Biocrusts mitigate the influence of aridity on carbon and nitrogen stocks

Aridification is threatening the capacity of drylands to support key ecosystem services such as carbon (C) sequestration, nutrient cycling, and plant primary productivity. Biocrusts are widespread in drylands and function in soil C and nitrogen (N) cycling, yet their role in modulating soil C and N stocks along aridity gradients remains unclear. Here, we examined how moss biocrusts influence surface soil C and N stocks (0–5 cm) across a ~700 km aridity gradient (aridity = 1 – AI, where AI is precipitation/potential evapotranspiration), ranging from 0.49 to 0.79 and spanning forestland, shrubland, and grassland ecosystems on the Chinese Loess Plateau. We compared soil organic C (SOC), dissolved organic C (DOC), microbial biomass C (MBC), total N (TN), and inorganic N (NO3– and NH4+) between biocrust-covered and adjacent bare soils. The results indicated that biocrust-covered soils stored substantially more C and N than bare soils. On average, SOC, DOC, and MBC stocks were 2.5-, 2.2-, and 3.8-fold higher, respectively, while TN, NO3–-N, and NH4+-N stocks were 1.9-, 3.0-, and 1.5-fold higher. These enhancements were strongest in shrubland ecosystems. Importantly, the negative relationship between aridity and soil C and N stocks were weaker in biocrust-covered soils than in bare soils. Analysis of paired differences (biocrust-covered soil minus bare soil) further revealed a nonlinear response of ΔSOC to aridity: ΔSOC decreased under lower aridity ( 0.66). In contrast, ΔTN increased monotonically with aridity and showed no statistically supported threshold. With increasing aridity, vascular plant cover declined while biocrusts development increased. Together with biocrust-induced enrichment in fine soil particles, these shifts enhanced the relative contribution of biocrusts to maintaining soil C and N stocks under drier conditions. Our findings highlight the critical buffering role of moss biocrusts in sustaining dryland soil fertility under ongoing climate drying.