Coastal wetland conversion to aquaculture pond reduced soil P availability by altering P fractions, phosphatase activity, and associated microbial properties

Reclamation and conversion of wetlands strongly affect nutrient cycling and ecosystem functions, while little attention has been paid to the effects of converting coastal wetland to aquaculture on the cycling and balance of soil phosphorus (P). Herein, we investigated soil P fractions, alkaline phosphatase (ALP) activity, and associated microbial properties following coastal wetland conversion in subtropical China. Soil P availability (especially resin-P) concentration and ALP activity in wetland were significantly higher than those in pond. The conversion of coastal wetlands to aquaculture significantly reduced the abundance and diversity of bacterial phoD genes and altered their community structure. The lower phosphatase activity and associated microbial properties after wetland conversion suggested a weaker capacity of microbes to transform organic P (Po) to inorganic P (Pi), consistent with the low P availability but the high Po:Pi ratio in pond. Structural equation modeling indicated that the conversion of the wetland to the pond decreased ALP activity and P availability by affecting soil variables such as bulk density, pH, the carbon: nitrogen ratio, and/or moisture. It was concluded that wetland conversion to pond reduced soil P availability and phosphatase activity, altered the abundance, diversity and community composition of the phoD gene, and ultimately affected coastal P cycles and balances. Moreover, an extended corollary is that the smaller amounts of variation in soil total P and lower labile P concentrations in pond than in wetland suggest that large amounts of P (introduced in feed and not harvested in shrimp) are "lost" from the system. Thus, aquaculture ponds might serve as a source of P for the surrounding environment. More investigations focusing on the P biogeochemical cycle and its potential impacts on adjacent ocean environments at regional and global scales is urgently needed, which could contribute to better management of coastal land uses.