Decomposition of mangrove litter under experimental nutrient loading in a fringe Rhizophora mangle (L.) forest

Abstract

Carbon (C) cycling is an important attribute of mangrove forests that relates to the structure, function, and resilience of mangroves under environmental change. Increased nutrient enrichment in tropical coastal waters may influence C cycling through organic C mineralization. For example, by alleviating nutrient limitation of the heterotrophic microbial community, nutrient enrichment may enhance C mineralization and facilitate a loss of within-stand C sequestration. Here, we enriched a coastal fringe Rhizophora mangle (L.) mangrove system for two years with two fertilizer regimes to mimic agriculture runoff (”+high” N:P ratio of 50:1) and urban runoff (”+moderate” N:P ratio of 16:1) scenarios as follows: (1) annual loading rate of 70 g N m −2 year −1 and 3.1 g P m −2 year −1 or (2) annual loading rate of 70 g N m −2 year −1 and 9.7 g P m −2 year −1. C mineralization was measured as microbial respiration rates from the forest floor and litter decomposition rates. While decomposing leaf litter and green leaves had lower molar C:N under the +moderate N:P fertilization course, neither fertilization scenario produced an effect on C mineralization processes compared with ambient conditions. Substrate CO 2 flux rates were not different among treatments and ranged from 1.15 to 1.81 μmol CO 2 m −2 s −1 (3.0–4.8 g CO 2 m −2 day −1) following 72 weeks of fertilization and 0.58–1.55 μmol CO 2 m −2 s −1 (1.5–4.1 g CO 2 m −2 day −1) 30 weeks following the end of the experiment. Time to 50% decay of above-ground leaf litter ranged from 61 to 110 days (average 79 days). Below-ground leaf litter material was fully decomposed by 22 months after burial. A15 N pulse-recovery suggests that the majority of the retained fertilizer (22.2 ± 4.4% at 10 months following spike) was taken up by fine roots, though this did not significantly affect CO 2 flux from the forest floor. This work demonstrates that nutrient enrichment by aqueous delivery does not strongly affect organic carbon mineralization in a coastal fringe mangrove within two years. Environmental conditions, substrate quality, and location may play a more substantial role in mangrove C dynamics compared with short-term aqueous-based nutrient enrichment.