AquaMEND: Reconciling multiple impacts of salinization on soil carbon biogeochemistry

Soil salinization, exacerbated by climate change, poses a global threat to coastal ecosystems and soil function. Salinity affects soil carbon cycling by directly impacting microbial activity and indirectly altering soil physicochemical properties. Current models inadequately represent these complexities, relying on linear reduction functions that overlook specific physicochemical changes induced by salinity. AquaMEND addresses this gap by integrating microbial-explicit carbon decomposition modeling with advanced geochemical processes. Through the incorporation of equilibrium chemistry via PHREEQC, AquaMEND accurately predicts soil chemistry responses to salinization and enables detailed simulations on how salinity impacts microbial processes. To represent microbial responses to salinity, we developed salt-sensitive and slat-resistant response functions, with microbial activity inhibited by 50% at 4 ppt and 55 ppt, respectively. While the choice of salinity response functions influences model outcomes, simulations revealed that respiration responses to salinization varied depend on the underlying microbial mechanisms. Increased microbial mortality and impaired extracellular enzyme activity led to decreased respiration, while reduced carbon use efficiency could enhance respiration unless substrate uptake was also inhibited by high salinity. These microbial processes interact in a coordinated manner with multiple abiotic factors, collectively determining both the direction and magnitude of soil carbon responses. These findings highlight the need for novel experiments to disentangle the complex interactions governing microbial and geochemical responses to salinity. AquaMEND's capability to model such interactions offers a versatile tool for studying and predicting the effects of soil salinization on belowground carbon cycling.