After-Effects of Hydrochar Amendment on Water Spinach Production, N Leaching, and N(2)O Emission from a Vegetable Soil under Varying N-Inputs

Biochar use in agriculture brings significant agronomic and environmental co-benefits, which are a function of biochar and crop types and nitrogen (N) rates. We here conducted a soil column experiment to evaluate the after-effects of hydrochar amendment at 0.5 and 2.0 wt% on vegetable production, N recovery and losses via leaching and nitrous oxide (N(2)O) emission from water-spinach (Ipomoea aquatica Forsk)-planted vegetable soil receiving three N inputs (120, 160, and 200 kg/ha). The results showed that hydrochar with 2.0 wt% significantly (p < 0.05) improved the biomass yield of water spinach, receiving 120–160 kg N/ha by 11.6–14.2%, compared with no change in the hydrochar treatment. Hydrochar had no effect on total N content of water spinach, and only increased the total N recovery under 2.0 wt% given hydrochar amended treatment with 120 kg N/ha. Neither pH or EC of leachate was changed with N reduction or hydrochar application. However, in some cases, hydrochar changes the NH(4)(+), NO(3)(−) and total N concentrations in leachate. When applied at 2.0 wt%, hydrochar significantly (p < 0.05) increased total N leaching losses by 28.9% and 57.1%, under 120 and 160 kg N/ha plot, respectively. Hydrochar applied at two rates increased the N(2)O emissions by 109–133% under 200 kg N/ha but decreased them by 46–67% under 160 kg N/ha. Therefore, after three years of application, hydrochar still improves the production of leafy vegetable, but the impacts on N leaching and N(2)O emission vary, depending on inorganic N and hydrochar application rates.