Biocrusts mediate seasonal warming effects of soil N transformation in drylands

Drylands are mostly covered by biocrusts and are sensitive to climate change, which will likely affect nitrogen (N) transformation. However, it remains unclear the response of N transformation-related variables (N transformation rates, microbial biomass, and enzyme activity) to warming in biocrust-dominated dryland ecosystems. Here, we examined three soil cover types (bare soil, cyanobacteria- and moss-dominated soil) over a full year as we conducted a warming treatment (open top chambers) in the Tengger Desert. In order to quantify the response of N transformation-related variables to warming, we defined the warming effects (WEs) as the increment of N transformation-related variable per- unit variation of temperature. Our results showed that the presence of biocrusts can significantly increase the WEs of soil N mineralization rates (Rmin), nitrification rates (Rnit), the content of microbial biomass carbon (MBC) and nitrogen (MBN), and the activities of soil nitrate reductase (S-NR) and urease (S-UE). Microbial biomass under biocrusts was more sensitive to warming followed by enzyme activity. Meanwhile, the WEs in spring and fall were higher than those in winter and summer. The cumulative rainfall was the driving factor affecting the seasonal change of WEs. Therefore, the defining and studying warming effects expand our understanding of seasonal dynamics of N transformation, microbial biomass and enzyme activity, and emphasize the important roles of biocrusts as modulators of N cycling under climate change in dryland ecosystems.