Nitrogen and Carbon Leaching in Repacked Sandy Soil with Added Fine Particulate Biochar

Biochar amendment to soil may affect N turnover and retention, and may cause translocation of dissolved and particulate C. We investigated effects of three fine particulate biochars made of wheat ( Triticum aestivum L.) straw (one by slow pyrolysis and two by fast pyrolysis) on N and C leaching from repacked sandy soil columns (length: 51 cm). Biochar (2 wt%), ammonium fertilizer (NH 4 + , amount corresponding to 300 kg N ha −1 ) and an inert tracer (bromide) were added to a 3‐cm top layer of sandy loam, and the columns were then irrigated with constant rate (36 mm d −1 ) for 15 d. The total amount of leachate came to about 3.0 water filled pore volumes (WFPVs). Our study revealed a high mobility of labile C components originating from the fine particulate fast pyrolysis biochar. This finding highlights a potential risk of C leaching coupled with the use of fast pyrolysis biochars for soil amendment on sandy soil. By contrast, C components from the slow pyrolysis biochar were fully retained in the topsoil. Contrary to our expectations, there were no overall effects of biochar amendment on the cumulative leaching of mineral N (NH 4 + plus NO 3 – ), that is, the biochars did not increase the N retention capacity of the soil. All three biochars caused a slight increase of NH 4 + leaching, while NO 3 – leaching was slightly decreased by addition of the fast pyrolysis biochars.