Saltwater intrusion enhances phosphorus availability

Globally sea-level rise is expected to intensify saltwater intrusion into tidal freshwater marshes, ye the time-dependent effects of salinity on phosphorus (P) biogeochemistry remains inadequately understood. Herein, we conducted a field transplant experiment simulating saltwater intrusion to examine how salinity influences soil P availability, organic P mineralization, and phoD‐harbouring bacterial community in subtropical tidal marshes. Our results showed that simulated saltwater intrusion significantly elevated soil pH, electrical conductivity, porewater salinity, and concentrations of major ions (SO42- and Cl-), effectively altering the abiotic conditions governing P dynamics. Soil P availability increased significantly, with resin-P and NaHCO3-Pi concentrations rising, while less available P fractions (NaOH-Pi and NaOH-Po) declined. Saltwater intrusion inhibited soil organic P mineralization, especially during the later stage of transplant incubation. Notably, the abundance and diversity of phoD-harboring bacterial communities increased in transplanted soils during the early incubation period, indicating a functional compensatory response to salt stress. Over time, these communities shifted toward brackish-like compositions, driven by salinity acting as a deterministic environmental filter. Correlation analysis indicated that saltwater intrusion–induced ionic strength, the abundance and diversity of phoD-harboring bacteria, redistribution and key microbial taxa jointly regulated P fraction and availability in tidal marshes. Overall, this study reveals a strong “two-phase” conceptual framework: an initial shock of enhanced P availability driven by abiotic mobilization and microbial functional shifts, followed by a longer-term transition toward a brackish-like state in both microbial communities and P fractions. These findings deepen our understanding of P cycling in coastal marshes under future seawater intrusion scenarios, highlighting the critical roles of P fraction redistribution and time-dependent responses in regulating P availability.全球海平面上升预计会加剧盐水入侵潮汐淡水沼泽的程度，但盐度对磷（P）生物地球化学的时变效应仍未得到充分阐明。本研究开展了模拟盐水入侵的野外移植实验，以探究盐度如何影响亚热带潮汐沼泽的土壤磷有效性、有机磷矿化以及携带phoD基因的细菌群落（phoD-harbouring bacterial community）。结果表明，模拟盐水入侵显著提升了土壤pH、电导率、孔隙水盐度以及主要离子（硫酸根SO42-和氯离子Cl-）的浓度，有效改变了调控磷动态的非生物环境条件。土壤磷有效性显著提升：树脂结合态磷（resin-P）与碳酸氢钠提取态无机磷（NaHCO3-Pi）浓度升高，而有效性较低的磷组分——氢氧化钠提取态无机磷（NaOH-Pi）与氢氧化钠提取态有机磷（NaOH-Po）则出现下降。盐水入侵抑制了土壤有机磷矿化，尤其在移植培养的后期阶段。值得注意的是，在培养初期，移植土壤中携带phoD基因的细菌群落的丰度与多样性均有所提升，这表明其对盐胁迫存在功能性补偿响应。随着培养时间推移，在盐度这一确定性环境过滤因子的驱动下，这些细菌群落逐渐向半咸水型群落组成转变。相关性分析显示，盐水入侵引发的离子强度变化、携带phoD基因的细菌的丰度与多样性、磷组分的再分配以及关键微生物类群，共同调控了潮汐沼泽中的磷组分与有效性。总体而言，本研究揭示了一套清晰的“两阶段”概念框架：首先是由非生物活化与微生物功能转变驱动的磷有效性提升的初始冲击阶段，随后是微生物群落与磷组分均向半咸水状态过渡的长期转变阶段。本研究结果加深了我们对未来海水入侵情景下海岸沼泽磷循环的认识，强调了磷组分再分配与时变响应在调控磷有效性过程中的关键作用。