Effect of field-aged biochar on fertilizer N retention and NO emissions: A field microplot experiment with N-labeled urea.

Biochar application is thought to improve crop yield and reduce N leaching and gas emissions; however, little is known about how field-aged biochar affects fertilizer N retention and NO emissions. Here, a field microplot experiment is established in the North China Plain at maize season by applying N-labeled urea to the sandy loam soil both with (Biochar) and without (Control) application of 3-year field-aged biochar at 12 t ha. Overall, 25.6-26.2% of the urea N was taken up by maize aboveground biomass, field-aged biochar did not affect yield or fertilizer N recovery efficiency. After maize harvest, the residual ratio of applied N in the soil profile (0-40 cm) was 21.6 and 20.3% under Control and Biochar treatment, respectively, with an increase of 10.2% in the topsoil (0-20 cm) and decrease of 37.2% in the subsoil (20-40 cm) following biochar amendment, probably due to reduced NO leaching. Cumulative NO emissions and urea N-induced NO emissions under Control treatment were 2.06 and 0.78 kg N ha, and significantly decreased to 1.89 and 0.74 kg N ha after Biochar treatment, respectively. NO emissions derived from the applied N accounted for 38.0 and 39.4% of the total emissions under Control and Biochar treatment, respectively. NO emissions from decomposition of soil organic N induced by the priming effect of the applied N was 0.69 and 0.56 kg N ha under Control and Biochar treatment, respectively, contributing 33.7 and 29.7% of the total emissions. Overall, our results suggest that field-aged biochar increased the retention of fertilizer N in the topsoil by reducing NO leaching, while effectively reduced NO emissions from fertilizer N and mineralization of organic N in the sandy loam soil.