Below-ground nitrogen cycling in relation to net canopy production in mangrove forests of southern Thailand

Rates of accumulation, transformation and availability of sediment nitrogen in four mangrove forests of different age and type in southern Thailand were examined in relation to forest net canopy production. Net ammonification (range: 0.3-2.3 mmol N m^-2 day^-1), nitrification (range: 0-0.7 mmol N m^-2 day^-1) and nitrogen fixation (range: 0-0.6 mmol N m^-2 day^-1) in surface sediments equated to <10% of canopy nitrogen demand (range: 7.5-32 mmol N m^-2 day^-1). By mass balance, we estimated that most of the nitrogen required for tree growth must be derived from root-associated nitrogen fixation and/or mineralisation processes occurring possibly to the maximum depth of live root penetration (75-100 cm). Denitrification, nitrification, rainfall and tidal exchange were comparatively small components of sediment nitrogen flow. Denitrification (range: 0-3.8 mmol N m^-2 day^-1) removed 3-6% of total nitrogen input at three Rhizophora forests, but removed 23% of total nitrogen input in a high-intertidal Ceriops forest. Nitrogen burial ranged from 4% to 12% of total nitrogen input, with the greatest burial rates in two forests receiving the least tidal inundation. Inputs of nitrogen to the forests were rapid (range: 11-37 mmol N m^-2 day^-1), likely originating from upstream sources such as agricultural and industrial lands, sewage and shrimp ponds. Our results indicate that ~70% to 90% of the nitrogen supplied to the forest floor is shunted via the ammonium pool to trees to sustain the rapid rates of net canopy production measured in these forests. Differences in plant-sediment nitrogen relations between the forests appeared to be a function of the interaction between intertidal position and stand age.