Combined effects of nitrogen fertilization and biochar incorporation on methane and nitrous oxide emissions from paddy fields in rice-wheat annual rotation system

【Objectives】The potentials of biochar application in mitigating global warming in agriculture systems need assessed through field experiments. The effects of combined N fertilization and biochar incorporation on the global warming potential(GWP)caused by CH4 and N2O emissions, the greenhouse gas intensities(GHGI), and grain yield are need to be investigated to provide a scientific base for the biochar application in a rice-wheat annual rotation system. 【Methods】Biochar used in the study was prepared by carbonization of wheat straw at 350-500℃. A field experiment was carried out during the wheat and rice seasons between November 2012 and October 2013. Five treatments were adopted in triplicate: no N fertilization without biochar amendment(N0B0), no N fertilization with 20 t/hm2 biochar amendment(N0B1), 250 kg/hm2 N fertilization without biochar amendment(N1B0), 250 kg/hm2 N fertilization with 20 t/hm2 biochar amendment(N1B1), 250 kg/hm2 N fertilization with 40 t/hm2 biochar amendment(N1B2). The CH4 and N2O gas emission fluxes were monitored with a static chamber and gas chromatography method.【Results】In N1B0 treatment, the yield of rice and wheat was increased by 82.8%, the CH4 and N2O emissions were 1.6 and 6.5 times of those in N0B0 treatment. In N0B1 treatment, the wheat and rice production was significantly increased by 25.4%, no pronounced difference in CH4 and N2O emissions was found with in the N0B0 treatment. In contrast with the N1B0 treatment, CH4 emission decreased by 3.7% and 11.3%(P＜0.05), N2O emission decreased by 6.1% and 20.9%(P＜0.05), the yield of rice and wheat increased by 21.6%(P＜0.05)and 10.0% in the N1B1 and N1B2 treatments, respectively. The N1B2 treatment significantly reduced the CH4 and N2O emissions than in N1B1 treatment. The mitigation effect on CH4 and N2O emissions became more noticeable with higher biochar amendment. Based on a 100 years horizon, the order of ranks in the annual total GWPs of CH4 and N2O total emissions over the entire rotation cycle was N1B0 N1B1 N1B2 N0B0 N0B1, the GWPs per unit crop grain yield were in order of N1B0 N1B1 N0B0 N1B2 N0B1. Significant difference in the GWPs existed between the treatments with and without N fertilizer, not in the GHGIs. There was no significant difference between the N0B0 and N0B1 treatments in the GWPs, but significant in the GHGIs. The noticeably higher GWP and GHGI were found in the N1B0 than in other treatments, which indicated that the single N fertilization could increase the GWP and GHGI. Both nitrogen and biochar combination treatments could reduce the GWP and GHGI. The single biochar amendment did not effectively reduce the GWP, but significantly increased crop yield and reduced GHGI. A two-way analysis of variance for treatments of N0B0, N0B1, N1B0 and N1B1 indicated that no obvious interaction between N fertilizer and biochar on CH4 and N2O emissions, crop yield, GWP and GHGI. All the single biochar application and the combined application with N fertilizer could reduce the GWPs and GHGIs, and biochar incorporation of 40 t/hm2 produced better results than that of 20 t/hm2. 【Conclusion】 The single N fertilization, and the biochar and N incorporation in wheat season increase the wheat and rice production, decrease CH4 and N2O emissions, thus simultaneously lowered GWP and GHGI in a rice-wheat rotation system. Biochar amendment of 40 t/hm2 could mitigate more GHG emissions than that of 20 t/hm2, while improved insignificant grain yields. Thus the two biochar amendments produce comparable GHGI. It is therefore an unanswered issue for decision when balanced between GHG mitigation and grain yield.