<b>Biocrusts mitigate the influence of aridity on carbon and nitrogen stocks</b>

Aridification is threatening the capacity of drylands to support key ecosystem services such as carbon sequestration, nutrient cycling and plant primary productivity. Biocrusts are widely distributed in global drylands and play significant roles in regulating carbon and nitrogen cycles. Yet, their roles as regulators of aridity are still under debate. Here, we conducted a survey of 70 sites (spanning forestland, shrubland, and grassland) across a 700-km transect in the Loess Plateau to examine the influences of biocrusts on soil carbon and nitrogen stocks along a gradient of increasing aridity from sub-humid to arid climates. Our results showed that, compared to unvegetated bare soil, biocrusts support greater stocks of soil organic carbon and total nitrogen and their inorganic fractions. The contribution of biocrust depended on ecosystem type, being especially important in shrublands. Most importantly, we found biocrusts mitigate the negative relationship between aridity and soil carbon and nitrogen stocks compared with bare soils, with greater biocrust and less vascular plant cover being present as aridity increases. Our findings highlight the crucial role of biocrusts in buffering the impacts of aridity, and thus of climate change on soil carbon and nitrogen sequestration in drylands and reveal a key aridity threshold that influences their impact on soil carbon stock.