The effects of rice husk char on ammonium, nitrate and phosphate retention and leaching in loamy soil

• Rice husk biochar can adsorb ammonium and nitrate, but not phosphate, from solution
• Rice husk biochar reduced nitrogen leaching from loamy soil and the effect is more significant on nitrate than on ammonium
• Application of rice husk biochar increased phosphate leaching from loamy soil

Understanding the nutrient retention and release characteristics of biochar before its application to agricultural soils is crucial to maximize the benefits and minimize the damage that might be posed. In this study, batch sorption experiments were carried out to determine the ability of biochar produced from rice husk to adsorb or release three major nutrient ions: ammonium (NH4+), nitrate (NO3−), and phosphate (PO43 −). Column leaching experiments were conducted to investigate the effects of rice husk char (RHC) application on soil nutrient leaching. The results showed that RHC could adsorb ammonium 4.7 mg N g− 1 and nitrate 2.1 mg N g− 1 in solutions with an initial concentration of 200 mg N L− 1. Almost no phosphate sorption was observed using the RHC. Instead, some phosphate was released into aqueous solution at lower initial concentrations (below 60 mg P L− 1). Application of RHC into loamy soils at a rate of 4% (w/w) reduced total ammonium leaching by 11% and total nitrate leaching by 23%, but increased phosphate leaching by 72%, compared with the control soil columns. Total leached phosphate throughout experimental period was 0.16 mg P from control soil and 0.27 mg P from soil with RHC application. These findings suggest that the application of RHC can be considered a potential strategy for mitigation of nitrogen leaching in loamy soil. The RHC, however, is unsuitable for phosphorus leaching mitigation. Instead, RHC addition is a possible method to increase the availability of phosphorus in a low P index soil.