Aims Drylands account for 40% of global net primary productivity, playing an important role in the global carbon (C) budget. As a key surface component, biocrusts are vital contributors to C sequestration in these regions. However, owing to the complexity of climate change, including concurrent changes in precipitation and temperature, their future role as regulators of the global C cycle is not well understood. Methods To address this gap, we conducted a study in the northern Loess Plateau of China, continuously monitoring the main C fluxes (photosynthesis (P-n), respiration (R-s), and net C flux (NCF)) of moss-dominated biocrusts under eight climatic scenarios. Experimental treatments included ambient controls, warming alone, and warming in combination with various precipitation scenarios, including reductions (- 50%, - 30%, and - 10%) and increases (+ 10%, + 30%, and + 50%) in rainfall. Results Warming and increased rainfall significantly enhanced R-s and P-n during the growing season, particularly under + 50% rainfall additions, while warming and rainfall reductions had the opposite effect. Due to the asymmetric responses of R-s and P-n, precipitation increases and reductions combined with warming led to enhanced and reduced C gains in biocrusts, respectively, resulting in a net C loss with rainfall reduction scenarios exceeding 30%. Structural equation modeling indicated that biocrust C fluxes were governed by moss density and soil water content. Conclusions We conclude that a threshold of 30% reduction in precipitation determines the shifts in biocrust net C flux induced by climate warming. This aids the accurate prediction of their future C sink/source dynamics.