Impact of biochar amendment on nutrient retention by riparian soils

Abstract

Establishment of riparian zones have the potential to reduce the effects of diffuse discharges of nutrients into waterways. Biochar is the carbon-rich product obtained by thermal decomposition of organic matter. Addition of biochar to soils in a riparian zone may improve the nutrient removal efficiency and extend the life of the riparian zone while sequestering carbon. The ability of biochar to improve nutrient retention in a simulated riparian zone was investigated by adding 30 t ha⁻¹ of Pinus radiata biochar (prepared at 350°C) to a Templeton Silt Loam soil and planted with the commonly recommended riparian zone native, Carex secta. Four replicates of four different treatments were applied; Water only, no biochar (WNC), Water only, with biochar (WBC), Effluent, no biochar (ENC), Effluent, with biochar (EBC). Addition of effluent had significant effects (p < 0.05) on biomass yield and mass loading of nutrients in the subsurface drainage collected. Biochar amendment significantly increased the mass load of NO₃⁻-N and NO₂⁻-N in the subsurface drainage from effluent by 210% and 430%, respectively, while NH₄⁺-N decreased by 54%. However, there was no effect on TN, indicating a significant increase in nitrification. There was a 35% decrease in TP in the subsurface drainage mass load from EBC compared to ENC (p < 0.1), probably due to less macropore flow and P immobilisation by microbes. There were no signficant effects of biochar addition on biomass yield, plant uptake of nutrients or on the mass load of DRP and DOC. In summary, addition of biochar significantly increased the rate of nitrification in the soil and decreased the amount of TP in the subsurface flow compared to the control.