Short- and long-term effects of nutrient enrichment on microbial exoenzyme activity in mangrove peat

Mangroves receive increasing quantities of nutrients as a result of coastal development, which could lead
to significant changes in carbon sequestration and soil subsidence. We hypothesised that mangroveproduced
tannins induce a nitrogen (N) limitation on microbial decomposition even when plant
growth is limited by phosphorus (P). As a result, increased N influx would lead to a net loss of
sequestered carbon negating the ability to compensate for sea level rise in P-limited mangroves. To
examine this, we quantified the short- and long-term effects of N and P enrichment on microbial biomass
and decomposition-related enzyme activities in a Rhizophora mangle-dominated mangrove, which had
been subjected to fertilisation treatments for a period of fifteen years. We compared microbial biomass,
elemental stoichiometry and potential enzyme activity in dwarf and fringe-type R. mangle-dominated
sites, where primary production is limited by P or N depending on the proximity to open water. Even in
P-limited mangroves, microbial activity was N-limited as indicated by stoichiometry and an increase in
enzymic activity upon N amendment. Nevertheless, microbial biomass increased upon field additions of
P, indicating that the carbon supply played even a larger role. Furthermore, we found that P amendment
suppressed phenol oxidase activity, while N amendment did not. The possible differential nutrient
limitations of microbial decomposers versus primary producers implies that the direction of the effect of
eutrophication on carbon sequestration is nutrient-specific. In addition, this study shows that phenol
oxidase activities in this system decrease through P, possibly strengthening the enzymic latch effect of
mangrove tannins. Furthermore, it is argued that the often used division between N-harvesting, P-harvesting,
and carbon-harvesting exoenzymes needs to be reconsidered