Influence of rice mechanical planting methods on methane and nitrous oxide emissions from paddy field

Abstract: Methane (CH4) and nitrous oxide (N2O) are 2 important long-lived greenhouse gases (GHGs) that contribute to global warming. Paddy soils have been identified as a dominant source of atmospheric CH4 and N2O. Little information is available on the impacts of rice mechanical planting methods on CH4 and N2O emissions and rice yield in high production rice-wheat double cropping system. A field experiment was conducted with super rice cultivar Nanjing 44 as materials during the rice growing season from 2011 to 2012 in Suzhou, Jiangsu Province. By using static chamber/gas chromatographic techniques, in this two-year field experiment CH4 and N2O emissions were simultaneously measured under 3 rice planting methods: mechanical direct-seeding (MD), mechanical transplanting (MT) and artificial transplanting (AT). Each planting treatment was combined with either wheat straw retention or wheat straw removal. The results indicated that all treatments exhibited comparable seasonality in CH4 fluxes, showing an increase at the beginning and a decline later on. High N2O emissions were triggered by the midseason drainage episode during the rice growing season in 2011 and 2012. CH4 accumulative emissions from transplantation to critical stage of effective tillering accounted for 76.49%-91.13% of the total emissions during the rice growing season. N2O accumulative emissions from critical stage of effective tillering to elongation stage represented 33.56%-49.41% of the seasonal N2O emissions. Compared with wheat straw removal, wheat straw retention significantly increased seasonal total CH4 emissions by 125.96%-138.31% in 2011 and by 108.63%-127.10% in 2012 (P<0.05), respectively, and reduced the seasonal total N2O emissions by 2.83%-12.50% in 2011 and by 3.39%-18.19% in 2012 (P>0.05), respectively. The MT slightly decreased CH4 emissions during the rice growing season by 3.25%-9.50% compared to the AT (P>0.05), while both treatments were significantly higher than that from the MD (P<0.05). The seasonal total CH4 emissions in the MD were respectively 15.69% and 18.43% lower than those in the MT and AT with wheat straw removal, and 14.54% and 22.66% lower than those with wheat straw retention in 2011. And in 2012, the seasonal total CH4 emissions in the MD were correspondingly 26.63% and 32.12% lower than those in the MT and AT with wheat straw removal, and 30.51% and 36.75% lower than those with wheat straw retention, respectively. Compared with the AT, MD significantly increased N2O emissions during the rice growing season by 0.16-0.97 kg/hm2 in 2011 and 2012 (P<0.05). The seasonal total N2O emissions were comparable between the MT and the AT (P>0.05). For the years of 2011 and 2012, the rice yields under the AT were the highest, followed by the MT, and the yield under the MD was the lowest. Compared with the AT, the MD significantly decreased rice yield by 8.43%-10.79% (P<0.05), while the MT slightly decreased yield by 1.27%-3.49% (P>0.05). CH4 was more important in the 2 GHGs in that the effect of the seasonal CH4 emissions from rice-wheat double cropping system on climate was approximately 4 times greater than that of N2O emissions. The global warming potential (GWP) of CH4 and N2O emissions in the plots with wheat straw retention was 116.23%-130.35% higher than that in the plots with wheat straw removal in 2011 and 87.72%-118.04% in 2012 (P<0.05). The GWP in the MD was significantly lower than those in the MT and the AT (P<0.05). The GWP per yield in the MD was respectively 12.02% and 28.71% lower than that in the AT with wheat straw retention in 2011 and 2012 (P<0.05). The overall results indicated that the MD could effectively decrease total CH4 emissions during the rice growing season; with wheat straw retention, the conversion from AT to MD would reduce the comprehensive greenhouse effect resulting from the CH4 and N2O emissions in high production rice-wheat double cropping system in the downstream of the Yangtze River in China.