Effect of rice husk biochar addition on nutrient leaching and microbial properties of Calcaric Cambisols

Abstract

The use of biochar to improve soil properties and decrease the leaching of nutrients has received attention, yet very little research has focused on its effects on Calcaric Cambisols. We evaluated the effect of rice (Oryza sativa L.) husk biochar at the highest pyrolysis temperatures of 450°C and 650°C (RH450 and RH650) on nutrient leaching, nutrient availability, and microbial properties of Calcaric Cambisols in the karst region of southwestern China. In a soil column leaching experiment, rice husk biochars produced under two temperatures were added at 1%, 2%, and 5% (weight/weight) to Calcaric Cambisols following application of fertilizer (ammonium nitrate [NH₄NO₃] and monopotassium phosphate [KH₂PO₄]) and incubated for 28 weeks. A control treatment (without rice husk biochar) was also included. After a 28-week incubation, both temperature rice husk biochars significantly reduced the cumulative amounts of ammonium (NH₄⁺) in leachate from Calcaric Cambisols, while they increased cumulative amounts of leachate phosphate (PO₄³⁻) and potassium (K⁺). Addition of 2% and 5% RH650 significantly reduced cumulative amounts of leachate nitrate (NO₃⁻), while RH450 amendment had no significant influence on NO₃⁻-N leaching. Addition of 5% RH450 significantly increased soil available nitrogen (N) content simultaneously accompanied by a significant increase in microbial biomass N, suggesting net N mineralization was enhanced and soil microorganisms utilized a portion of mineral N for their own cell building. Both temperature rice husk biochar additions caused a significant increase in soil available phosphorus (P) and K. The highest P availability was observed in RH450-treated soil samples at 5% rate. Compared with RH450 treatment, higher K availability was found in RH650-treated soils. Both rice husk biochar additions significantly increased soil microbial biomass carbon (C), and RH450 addition also significantly increased soil microbial biomass P and soil fluorescein diacetate activity (FDA). This is largely due to the more labile C in RH450 (high oxygen [O]/C and hydrogen [H]/C ratios). The results suggest that high-temperature rice husk biochar amendment may be an effective strategy for reducing N leaching of Calcaric Cambisols, whereas low-temperature rice husk biochar has more beneficial effect on soil microbial biomass and microbial activity.