Direct and indirect pathways of land management effects on wetland plant litter decomposition

Litter decomposition is a fundamental process underpinning multiple ecosystem services. Despite a long history of research on decomposition, direct and indirect effects of multiple interactive land management on wetland decomposition yet remain less well understood. Here, we used a long-term whole-ecosystem wetland experiment in south-central Florida to investigate interactive effects of land-use intensification, cattle grazing and prescribed fire on in situ wetland plant litter decomposition. We further examined the direct and indirect pathways of land management effects on litter decomposition through changes in associated litter traits, soil properties, and soil microbial attributes using structural equation models. We used the litterbag technique that quantifies decomposition rates (k-values) and recalcitrant fractions (A-values). Our results showed that land-use intensification increased k-values in ungrazed wetlands and decreased k-values in grazed wetlands, but consistently reduced A-values regardless of other treatments. Prescribed fire individually suppressed litter decomposition by reducing k and increasing A. Further, these effects occurred through altering litter, soil, and microbial properties. Our results revealed that litter traits and soil properties were the first two strongest factors in determining wetland decomposition processes. Particularly, litter P and Mg contents and soil P and K contents were the best predictors for k, while litter Ca and lignin contents and soil pH, N and water content best predicted A. Moreover, microbial traits exhibited interactive effects with litter and soil properties to affect wetland litter decomposition. Our research suggests that cattle grazing could buffer against stimulating effect of land-use intensification on decomposition rates and thus avoid nutrient releases pulses. Our study further indicates that land-use intensification and fire suppression in subtropical wetlands could promote organic matter depletion and thus nutrient loss, highlighting the need to reduce anthropogenic disturbances to natural wetlands to maintain their capacity for providing associated regulating and supporting services.