Patterns and drivers of the degradability of dissolved organic matter in dryland soils on the Tibetan Plateau

Abstract Dryland soils consistently exhibit a low capacity for the long‐term accumulation and storage of organic matter, which has been primarily attributed to low plant biomass inputs under drought suppression. Whether, and how, soil organic matter (SOM) compositions contribute to the consistently low SOM storage have been puzzling. A fundamental understanding of this mechanism is particularly essential to achieve the aspiration of ‘4 per mille Soils for Food Security and Climate’. By screening the molecular composition of dissolved organic matter (DOM), the gatekeeper of SOM decomposition, we explored the transformation processes among the pools of SOM, DOM and microbial biomass carbon (MBC) in soils along a precipitation gradient on dryland grasslands of the Tibetan Plateau. The results revealed that the number and mean weight of DOM molecules significantly decreased, and the soil DOM composition gradually shifted to be more labile along the transition from meadow, steppe, to desert with decreasing precipitation, coinciding with the substantial reduction in SOM. Compared with meadow soils, DOM degradability increased by 8.7% in steppe soils and by 23.4% in desert soils. The ratio of soil MBC to total organic carbon was threefold higher in desert than in meadow, and positively correlated with DOM degradability, indicating that labile DOM accelerated microbial growth and SOM decomposition in desert soils. Structural equation model and correlation analyses demonstrated that the DOM degradability was primarily controlled by soil dissolved nitrogen and soil organic C and soil DOC/DN ratio. Synthesis and application . This study at a molecular level provides a novel insight into the important role of the degradability of dissolved organic matter in carbon accumulation in dryland soils with consistently low organic matter storage. The findings will inform better global managements of soil organic matter under consideration of both food security and climate change.