Saltwater intrusion into tidal freshwater marshes alters the biogeochemical processing of organic carbon

[摘要] Environmental perturbations in wetlands affect the integratedplant-microbial-soil system, causing biogeochemical responses that canmanifest at local to global scales. The objective of this study was todetermine how saltwater intrusion affects carbon mineralization andgreenhouse gas production in coastal wetlands. Working with tidal freshwatermarsh soils that had experienced ~ 3.5 yr of in situsaltwater additions, we quantified changes in soil properties, measuredextracellular enzyme activity associated with organic matter breakdown, anddetermined potential rates of anaerobic carbon dioxide (CO₂) andmethane (CH₄) production. Soils from the field plots treated withbrackish water had lower carbon content and higher C : N ratios than soilsfrom freshwater plots, indicating that saltwater intrusion reduced carbonavailability and increased organic matter recalcitrance. This was reflectedin reduced activities of enzymes associated with the hydrolysis of celluloseand the oxidation of lignin, leading to reduced rates of soil CO₂ andCH₄ production. The effects of long-term saltwater additions contrastedwith the effects of short-term exposure to brackish water during three-daylaboratory incubations, which increased rates of CO₂ production butlowered rates of CH₄ production. Collectively, our data suggest thatthe long-term effect of saltwater intrusion on soil CO₂ production isindirect, mediated through the effects of elevated salinity on the quantityand quality of autochthonous organic matter inputs to the soil. In contrast,salinity, organic matter content, and enzyme activities directly influenceCH₄ production. Our analyses demonstrate that saltwater intrusion intotidal freshwater marshes affects the entire process of carbonmineralization, from the availability of organic carbon through its terminalmetabolism to CO₂ and/or CH₄, and illustrate that long-term shiftsin biogeochemical functioning are not necessarily consistent with short-termdisturbance-type responses.

[学科分类] 地球化学与岩石