Universal microbial reworking of dissolved organic matter along environmental gradients

Soils are losing increasing amounts of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can increasingly offset their carbon sink capacity. DOM is more susceptible to degradation closer to its source and becomes increasingly dominated by the same (i.e., universal), difficult-to-degrade compounds as degradation proceeds. However, the processes underlying DOM degradation across environments are poorly understood. Here we found DOM changed similarly along soil-aquatic gradients irrespective of differences in environmental conditions. Using ultrahigh-resolution mass spectrometry, we tracked DOM along soil depths and hillslope positions in forest headwater catchments and related its composition to soil microbiomes and physico-chemical conditions. Along depths and hillslopes, carbohydrate-like and unsaturated hydrocarbon-like compounds increased in abundance-weighted mass, suggestive of microbial reworking of plant material. More than half of the variation in the abundance of these compounds was related to the expression of genes essential for degrading plant-derived carbohydrates. Our results implicate continuous microbial reworking in shifting DOM towards universal compounds in soils. By synthesising data from the land-to-ocean continuum, we suggest these processes can be generalised across ecosystems and spatiotemporal scales. Such general degradation patterns can be leveraged to predict DOM composition and its downstream reactivity along environmental gradients to inform management of soil-to-stream carbon losses.土壤每年以溶解性有机质（dissolved organic matter, DOM）的形式向淡水生态系统流失的碳量持续攀升，若此类有机质发生降解，将进一步抵消土壤的碳汇能力。DOM在其生成源附近更易发生降解，且随着降解过程的推进，其组成会愈发被同类（即普遍性）难降解化合物所主导。然而，目前学界对不同环境中DOM降解的潜在过程仍缺乏深入认知。本研究发现，无论区域环境条件存在何种差异，DOM沿土壤-水生梯度的变化模式均保持一致。我们依托超高分辨率质谱（ultrahigh-resolution mass spectrometry）技术，在森林源头集水区中沿土壤深度与坡位梯度追踪DOM的动态变化，并将其分子组成与土壤微生物组（soil microbiomes）及土壤理化条件进行关联分析。沿土壤深度与坡位梯度，类碳水化合物与类不饱和烃化合物的丰度加权质量占比均显著提升，这一现象暗示植物源有机质经历了微生物的改造与再加工。上述两类化合物的丰度变异中，超过一半与降解植物源碳水化合物所必需的功能基因表达水平显著相关。本研究结果表明，持续的微生物改造作用是推动土壤中DOM向普遍性难降解化合物转化的核心过程。通过整合陆-海连续体（land-to-ocean continuum）的多源观测数据，我们认为此类降解过程可在不同生态系统与时空尺度上具备普适性。此类普适性降解模式可用于预测环境梯度下DOM的分子组成及其下游反应性，从而为土壤-溪流碳流失的管控提供科学支撑。