Effect of biochar combined with ammonium sulfate on nitrogen release performance in soil

Biochar-based fertilizers can be prepared to mix or compound with the fertilizers and biochar as a carrier. The nutrients can also be provided for the crops. The function of slow-release to conventional fertilizers can also reduce the soil nutrient leaching, in order to improve the efficiency of fertilizer utilization. However, urea is normally used as the nitrogen-supplying fertilizer in most biochar-based fertilizers at present. Only a few studies have reported on biochar-based fertilizers for ammonium sulphate. This study aims to investigate the effect of biochar addition on the nitrogen slow-release performance of ammonium sulfate fertilizer. Soil nitrogen loss and soil nutrient sequestration were determined by soil column leaching simulation. Fourier infrared (FIR) spectroscopy and scanning electron microscopy (SEM) were used to observe the surface morphology and structure of biochar-ammonium sulfate fertilizer. A systematic investigation was made on the nitrogen loss of biochar-ammonium sulfate fertilizers (BC-AS_1/3, BC-AS_1/4, BC-AS_1/5) that were prepared under different charcoal-fertilizer ratios (the mass ratios of biochar to ammonium sulfate fertilizer were 1:3, 1:4 and 1:5). The results showed that: 1) Addition of biochar was effectively adsorbed the ammonium sulfate fertilizer, and then slowed down the nitrogen release rate of ammonium sulfate. According to the three charcoal-fertilizer ratios, the BC-AS_1/4 treatment shared the smoothest and tightest surface structure, while the combination of biochar and ammonium sulfate had the best molding properties. 2) Compared with the conventional ammonium sulfate treatment AS, the different biochar-ammonium sulphate treatments significantly improved the soil water and fertilizer retention performance, but reduced the soil inorganic nitrogen leaching. Among them, the cumulative ammonium and nitrate nitrogen leaching of BC-AS_1/4 treatment was reduced by 80.83% and 11.56%, respectively, compared with the AS treatment, which showed the smallest leaching rate and cumulative leaching amount, and the best slow-release effect. 3) Various treatments of biochar-ammonium sulphate significantly increased the soil nitrogen accumulation in the soil column, compared with the conventional AS treatment. Among them, the BC-AS_1/4 treatment showed that the best nutrient sequestration increased nitrogen accumulation in the soil column, compared with the AS treatment. There was a significant increase in the content of nitrate nitrogen in the soil layers of 0-10 cm and 10-20 cm compared with the AS treatment. 4) In the leaching test, the content of soil organic carbon in each soil layer was positively correlated with the soil's total nitrogen, quick-acting potassium content, and soil conductivity. There was a positive correlation with the content of soil effective phosphorus in the 0-20 cm soil layer, whereas, a negative correlation was observed with the content of soil effective phosphorus in the 20-40 cm soil layer. The input of carbon in the biochar-ammonium sulfate fertilizer effectively improved the nutrient-holding capacity of the soil. In conclusion, the biochar-ammonium sulfate fertilizer effectively slowed down the release rate of nitrogen nutrients, indicating better slow-release performance and soil nutrient-holding capacity. The nitrogen nutrient leaching was reduced to improve the fertilizer utilization for the less risk of environmental pollution. At the same time, the biochar-ammonium sulfate fertilizer can be applied to increase the content of soil organic carbon, holding nitrogen, phosphorus, and potassium nutrients, in order to cultivate the effect of fertility. Among them, the combination of biochar and ammonium sulphate performed the best, when the charcoal-fertilizer ratio was 1:4, indicating the best slow-release performance and fertilizer-building. It is of great significance to realize the efficient use of industrial by-product ammonium sulphate fertilizer.