No abstract

Purpose Overuse of chemical fertilizer in agriculture has caused serious nitrogen (N) loss and water pollution problems in China. Biochar has the potential ability to reduce N loss and increase crop yield. However, there is still limited knowledge of the impacts of different biochars on N loss and crop yield over agriculturally relevant time frames. In this study, we compared the effects of amendment with bamboo biochar and rice straw biochar on the N retention and rice productivity in paddy fields, over an agriculturally relevant time span of 2 years. Materials and methods A 2-year field study was conducted to investigate the effects of bamboo biochar and rice straw biochar amendment at a rate of 22.5 t ha −1 (with or without urea) on N retention and rice growth. Total nitrogen (TN), ammonia (NH 4 + -N), and nitrate (NO 3 − -N) in soil and surface water were determined after biochar application. Stem lengths and rice yield were monitored during the experiment. Results and discussion Amendment with rice straw biochar resulted in higher rice yields than in paddy soils that had bamboo biochar amendments. Incorporating rice straw biochar into a paddy field increased the rice yield by 19.8 % in 2009 and 21.6 % in 2010 without urea (P<0.05) and by 11.3 % in 2009 (P<0.05) and 14.4 % in 2010 with urea, compared with their corresponding control treatments. Although there were no significant impacts on the surface water N runoff potential, biochar amendment did result in a significant increase in the NO 3 − -N content of rhizosphere soil-121.2-135.7 % with urea and 89.7-102.2 % without urea, respectively, at the tillering stage in the first year (P<0.05).Conclusions These results show that carbonizing rice straw residue into biochar and incorporating it into soil has the potential to enhance rice productivity and N retention in a paddy field.

Conversion of rice straw into biochar for soil amendment appears to be a promising method to increase long-term carbon sequestration and reduce greenhouse gas (GHG) emissions. The stability of biochar in paddy soil, which is the major determining factor of carbon sequestration effect, depends mainly on soil properties and plant functions. However, the influence of plants on biochar stability in paddy soil remains unclear. In this study, bulk and surface characteristics of the biochars incubated without rice plants were compared with those incubated with rice plants using a suite of analytical techniques. Results showed that although rice plants had no significant influence on the bulk characteristics and decomposition rates of the biochar, the surface oxidation of biochar particles was enhanced by rice plants. Using 13C labeling we observed that rice plants could significantly increase carbon incorporation from biochar into soil microbial biomass. About 0.047% of the carbon in biochar was incorporated into the rice plants during the whole rice growing cycle. These results inferred that root exudates and transportation of biochar particles into rice plants might decrease the stability of biochar in paddy soil. Impact of plants should be considered when predicting carbon sequestration potential of biochar in soil systems.