Effects of tide and season changes on the iron-sulfur-phosphorus biogeochemistry in sediment porewater of a mangrove coast

To better understand the biogeochemical processes of iron, sulfur, and phosphorus and their coupled relationships, we used the in situ high-resolution diffusive gradients in thin films (DGT) technique to characterize labile Fe, S, and P in the sediment porewater of a mangrove coast in the Jiulong River Estuary, China. The distributions of DGT-labile Fe and S indicated that microbial sulfate reduction (MSR) and microbial iron reduction (MIR) exhibited an intense competitive relationship and temperature-dependent behavior. Thus, in addition to the expected MSR, MIR is an important or even predominant biogeochemical process for organic matter (OM) mineralization in this Fe-rich mangrove coast. Labile Fe, S, and P distributions and variations demonstrate that Fe redox cycling plays a crucial role in regulating P behavior and P cycling. An additional new finding is that the flux of labile S in the intertidal zone increases during flood tides and decreases during ebb tides, indicating that the formation and reoxidation of sulfide in surface layers are regulated by tide-induced redox changes. The net P flux associated with recirculated seawater was estimated to range from -0.306 +/- 0.001 to 0.343 +/- 0.013 mmol m(-2) d(-1) in the winter survey, suggesting that sediment porewater tends to be a P source in the subtidal zone and a P sink in the intertidal zone for the overlying seawater in winter. The continuous accumulation of labile P in surface sediment could be intensively released into the overlying seawater through prevailing MIR process in the subsequent riverine flood season, which may promote eutrophication of the adjacent Xiamen Bay.