Coupled Effects of Reduced Chemical Fertilization and Biochar Supplementation on Availability and Transformations of Nitrogen and Phosphorus in Vegetable Farmland Soil: An In Situ Study in Southern China

Reduced fertilization technology is an eco-friendly strategy to minimize nitrogen (N) and phosphorus (P) surpluses and losses in vegetable production. However, little is known about the performance of chemical fertilizer reduction when supplemented with palm silk biochar (PSB) in subtropical acid soils. A short-term (60 d) field investigation under conditions of in situ incubation was conducted in vegetable farmland in southern China. The treatments included no fertilization (Control), 100% conventional fertilization (CF 100 ), 90% conventional fertilization plus 10% PSB-based fertilization (CF 90 B 10 ), 85% conventional fertilization plus 15% PSB-based fertilization (CF 85 B 15 ), and 80% conventional fertilization plus 20% PSB-based fertilization (CF 80 B 20 ). The CF 90 B 10 , CF 85 B 15 , and CF 80 B 20 treatments had the same inputs of total N and P as the CF 100 treatment. Reduced chemical fertilization generally decreased the soil NH 4 + -N regardless of the PSB substitution rate (10%, 15%, or 20%), incubation condition (top-covered or top-open: preventing or allowing the leaching process, respectively), and sampling time (1 day or 60 days). Conversely, compared with the CF 100 treatment, both the CF 85 B 15 and CF 80 B 20 treatments did not lead to a significant decrease in the NO 3 − -N concentration in soil under top-open incubation conditions, but significantly ( p < 0.05) increased soil NO 3 − -N under top-covered incubation conditions. The CF 80 B 20 treatment significantly ( p < 0.05) decreased soil Olsen-P in comparison with the CF 100 treatment, regardless of the incubation condition and sampling time. After applying chemical fertilizer in combination with PSB, soil net ammonification and N mineralization tended to be reduced considerably, with substantial reductions of 39–76% and 24–45%, respectively; reversely, soil net nitrification was stimulated by an increased PSB substitution rate. As the rate of chemical fertilization decreased, the trends in NH 4 + -N and NO 3 − -N losses from the soil were similar to the trends observed in soil net ammonification and net nitrification, respectively. Additionally, there were no significant differences in the soil net P mineralization and Olsen-P loss between chemical fertilization alone and in combination with PSB application. Generally, the partial substitution of chemical fertilizer with PSB at a low application rate may not substantially reduce plant-available NO 3 − -N and Olsen-P. It can also contribute to the sustainable availability of N and P in vegetable farmland soil via a variety of transformation processes, such as mineralization, immobilization, and loss.