Biochar addition to soil changed microbial community structure and decreased microbial biomass carbon and net inorganic nitrogen mineralised

Biochar is a recalcitrant carbon rich soil amendment proposed to improve soil fertility. We investigated changes in soil microbial biomass, community structure and function and net inorganic nitrogen changed resulting from its addition. We homogenously incorporated biochar at rates of 0, 5, and 25 t/ha into a coarse textured sand in a glasshouse trial. Three nitrogen treatments were added: organic nitrogen (a pelleted pig manure and wheat straw compost), inorganic nitrogen (33 mg kg @1 of N as NH 4NO 3 at 3 and 6 weeks), and a control treatment. Wheat ( Triticum aestivum cv. Calingiri) was grown for 70 days, after which samples were taken. Microbial biomass carbon (MBC) decreased with biochar addition (P=0.05). Microbial carbon to nitrogen ratio and microbial community structure changed significantly with biochar addition at 25 t/ha but not at 5 t/ha. The decrease in microbial C: N ratio (from 8:1 to 5:1) suggests a shift to a more bacteria dominant community. Net nitrogen mineralisation decreased from an average of 11 mg N/kg dry soil after 10 weeks of incubation without biochar to 1 mg N/kg dry soil with addition at 25 t/ha. Sorption of nitrogen to biochar at these rates could not explain the result.