnerable to these pressures due to low water and resource (i.e., carbon and nutrient) availability. Moreover, the expansive nature of drylands and the creation of “fertile islands” beneath shrub canopies support substantial biogeochemical heterogeneity at regional and fine spatial scales, further complicating the influence of global change drivers. To examine the effects of climate, drought, and disturbance on dryland soil biogeochemistry, we implemented drought and disturbance treatments (n=10) in a full factorial design across three North American deserts representing a gradient of aridity - using rainout shelters to mimic 100-year drought conditions and physically disturbing plots with fracturing and compression. We collected soil from beneath shrubs and shrub interspaces prior to treatments and six months and one year post-treatment and measured concentrations of total and extractable carbon and nitrogen, bioavailable nitrogen and phosphorus, and microbial biomass carbon and nitrogen.

Results/Conclusions

Concentrations of total soil carbon and nitrogen and microbial biomass carbon and nitrogen were between 2 and 5× greater in the wettest site (i.e., Great Basin Desert) relative to the driest site (i.e., Chihuahuan Desert). Fertile Island effects were prominent regardless of climate, but the largest and most consistent differences in soil carbon and nutrient concentrations between shrub and interspace soils occurred in the Chihuahuan Desert. Physical disturbance had no independent effects on soil biogeochemistry in the Chihuahuan Desert, but, after one year, drought conditions increased extractable carbon and nitrogen and bioavailable nitrogen in interspace soils (P=0.04, <0.001, and <0.001, respectively) and decreased microbial biomass carbon in shrub soils (P=0.04). These findings suggest that, after a single drought year, arid soils experienced a marked decline in microbial community size and activity that overshadowed physical disturbance effects. In contrast, drought had no independent effects in the Great Basin, but physical disturbance increased bioavailable nitrogen (P=0.02) and decreased microbial biomass nitrogen (P=0.02) under shrubs. Taken together, our results suggest that the strength and direction of drought and physical disturbance effects, and their interactions, on dryland soil biogeochemistry are highly dependent on climate.