What happens to soil chemical properties after mangrove plants colonize?

Understanding soil chemical properties is necessary to characterize the basic properties of ecosystems. In mangrove ecosystems, soil iron, phosphorus, methane and nitrogen have been well studied under field conditions. However, it is difficult to understand fundamental relationships between mangrove root functions and soil chemical properties, because of the multiple factors present in field data. The aim of this study was to clarify what will happen to soil chemical properties after mangrove plant colonize. To examine the effect of mangrove roots on these soil properties, three representative mangrove species (Avicennia marina, Rhizophora stylosa and Bruguiera gymnorrhiza) were cultivated in a greenhouse and selected soil chemical properties were monitored in comparison with those in unplanted soil. We detected oxidative effects in all three species, including deposition of iron oxide on root surfaces, lowered methane concentrations and increased oxidized inorganic nitrogen concentrations in the soil pore-water, suggesting that radial oxygen loss from mangrove roots had affected these soil chemical properties. Besides the oxidative effects, enhanced Fe2+ concentrations in the soil pore-water were present in A. marina, and enhanced phosphorus concentrations in the soil pore-water were present in all three species, suggesting that mangrove roots provide Fe- and phosphate-solubilizing substrates. The most remarkable change was in soil nitrogen enrichment. During the experimental period, amounts of nitrogen in the mangrove soils increased four times more than in uncolonized soil. Six months from the start of cultivation, bacterial nitrogen fixation (nitrogenase activity) was significantly higher in soil colonized by mangrove plants than in uncolonized soil, suggesting that mangrove roots stimulated bacterial nitrogen fixation. Among these properties, Phosphate mobilization and soil nitrogen enrichment are likely to be particularly important for the growth of mangrove plants, because phosphate and nitrogen are generally limited in mangrove ecosystems. This self-supporting ability of mangroves observed in this study could be one key to the high productivity of mangrove ecosystems.