Regeneration of nitrogen (15N) from seagrass litter in tropical Indo-Pacific meadows

The input of nitrogen (N) into litter of the seagrasses Thalassia hemprichii, Halodule uninervis and Cymodocea rotundata during decomposition and the uptake of N released from this litter by the surrounding seagrasses were examined Simultaneously using N-15-enriched seagrass leaf material (delta N-15 approximate to 500%) placed in litterbags in seagrass meadows. The biomass (dry weight) decomposition rates (k) for these tropical seagrasses were high (0.023 to 0.070 d(-1)). Litter N concentration declined during decomposition, indicating an overall release of N during decomposition, Besides release of litter N there was an input of non-litter N, indicated by a decline of delta N-15 values. This exogenous N made tip an important part of litter N, consisting of ca. 25%, of the total standing stock litter N. The increased 515 N values of the surrounding seagrasses were used to calculate the uptake of N released from the macrophyte litter. The seagrasses efficiently took up the released N. The canopy thickness (aboveground biomass) of the seagrass meadow played a role in the uptake efficiency of N released from litter, increasing the N retention in these tropical meadows. The high litter decomposition rates, substantial input of exogenous N onto litter and efficient uptake of N released from litter suggest that N cycling, outside the living plant but within the meadow via the detrital pathway, is important to retain N in these nutrient-poor tropical meadows, despite the open and highly dynamic shallow coastal environments from which an easy loss of N, either dissolved in the water Column or associated with the export of leaf litter, would be expected.