Short-term changes in plant dissolved organic carbon inputs are fundamental inmediating soil organic carbon dynamics

Climate change and forest management are expected to alter plant carbon allocation and dissolved organic carbon (DOC) inputs to soils, with potentially strong but poorly constrained consequences for soil organic carbon (SOC) dynamics. While DOC inputs can stimulate microbial decomposition through priming effects, they may also contribute to SOC stabilization via microbial processing and mineral association. However, the relative roles of aboveground and belowground DOC inputs, their chemistry, and interactions with soil biotic communities across forest types and soil depths remain insufficiently understood.

Here, we investigated the effects of plant-derived DOC inputs on SOC dynamics using a field manipulation experiment that disentangled aboveground (leaf leachates) and belowground (root exudates) DOC inputs in broadleaf (European beech) and coniferous (Norway spruce) forest stands. We quantified short-term responses of SOC fractions—free particulate organic matter (fPOM), occluded particulate organic matter (oPOM), and mineral-associated organic matter (MAOM)—in mineral topsoil (0–10 cm) and subsoil (50–60 cm), and related these responses to DOC input chemistry as well as microbial and faunal properties.

Despite the relatively short experimental duration (two years), DOC inputs exerted pronounced effects on SOC fraction dynamics, with responses strongly dependent on organic matter fraction and soil depth. Both leaf leachates and root exudates induced SOC formation as well as loss, depending on the fraction considered. Regarding soil depth, SOC fractions in the subsoil were generally less responsive to DOC inputs than those in the topsoil, indicating greater short-term vulnerability of topsoil SOC to DOC-induced losses. In contrast, forest type had only minor influence on DOC-driven SOC dynamics, suggesting that DOC input chemistry and point of entry outweigh tree species identity as short-term controls of SOC dynamics.

Our results demonstrate that short-term SOC responses to DOC inputs are governed primarily by DOC input chemistry, organic matter fraction stability, and soil depth rather than forest type. Explicit consideration of above- and belowground DOC pathways and microbial mediators is therefore essential for predicting forest SOC responses to environmental change.