Effects of the addition of rice-straw-based biochar on leaching and retention of fertilizer N in highly fertilized cropland soils

Abstract

Little is known about the effects of adding crop straw biochar on the leaching and retention of fertilizer nitrogen (N) in intensively managed cropland soils. In this study, the relationship between N leaching and retention from or in soils was investigated after adding 5% (by weight) rice (Oryza sativa L.) straw-derived biochar to four cropland soils with varying properties through an experiment involving 300 mg N kg–1 15N labeled urea and soil column leaching. The 15N results from the 120-d experiment showed that fertilizer N retention increased by 5.6%~26% (of added N; 8.67~40.0 mg N per column) and fertilizer N leaching decreased by 26%~49% (41.0~76.7 mg N) after biochar was added to the four soils (compared to that in the urea-only treatments). Biochar addition consistently reduced N recovery (retention plus leaching) by 4.5%~38% (7.07~59.5 mg) in the soils. These results indicated that the decreased leaching after the addition of biochar could not be entirely explained by increase of N retention in soils; this suggests that the reduced N leaching may result from both improved N retention in the soil and increased unaccounted-for N (increased gaseous losses). The addition of rice straw-based biochar also significantly affected ammonia (NH3) volatilization related to crop growth in a pot experiment. Our results suggest a potential negative effect of biochar application on N gaseous losses in heavily fertilized cropland soils, which should be investigated further in future.