| Agricultural greenhouse gases emissions play an important role of the global climate change.Methane(CH4)and nitrous oxide(N2O)have been recognized as the most important longlived greenhouse gases that significantly contributed to global warming in addition to atmospheric carbon dioxide(CO2).Their emissions represent the net results ofproduction,consumption and transformation in paddy soil profile.Understanding the fate of CH4 and N2O in the subsoil is important for accurately assessing the distribution and storage conditions under rice-wheat annual rotation in southeastern China.Moreover,defining the CH4 and N2O emissions with biochar amendment can provide more scientific evidences of carbon sequestration and emission reductions in the paddy fields.Field experiments were carried out to make a comprehensive evaluation on soil CH4 and N2O concentration profiles,diffusion,and emissions by biochar application.An in situ soil gas device was adapted to measure CH4 and N2O concentrations at depths of 7,15,30 and 50 cm and combined the monitoring of their emissions was performed using a static chamber method to make a comprehensive evaluation of soil CH4 and N2O distributions,diffusions and eimissions.The experiment lasted two years from June 2013 to June 2015.Five treatments with three replicates were installed as N0B0,N0B1,N1B0,N1B1,and N1B2(B0,B1,and B2 designated as biochar at 0,20,and 40t ha-1,respectively,while NO and N1 as nitrogen at 0 and 250 kg N ha-1 crop-1,respectively).The results showed that CH4 productions and emissions were mainly occured during the rice seasons.Soil CH4 concentrations at depth of 7cm were the highest and decreased with the depth during rice season(P<0.05),however without difference in the wheat season.Mean methane diffusive fluxes to the 7 cm depth were positive in the rice season and negative in the wheat season,resulting in the paddy field being a source and weak sink,respectively.N fertilization significantly increased the soil CH4 concentrations and emissions during rice season,while without influence during wheat season.Relative to N1B0,N1B1 and N1B2 significantly depressed the mean CH4 emissions and soil CH4 concentration profiles during rice season in 2013,respectively(P<0.05).Compared to N1B0,N1B2 significantly reduced CH4 emissions,and,N1B1 and N1B2 significantly reduced soil CH4 concentrations at the depth of 7,15 and 30 cm during rice season in 2014.There was no significant difference between N1B1 and N1B2.Soil N2O concentrations were higher in the 7 cm depths than other depths during rice season and were higher in the 15 and 30 cm depths than other depths during wheat season.With positive diffusive fluxes,the depth of 0-7 cm in the rice season and 7-30 cm in the wheat season were main production sites.N fertilization significantly increased the soil N2O concentrations and emissions during the two rice-wheat annual rotations(P<0.05).Relative to N1B0,N1B2 decreased N2O concentration and surface emissions whithin the two wheat seasons in 2013 to 2015,the effect decreased with soil depth(P<0.05).However,N1B 1 just decreased N2O surface emissions in the first wheat season,while without effects on soil N2O concentrations or emissions at other periods.The surface CH4 emissions and diffusive fluxes to 7 cm depth showed good agreement when N was applied but the cumulative CH4 diffusive fluxes were much lower than the emissions(P<0.05).The surface N2O emissions and topsoil diffusive fluxes showed good agreement in the two rice-wheat annual rotation systems and without significant difference.As such,appropriate rate of biochar can decrease soil CH4 and N2O production and emission when N was applied.The gas gradient method is suitable for define CH4 and N2O production sites and distribution.The method is also effective for estimating N2O emissions in the rice-wheat annual rotation system. |
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