Subsurface leaf and root litter enhance soil carbon formation compared to surface litter: Direct evidence from a <scp> ¹³ C </scp> ‐labelled incubation experiment

Abstract Litter inputs are a primary source of soil carbon (C), and balancing C sequestration against litter‐induced priming effects (PE) is critical for maintaining soil C dynamics. External disturbances (e.g. grazing or global change) can alter the spatial position of litter; however, how litter position (soil surface vs. subsurface) regulates litter‐derived C formation and PE through interactions with litter chemical, morphological and microbial traits remains unclear. We tracked the fate of litter‐derived C in a laboratory microcosm experiment using 12 types of 13 C‐labelled litter from the leaves and roots of six representative temperate grassland species spanning three functional groups: grasses, legumes and forbs. Leaf litter was incubated both on the soil surface and buried in the soil (subsurface litter), while root litter was buried only. Two destructive samplings were conducted throughout the 6‐month incubation period. We found that the effects of litter quality on litter‐derived C transformation were modulated by litter position and decomposition duration. High‐quality litter (characterized by high N concentration, low C:N ratio and low lignin concentration) promoted early accumulation of litter‐derived SOC and net C gain within 30 days of decomposition. However, as decomposition progressed to 6 months, the beneficial effects of litter quality on litter C transformation diminished. Litter position governed the relationship between litter quality and both litter‐derived SOC and net C balance; when high‐quality leaf litter was buried in the soil, the positive association between litter quality and litter‐derived C dynamics disappeared. Litter leaf morphological traits associated with resource acquisition strategies (e.g. larger specific leaf area) contributed to increased litter‐derived SOC and net C gain, whereas no corresponding relationship was observed for root morphological traits. Litter‐derived SOC was primarily governed by initial chemical traits in the surface leaf and buried root treatments, while microbial traits dominated in the buried leaf treatment. Microbial traits also emerged as key regulators of PE. Synthesis . Our study demonstrates that litter position mediates quality‐dependent C dynamics. Buried litter promotes litter‐derived SOC formation while masking the positive effects of litter quality or morphological traits. We further highlight the critical role of microbial traits in transferring high‐quality leaf‐litter C to soil and regulating decomposition, thereby providing insights to enhance mechanistic understanding and inform C management in sustainable grassland ecosystems. Read the free Plain Language Summary for this article on the Journal blog.