Short-lived effects of nutrient enrichment on Avicennia germinans decomposition in a saltmarsh-mangrove ecotone

Abstract

Rates of litter decomposition, an important component of the global carbon (C) cycle, often depend on litter quality and decomposer activity, which can then drive differences in C sequestration and storage in ecosystems. Nutrient enrichment may alter C storage in wetlands through its effects on litter quality. We investigated the effects of long-term nitrogen (N) and phosphorus (P) enrichment on Avicennia germinans litter quality and above- and belowground decomposition rates in a saltmarsh-mangrove ecotone in St. Augustine, Florida, U.S.A. Senescent leaf litter collected from Control, +N-, and +P-fertilized shrubs was placed in litterbags for use in a reciprocal transplant experiment, with each litter treatment placed on the soil adjacent to each fertilized shrub (Control, +N, and +P). Leaf litterbags were collected at 0, 14, 30, 60 and 180 days, while roots collected from a common, unfertilized location were incubated belowground at each shrub for 180 and 365 days to determine mass loss and decay rates. Nutrient enrichment did not alter %N, C:N, or %lignin in treatment leaves, and there were subsequently no significant differences in decay rates of treatment leaves, likely due to the homogeneity of litter quality. Initial decay rates (30 and 60 days) were significantly higher in +N and +P fertilized soils compared to controls, but by 180 days decay rates had stabilized to 0.009 ± 0.0003 k day−1. Labile fractions were rapidly decomposed and net immobilization of litter was favored after 60 days of incubation irrespective of nutrient treatments. Additionally, belowground root decomposition with roots from unfertilized plants did not vary across nutrient treatments. Thus, nutrient enrichment of A. germinans did not alter litter quality considerably or increase decomposition in the longer term, and therefore, we do not expect a substantial decline in sediment C sequestration as a result of eutrophication in this mangrove system.