<b>Depth-Dependence of Soil Stoichiometry Decoupling in China’s Croplands over the Last Four Decades</b>

Soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry is an integrative metric for the balance and restrictive relationships of soil elements. However, how climate change and nutrient inputs affect cropland C:N:P ratios along soil profiles at large scales remains largely unknown. Here, we assess variations in soil C:N:P ratios across a 0-100 cm soil profile in China’s croplands by conducting repeated soil C, N, and P measurements at 305 resampling sites in 1980s and 2023, totaling 6,405 soil profiles. Over the past four decades, the mean soil C:N ratio across the entire profile increases by 20.18%, the C:P ratio increases by 4.49%, while the N:P ratio decreases by 9.02%. Spatially, changes in C:N and N:P ratios tend to weaken with increasing latitude. These changes exhibit clear depth-dependent patterns. The soil C:N ratios increase across the entire soil profile, while the C:P and N:P ratios increase in the topsoil (0-40 cm) but decrease in the subsoil (40-100 cm), primarily driven by organic C and nutrient inputs, influenced by initial soil conditions and climate. Our results highlight the depth-dependent decoupling of soil stoichiometry in response to long-term management practices and its implications for ecosystem services.土壤碳（C）、氮（N）与磷（P）化学计量比（soil C:N:P stoichiometry）是反映土壤元素平衡与限制关系的综合指标。目前，气候变化与养分输入如何影响大尺度农田土壤剖面的C:N:P化学计量比，仍尚未厘清。 本研究通过对1980年代与2023年的305个重采样样点（resampling site）开展重复土壤C、N、P含量测定，共获取6405个0-100 cm农田土壤剖面，以此解析中国农田0-100 cm土层的C:N:P化学计量比变化特征。 过去四十年来，全剖面土壤的平均C:N比提升20.18%，C:P比提升4.49%，而N:P比下降9.02%。空间维度上，C:N与N:P比的变化幅度随纬度升高而逐渐减弱。上述变化呈现显著的深度依赖模式：土壤C:N比在全剖面均呈上升趋势，而C:P与N:P比在0-40 cm表层土壤呈上升趋势，在40-100 cm亚表层土壤则呈下降趋势。该变化主要由有机碳与养分输入主导，并受初始土壤条件与气候的调控。 本研究结果凸显了土壤化学计量比响应长期管理措施的深度解耦特征，及其对生态系统服务的潜在影响。