Annual CH4 and N2O Emissions from Double Rice Cropping Systems under Various Fertilizer Regimes in Hunan Province, China

Annual CH4 and N2O emission fluxes were measured from double rice cropping systems under six fertilization treatments by using the method of static chamber-gas chromatograph techniques. The different treatments including non-fertilizer(CK), conventional chemical fertilizers (NPK), fresh rice straw with chemical fertilizers (RS+NPK1), mushroom residue with chemical fertilizers (MR+NPK1), fresh cow dung with chemical fertilizers (CD+NPK2), and biogas digestion with chemical fertilizers (BD+NPK2) were dedicated to this experiment in order to study the characteristics of CH4 and N2O emissions and the relationship between gas emission fluxes and related environmental factors. The results showed that, seasonal variation of CH4 emission fluxes under different treatments displayed the same trend, which can be described as “lower in early rice growing period, higher in late rice growing stage, and negligible in the fallow season”. The majority of N2O emissions (more than 78%) occurred in rice growing periods, and small amounts of N2O emission occurred in the fallow season. Although the annual accumulated CH4 emission amount for NPK was increased by 35% compared with CK, the difference between them was not significant at 0.05 level. Among different treatments of organic and mineral fertilizers, the annual accumulated CH4 emission amount for RS+NPK1 was 2.44 times more than that for MR+NPK1 (P<0.05, P is the significance level), while the annual N2O emission amount for the latter was 59% higher than that for the former (P>0.05). The annual accumulated CH4 emission amount for CD+NPK2 was 2.45 times more than that for BD+NPK2 (P<0.05), but the annual N2O emission amount for the latter increased by 102% compared with CD+NPK2 (P>005). The fertilizer-induced direct N2O emission factor (E) was reduced by 45%－80% under the treatments of combined application of chemical and organic fertilizers in paddy fields compared to NPK. In this analysis, water status and temperature were main driving factors which affected the seasonal variation of CH4 emission fluxes. However, N2O emission fluxes did not have direct relation with water status in double rice paddies and a temporal trade-off relationship between the two gas emission fluxes was not observed in this study. CH4 was the dominant greenhouse gas which contributed more than 75% to the integrated global warming potential of CH4 and N2O emissions in local double rice fields. By using composted mushroom residue and biogas digestion to substitute for fresh rice straw and cow dung for returning fields, CH4 emission and integrated global warming potential of CH4 and N2O emissions reduced by approximately 60% and 50%, respectively, and the grain yield will not be affected.