A large imbalance in soil nitrogen (N) and phosphorus (P) inputs induced by anthropogenic activities is anticipated to profoundly influence soil carbon (C) budgets in salt marshes. In this study, we hypothesized that imbalances in the nitrogen–phosphorus (N–P) input would result in the nonlinear response of soil organic carbon (SOC) content, fractions and mineralization to the N–P input ratio. We applied three N–P input ratios (low (5:1), medium (15:1), high (45:1)) in a salt marsh of the Yellow River Delta (YRD) for four years (in which N added increased from 8.67 to 26.01 g N m⁻² y⁻¹ and P added decreased from 1.73 to 0.58 g P m⁻² y⁻¹) and quantified their impacts on SOC fractions and SOC mineralisation. The control treatment did not receive fertilization. The results showed that the N and P input led to overall increases in the availability of soil nutrients (i.e., inorganic N (IN) and available P (AP)), stimulation of plant biomass and changes of microbial community structure (i.e., γ- and δ-Proteobacteria and Acidobacteria). N and P input increased soil dissolved organic carbon (DOC) and decreased aromatic DOC components through improving N availability and stimulating plant growth. Notably, though, there may be a threshold N–P input ratio between 15:1 and 45:1 that, once crossed, triggers the loss of SOC. Appropriate increase in N availability induced by low and medium N-P input ratios would stimulate the SOC mineralization. However, excessive N-P input ratio would reduce SOC mineralization. Path analysis indicated that N–P input ratios dominantly regulate SOC mineralisation by changing soil DOC and microbial biomass (MBC)contents and microbial community structure. Thus, we speculate that the continuous increase in N input causes a growing N–P imbalance that reduces SOC stocks, despite a reduction in SOC mineralisation.

预计人为活动引起的土壤氮 (N) 和磷 (P) 输入的严重失衡将深刻影响盐沼中的土壤碳 (C) 收支。在这项研究中，我们假设氮磷 (N-P) 输入的不平衡将导致土壤有机碳 (SOC) 含量、分数和矿化对 N-P 输入比率的非线性响应。我们在黄河三角洲 (YRD) 的盐沼中应用了三种 N-P 投入比（低 (5:1)、中 (15:1)、高 (45:1)）四年（其中 N 添加从 8.67 g N m ^-2 y ^-1增加到 26.01 g N m ^-2 y ^-1并且添加的 P 从 1.73 g P m ^-2 y ^-1减少到 0.58 g P m ^-2 y ^-1) 并量化它们对 SOC 分数和 SOC 矿化的影响。对照处理不接受施肥。结果表明，氮磷输入导致土壤养分有效性（即无机氮（IN）和有效磷（AP））的整体增加、植物生物量的刺激和微生物群落结构的变化（即γ-和δ-Proteobacteria和Acidobacteria）。N 和 P 输入通过提高 N 可用性和刺激植物生长增加了土壤溶解有机碳 (DOC) 并减少了芳香 DOC 成分。但值得注意的是，可能存在 15:1 和 45:1 之间的阈值 N-P 输入比率，一旦超过，就会触发 SOC 损失。由中低 NP 输入比率引起的 N 可用性的适当增加将刺激 SOC 矿化。然而，过多的NP输入比例会降低SOC矿化。通径分析表明，N-P 输入比通过改变土壤 DOC 和微生物生物量 (MBC) 含量以及微生物群落结构来显着调节 SOC 矿化。因此，我们推测，尽管 SOC 矿化减少，但 N 输入的持续增加会导致 N-P 不平衡加剧，从而减少 SOC 储量。