Biocrusts drive soil respiration across seasons and depths in a cold-winter desert

Biocrusts are known to regulate soil respiration in drylands. However, how biocrusts influence the changes in soil respiration across seasons and soil depths is far less understood. This knowledge gap hampers our ability to accurately predict the impacts of biocrust development or disturbance on soil carbon (C) balance in drylands, which covers almost half of the planet. Here, we used solid-state CO2 sensors in a cold-winter desert on the Loess Plateau in northern China to investigate the impacts of biocrusts on C flux across seasons and soil depths. In particular, we measured C fluxes from 5, 15, and 30 cm depths in biocrust-covered and bare soils during two continuous years and multiple seasons. Our high-time-resolution approach indicated that biocrusts release 55% (i.e., 490 g C m(-2)) more C per year than bare soil. This result is attributed to the improvements of organic C pools, microbial abundance and diversity, and hydrothermal environments induced by biocrusts. Soil C flux of biocrusts have significant seasonal variations due to the differences of biocrust responses to soil temperature and water content and their effects on microbial activities in different seasons. Meanwhile, the seasonal variability of C flux in biocrusts was higher than that in bare soil (coefficient of variation = 0.48 vs. 0.43). Biocrusts contributed the most to annual C efflux in cold season (winter: 32%) followed by warm season (summer: 29%), and the reason is that the duration of winter during cold season (averaged 175 days over two years) is about 3 times longer than that of summer during warm season (67 days) in the cold desert. Furthermore, although the total soil C efflux was mainly from 0 to 15 cm depth (>95%), biocrusts still increased the contribution (3%) of deeper soils (15-30 cm) to total C efflux compared with bare soil. Our findings highlight the fundamental role of biocrusts as major C regulators in cold deserts by providing high temporal resolution and soil depth data. This knowledge is critical to understand the consequences of biocrust development, or biocrust disturbance, for C cycling when accurately assessing the contribution of cold deserts to terrestrial ecosystem under global environmental changes.