<b>High litter quality enhances plant energy channelling by soil macro-detritivores and lowers their trophic position</b>

Detritus-based resources, i.e., plant litter, are a major energy source for many living organisms and considered to be a key determinant of primary production and nutrient cycling. Earthworms are among the most important macro-detritivores in terrestrial food webs and play a crucial role in facilitating these processes in terrestrial ecosystems. Yet, the influence of litter quality on earthworm nutrition, and consequently on soil food web dynamics, has remained largely underexplored, mainly for methodological reasons. Here, we combined bulk and novel compound-specific stable isotope analysis of amino acids to investigate the dietary contribution of litter resources of differing quality to earthworm species of different ecological groups. Our findings show that earthworms acquired most essential amino acids from bacterial (~60%) and plant (~30%) resources, with the latter increasing in importance with higher litter quality, resulting in lower trophic positions across earthworm species. The high bacterial contribution to earthworms corresponds to the dominance of bacteria in the experimental soil, suggesting that bacteria served as important intermediate link in transferring detritus-based resources to earthworms. Bacterial contributions were notably higher in the soil-feeding earthworm species than in the litter-feeding earthworm species, likely due to more pronounced ingestion of soil by soil-feeding earthworms. Overall, our study indicates that a major group of soil macro-detritivores, earthworms, receive detrital resources via bacteria. Further, it underscores the important role of litter quality in shaping the trophic niches of detritivores, thereby influencing the overall structure of soil food webs.以碎屑为基础的资源（即植物凋落物）是众多生物的核心能量来源，同时被视为初级生产与养分循环的关键决定因素。蚯蚓是陆地食物网中最重要的大型碎屑食性动物之一，在陆地生态系统的上述过程调控中发挥关键作用。然而，凋落物质量对蚯蚓营养的影响，进而对土壤食物网动态的影响，在很大程度上仍未得到充分探索，这主要源于方法学层面的限制。本研究结合总样品分析（bulk analysis）与新型氨基酸化合物特异性稳定同位素分析技术，探究不同质量等级的凋落物资源对不同生态类群蚯蚓物种的膳食贡献份额。研究结果显示，蚯蚓获取的大部分必需氨基酸来自细菌（约60%）与植物（约30%）资源，且植物资源的贡献占比随凋落物质量提升而升高，进而导致各蚯蚓物种的营养级位整体偏低。蚯蚓膳食中较高的细菌资源占比，与实验土壤中细菌的优势地位相契合，表明细菌是介导碎屑基资源向蚯蚓转移的重要中间环节。取食土壤的蚯蚓物种，其膳食中的细菌贡献占比显著高于取食凋落物的蚯蚓物种，这可能源于取食土壤的蚯蚓会更大量地摄入土壤。综上，本研究表明，作为大型土壤碎屑食性动物的核心类群，蚯蚓通过细菌介导获取碎屑类资源。此外，本研究还凸显了凋落物质量在塑造碎屑食性动物营养生态位中的关键作用，进而影响土壤食物网的整体结构。