Using biochar capping to reduce nitrogen release from sediments in eutrophic lakes.

The effects of reduced nitrogen release from sediments were studied using biochar (BC) capping in simulated water-sediment systems. Dried solid waste of Phyllostachys pubescens was used to produce BC, which was then pyrolyzed at 500 °C. Subsequently, 14 sediment cores were collected, including the sediment-water interface and some overlying water, from two sites in Baiyangdian Lake (China). The sediment cores were split into two batches (A and B), and then two each were capped with soil, BC or a BC/soil mixture, and incubated for 30 days. In the BC capped cores, the ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3--N) and total nitrogen (TN) concentrations decreased from 0.90 mg·L-1 to 0.05 mg·L-1, 0.88 mg·L-1 to 0.18 mg·L-1, 6.93 mg·L-1 to 2.81 mg·L-1, respectively, in batch A and 3.51 mg·L-1 to 0.11 mg·L-1, 0.92 mg·L-1 to 0.61 mg·L-1, 8.88 mg·L-1 to 3.32 mg·L-1, respectively, in batch B. The sediments to water fluxes of NH4+-N, NO3--N and TN were greatly reduced or reversed. Compared with other cappings, the BC layer was shown to absorb more NH4+-N from the pore water, thereby breaking the diffusion gradient of NH4+-N at the sediment-water interface, and has a good inhibitory effect on the endogenous release of NH4+-N from the sediments. Additionally, in the BC capped cores, the redox potential remarkably increased and dissolved oxygen was comparatively high. This study suggests that BC capping can reduce the amount of nitrogen released from polluted sediments because the diffusion of nitrogen to the overlying water is chemically blocked by the cap.