Banana, pineapple, cassava and sugarcane residue biochars cannot mitigate ammonia volatilization from latosols in tropical farmland

Ammonia (NH3) volatilization is a major pathway of soil nitrogen loss in tropical farmland, causing many environmental issues. Biochar can improve soil quality and affect soil NH3 volatilization. However, little is known about the effects of tropical crop residue biochar on soil NH3 volatilization in tropical farmland. Therefore, a laboratory incubation study was conducted using four kinds of tropical crop residue biochar (pineapple straw (stem and leaves), banana straw, cassava straw and sugarcane bagasse pyrolyzed at 500 °C) with five addition rates (0.5%, 1%, 2%, 4%, and 6%) to evaluate their impact on NH3 volatilization from tropical latosols. The results showed that NH3 volatilization peaked twice under biochar application, once at 1–5 days and again at 12–16 days. The cumulative NH3 volatilization (0.14–0.47 mg kg−1) of the 20 biochar treatments was higher than that of the control (0.12 mg kg−1). With the increase in the biochar addition rate, the soil pH, soil organic matter (SOM), urease activity, nitrate nitrogen content (NO3−-N), nitrification rate and cumulative NH3 volatilization increased gradually, and the 6% biochar treatment resulted in the highest NH3 volatilization loss (0.19–0.47 mg kg−1). The type of biochar is also a main factor affecting soil NH3 volatilization. The cumulative NH3 volatilization was the highest under pineapple straw biochar, as it was 19–43% higher than when the other three biochars were applied. However, sugarcane bagasse biochar had the lowest cumulative NH3 volatilization due to its low quartz, sylvite and calcite contents, lack of -OH hydroxyl groups and high adsorbability. NH3 volatilization was positively correlated with the soil pH, SOM, urease activity, NO3−-N and nitrification rate. In conclusion, four tropical crop residue biochars can increase NH3 volatilization in tropical latosols, so reducing NH3 volatilization needs to be further considered in tropical crop residue biochar applications.