Aims <p>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.</p> Methods <p>To address this gap, we conducted a study in the northern Loess Plateau of China, continuously monitoring the main C fluxes (photosynthesis (<i>P</i>_n), respiration (<i>R</i>_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.</p> Results <p>Warming and increased rainfall significantly enhanced <i>R</i>_s and <i>P</i>_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 <i>R</i>_s and <i>P</i>_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.</p> Conclusions <p>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.</p>