Investigation On Soil Carbon Sequestration And Mitigation Of Greenhouse Gas Emission By The Integrated Rice-Azolla Cropping System In Double Rice Cropped Region

Sustainable rice（Oryza sativa L.）production is of great importance for Chinese food safety.Rice paddy is an important source of greenhouse gases and paly a significant role in global budget in anthropogenic greenhouse gas emission.Rice intensification,excessive chemical fertilizer application and irrational fertilization struction imposed an important influence on greenhouse gas emission.In rice cultivation,green manure Azolla is used historily a dual crop along with rice plants,having great agricultural potential.The dual cropping of Azolla with rice plants is either economically feasible or environmentally desirable.The current study established the integrated rice-Azolla cropping system（IRAC）through dual cropping of Azolla along early and late rice in double-rice cropping system in suthern China.A 4-year successive field experiment was conducted to study the effect of IRAC on soil productivity,rice prodcution,methane（CH4）and nitrous oxide（N2O）emissions and soil organic carbon storage.Moreover,a pot experiment was conducted to explore the underlying mechanism for the effect of dual cropping of Azolla on CH4 and N2O emissions.The main results are summarized as follows:Ⅰ.In double-rice cropping system,the dual cropping of Azolla improved soil physical properties through declining soil bulk density and increasing soil porosity,and fertilized the paddy soil through increasing soil organic carbon and NH4+-N content.The IRAC only or in conjunction with havling nitrogenous fertilizer at 100 kg ha^-1 N per rice crop declineded soil bulk density by 6.3%and 7.7%,and increased soil porosity by 5.7%and 7.0%,respectively.Ⅱ.In comparison with the conventional rice cropping,the IRAC increased early and late-rice grain yield by 4.7%-8.9%and 2.3%-7.4%,and hence the annual double-rice grain yield totalled 12549.8^-13006.6 kg ha^-1,which was 3.2%-8.0%higher than the conventional rice cropping did.Under different nitrogen level,the IRAC presented the increment of rice grain yield compared with the conventional rice cropping.The increase in the early and late-rice grain yield were caused by the increase in.The IRAC system increased the spikelets per panicles by 3.3%^-15.9%and 2.8%^-15.2%over the conventional rice cropping for the early and late riceⅢ.A 3-year successive measurements were performed for CH4 and N2O emissions from double-rice cropped field.The result indicated that the IRAC system significantly decreased CH4 emissions from early and late rice paddies as compared with the conventional rice cropping（P<0.05）.Over the early and late rice seasons,CH4 emissions decreased drastically by 21.2%-49.9%and 19.2%-37.3%.However,in the early and late rice growing seasons,the N2O emissions from the plots under the IRAC system were 10.5%-63.8%and 14.5%-62.3%higher than from the plots under the conventional rice cropping system.Global warming potential（GWP）was dominated by CH4 emission with a larger percentage of>85%.The GWP and greenhouse gas intensity（GHGI）under the IRAC system was lower than the conventional rice cropping system.IV.The carbon footprint（CF）of the two cropping system IRAC and conventional was estimated in the proceeding double-rice production.The IRAC was labor-intensive and increased labor.However,the IRAC decreased the input of chemical pesticide.Under the IRAC,the indirect CO2-eq emission from field management practices totaled respectively 403.17 and 514.59 kg C02-eq ha^-1 for the early and late rice,which was 0.36 and 0.54 kg CO2-eq ha^-1 larger than the conventional rice cropping system did.In terms of agricultural inputs,the IRAC system decreased indirect CO2-eq emissions by 13.19 and 13.20 kg CO2-eq ha^-1.V.Pot experiment revealed that midseason drainage,Azolla inoculation and N addition significantly decreased CH4 emissions.Specifically,during the early and late rice growing seasons,the midseason drainage drastically reduced CH4 emissions by 22.1%and 20.3%relative to the continuous flooding;the pots with Azolla decreased profoundly CH4 emissions by 35.2%and 28.3%over the pots without Azolla;the pots with N addition mitigated remarkably CH4 emissions by 22.2%and 19.4%relative to the unfertilized pots.The reason for the reduction in CH4 emissions associated with the presence of Azolla could get explained by the increase in dissolved oxygen in standing water and hence raising soil redox potential.VI.The IRAC increased soil organic carbon storage due to the raising soil organic carbon content.The carbon sequestration rate was faster than the conventional rice cropping system.The maximum carbon sequestration rate across all treatments was observed under the IRAC system in combination with halving N fertilizer at 100 kg ha^-1 N per rice crop,up to 3148.9 kg C02-eq ha^-1 a^-1.Further,the CF value under IRAC system in conjunction with N fertilizer at 100 kg ha^-1 N per rice crop was estimated to be 0.57 kg CO2-eq kg^-1 grain yield a^-1,which was 37.8%lower than the conventional rice cropping system with common N fertilizer application at 200 kg ha^-1 N per rice crop.Therefore,the IRAC system applied N fertilizer at 100 kg ha^-1 N per rice crop could be an optimum option for double rice production.