The contrasting effects of nutrient enrichment on growth, biomass allocation and decomposition of plant tissue in coastal wetlands

Aims: Eutrophication of coastal waters can have consequences for the growth, function and soil processes of coastal wetlands. Our aims were to assess how nutrient enrichment affects growth, biomass allocation and decomposition of plant tissues of a common and widespread mangrove, Avicennia marina, and how eutrophication drives changes in below-ground carbon sequestration. Methods: We assessed this through the measurement of above- and belowground growth and decomposition rates of plants and plant tissue in unenriched or nutrient enriched treatments. Results: Nutrient enrichment increased biomass allocation above-ground compared to below-ground in seedlings but not in fully developed, mature trees where we observed the opposite pattern. Experiments to assess root decomposition found that 40–50% of biomass was lost within six months with little change between 12 and 18\ua0months, indicating a high potential for accumulation of organic matter over time. We estimate root-derived carbon sequestration rates of 53, 250 and 94\ua0g C m\ua0year for unenriched control, N and P enriched treatments, respectively. Conclusions: These results show coastal eutrophication can be beneficial and detrimental to ecosystem function of coastal plants. Eutrophication stimulates root growth in fully developed trees, increasing organic matter input to soils. Our data suggests that organic matter accumulation will increase in areas with high nutrient availability where root growth is increased and rates of decomposition are low