Effect of mineral loaded biochar on the leaching performances of nitrate and phosphate in two contrasting soils from the coastal estuary area.

Coastal estuary area is an important sink for the land-based or/and atmosphere-based nutrients, and is suffering a serious destruction derived from the intensifying human activities, which subsequently threatens the marine environment. Therefore, increasing soil retention capacities of nitrogen (N) and phosphorous (P) and reducing their leaching amount to sea water become a critical issue needed to be urgently addressed. In this study, a 38-day incubation and leaching experiment was conducted with two contrasting soils taken from the coastal estuary area, including the wetland and agricultural soils. Four kinds of biochars (BC), including one pure reed straw BC (BC), and three mineral loaded BCs produced through the co-pyrolysis of reed straw with CaO (BC), MgO (BC), and shell powder (BC), respectively, were used to explore their effects on the leaching performances of nitrate-N and phosphate-P. The results demonstrated that the application of mineral loaded BCs could generally decrease the leaching amount of phosphate-P, while showed little effect on the nitrate-N leaching, compared to the controls. The positive improvement in soil nutrient retention capacity, mostly contributed by the increased adsorption on BC surface and into aperture, was suggested as the main mechanism for the decrease in nitrate-N and phosphate-P leaching. Compared to the agricultural soil, high clay content in the wetland soil could weaken the reduction potential in leaching losses of nitrate-N and phosphate-P derived from the newly introduced minerals with BC application. Furthermore, our results also indicated that the mineral loaded BCs may slow down the conversion rate of nutrients from organic forms to inorganic forms supported by the decreased enzymatic activity, which would be beneficial to the long term retention of nutrients in soil. Overall, based on the findings in the present study, the BC and Ca loaded BCs were respectively recommended for the wetland and agricultural soils.