Unveiling the role of saltmarshes as coastal potassium sinks: A perspective from porewater-derived potassium exchange

Saltmarshes serve as repositories for various metal species, primarily due to vegetation removal and mineralization processes. However, the significance of potassium (K), one of the three major nutrients (nitrogen, phosphorus, and K) essential for plant growth, has often been overlooked, particularly in the context of saltmarshes where the mechanisms of K transport via porewater exchange remain poorly understood. To address this knowledge gap, we conducted field observations and laboratory analysis, and developed a ²²²Rn mass balance model to quantify K fluxes via porewater exchange under physical, biological, and anthropogenic drivers. Our findings revealed that saltmarshes function as highly effective K sinks, with porewater exchange rates ranging from 12.2 to 44.5 cm d^-1 and related K fluxes spanning -122 to -1260 mmol m^-2 d^-1. Interestingly, wet season K fluxes were found to be ∼4.3 times higher than those observed during the dry season. Moreover, we observed that wet season K fluxes peaked during the neap tide, while dry season K fluxes reached their maximum during the spring tide. This suggests that K transport via porewater exchange is influenced by both crab burrow bioturbation and spring-neap exchange mechanisms. In addition, anthropogenic activities, such as biomass burning, also impact K dynamics in addition to physical and biological drivers. Overall, our study highlights the pivotal role of porewater exchange in driving the K cycle within saltmarshes. This exchange mechanism not only facilitates plant growth but also contributes to important mineralogical processes, including cation exchange and reverse weathering, occurring within the saltmarsh ecosystems. By shedding light on the K cycle in saltmarshes, our research contributes to a better understanding of the functioning of coastal wetlands and their implications for the oceanic K budget.