Effect of pine waste and pine biochar on nitrogen mobility in biosolids

Abstract

Humanity produces ~27 kg of dry matter in biosolids per person per year. Land application of biosolids can improve crop production and remediate soils but may result in excessive nitrate N (NO₃⁻N) leaching. Carbonaceous materials can reduce the environmental impact of biosolids application. We aimed to ascertain and compare the potentials for Monterey pine (Pinus radiata D. Don)-sawdust-derived biochars and raw sawdust to reduce NO₃⁻-N leaching from biosolids. We used batch sorption experiments 1:10 ratio of material to solution (100 mg kg⁻¹ of NH₄⁺ or NO₃⁻) and column leaching experiments with columns containing biosolids (2.7% total N, 130 mg kg⁻¹ NH₄⁺ and 1350 mg kg⁻¹ NO₃⁻) mixed with soil, biochar, or sawdust. One type of low-temperature (350°C) biochar sorbed 335 mg kg⁻¹ NH₄⁺, while the other biochars and sawdust sorbed <200 mg kg⁻¹ NH₄⁺. None of the materials sorbed NO₃⁻. Biochar added at rates of 20 to 50% reduced NH₄⁺-N (<1% of total N) leaching from columns by 40 to 80%. Nitrate leaching (<7% of total N) varied little with biochar form or rate but was reduced by sawdust. Incorporating dried sawdust with biosolids showed promise for mitigating NO₃⁻-N leaching. This effect likely is due to sorption into the pores of the biochar combined with denitrification and immobilization of N rather than chemical sorption onto surfaces.