Greenhouse Gases Emissions And Net Global Warming Potential In Croplands As Affected By Different Rotation Systems And Nitrogen Management In Sichuan Basin

A field experiment was conducted at Jiangjin District of Chongqing City from 2012 to 2014, considering excessive- and mis-use of nitrogen （N） fertilizer and shortage of water being the major limiting factors in different cropping systems in Sichuan basin, to examine fluxes of nitrous oxide （N₂O） and methane （CH₄） and carbon dioxide （CO₂） and the equivalent of CO₂ （CO₂-eq） emissions from the input of agricultural productionin the two-year study, and then calculated the net global warming potential （net GWP） and greenhouse gas intensity （GHGI） in different rotation systems using soil-based approach. There were three cropping systems as main treatments which were maize-wheat （MW）, rice-wheat （RW） and rice-winter flooded fallow （RF） cropping systems. Three N treatments of each cropping system, as assistent treatments, were NO （no N application）, Nopt （96 kg N ha-1 in wheat season; 150 kg N ha-1 in maize and/or rice season） and Ncon （180 kg N ha-1 in wheat season; 225 kg N ha-1 in maize and/or rice season）, respectively. N₂O, CH₄ and CO₂ were collected and measured one to three times every week by static chamber-gas chromatography system in the two-year study. The main results are summarized as follows:（1） Nitrogen fertilizer application was the main source of N₂O emissions in the Sichuan basin, determined by increased soil inorganic N from N fertilizer application. N₂O emissions showed large inter-annual variation in maize season because of the difference of rainfall intensity between the two years. N₂O emissions were ranked by MW>RW>RF systems, with higher emissions in Ncon than Nopt and much higher than NO. The emission factor derived from fertilizer （EFd） of Ncon and Nopt treatments in wheat, maize, rice season were,0.68-0.82%,3.16%、0.35～0.36% and 0.48～0.73%,2.23%, 0.21～0.22%, respectively. There were larger risks of N₂O emissions under conventional fertilizer treatment and the application of ammonium-based N fertilizers.（2） The CH₄ emissions were highest in RF system, followed by RW system and lowest in MW system. Soil temperature （5 cm） only explain 6%,13%,14% of CH₄ emissions in wheat, maize and rice, respectivlely, because it was affected by fertilizer, irrigation and precipitation. Flooded water depth was an important factor of CH₄ emissions in rice seasons because the determination coefficient 19%。The peak emissions of CH₄ in MW system appeared for the first year （2012-2013） when the single rice paddy was changed for upland-crop based cropping systems but no peak emissions were found in the second year （2013-2014） across all cropping systems. There was no significant difference for annual variation of CH₄ fluxes in RW system. When the single rice paddy was transformed for RF system, the CH₄ fluxes increased rapidly in the second year. The maize season was the main emission source of CH₄ in MW system while the rice season was the main emission source of CH₄ in both RW and RF systems.The CH₄ emissions decreased in the first year of RW and MW systems with increased N fertilizer applied. No significant difference for CH₄ fluxes in RF system under different N fertilzer treatments was detected/The annual average CH₄emissions from MW, RW, RF systems were 13.5,26.7, 89.8 kg CH₄-C ha-1 in the first year; and they were 0.8,22.7,236.3 kg CH₄-C ha-1 in the second year, respectively. Based on the two-year study, the CH₄ fluxes of MW, RW, RF systems for NO averaged 9.1, 28.2,156.6 kg CH₄-C ha-1, respectively; and those for Nopt and Ncon treatments were 91.3%,107.9%, 108.9%, and 46.1%,54.5%,103.5% of the corresponding NO treatment, respectively.（3） The mean agri-ecosystem respiration （CO₂ emissions） in the first year were higher than the secondyrear; the highest agri-ecosystem respiration was MW system in all systems while the lowest agri-ecosystem respiration was RF. Compared with no fertilizer treatment, N fertilizer increased significantly the yield. The change of soil agri-ecosystem respiration was mainly controlled by temperature in the chamber or 5-cm-depth soil temperature in Sichuan Basin. The temperature in the chamber explained 38.3% of agri-ecosystem respiration in wheat seson, and soil temperature （5 cm） explained 29%,49% of agri-ecosystem respiration in maize and rice sesons, respectively. Negative linear correlations were found between WFPS and agri-ecosystem respiration CO₂ emissions both in wheat and maize seasons, and they only explained 9.3%,8.5% of agri-ecosystem respiration CO₂ emissions in wheat and maize seasons, respectivley.（4） N₂O emissions in N fertilizer treatments accounted for approximately 90% of the GWP in MW system, and up to 40% of the GWP in RW systems, and less than 8% of the GWP in RF system. Net GWP and net GHGI were lowest in RW system and highest in RF system. Besides equal CC2-eq emissions of GHG gases which were measured, the input of fertilizers were very importantcontributor in net GWP of MW and RW systems due to low production efficiency. GWP of irrigation electricity was the main source of net GWP in rice season of RW system under drought condition. GWP of CH₄ was main source of net GWP in RF system. Compared with original SOC, the SOC decresed in MW systmes, increased in RF systems and weakerly increased in RW systems. The net GWP and GHGI was highest in MW system while was lowest in RF system.