| Nitrous oxide(N2O)has been considered an important greenhouse gas causing global warming and contributes to ozone depletion.The mitigation of N2O has attracted the attention of researchers.In recent years,it has been found that paddy field is one of the main sources of N2O emission,which could be affected by crop growth,environmental factors and field management.However,researches comparing inter-field N2O emission under various paddy planting methods and exploring the mechanism dominated by microorganisms are very scarce,especially,under different cropping.Nitrogen fertilizer application is a key factor impacting the growth of crops,consequently affecting N2O emission.There have been many studies on the characteristics of N2O emission from rice fields after nitrogen fertilizer application in the current season,but there are few reports on the effect of prior nitrogen(N)fertilizer application on N2O emission in the later season.In this study,two farmland experimental stations in central China that has typical paddy planting modes,were selected to study the effects of different crop planting patterns on soil N2O emission during crop-growing season.In Huanggang experimental station,the effects of different upland crop planting and rice crop planting on soil N2O emission in rice-growing season were investigated.In addition,the N balance and the contribution of N2O to N cycle in paddy system under different cropping patterns were compared.While,the effect of prior N application on soil N2O emission was studied in a rapeseed-rice rotation system in the experimental station of Huazhong Agricultural University in Wuhan.During the study period,N2O emission flux was monitored in situ,and soil samples were collected to determine soil physicochemical properties and the abundance of some genes related to nitrification(AOA-amo A and AOB amo A)and denitrification(nir S,nir K and nos Z)to reveal the key regulatory factors affecting N2O emission.The main results of the study were as follows:1.The cumulative N2O emission from three rice-growing seasons under double cropping rice-fallow(RF),rice-rapeseed(RR)and rice-wheat(RR)cropping patterns were 1.32±0.12,0.82±0.09 and 2.97±0.18,1.38±0.16,0.91±0.15 and 1.06±0.04 and 0.95±0.07,0.51±0.09 and 0.74±0.08 kg N ha-1,respectively.The cumulative N2O emissions from rice seasons under RW were significantly lower than those of the other two paddy field planting patterns(p<0.05);But there was no significant difference in N2O emission for first two rice seasons between RF and RR system(p>0.05),the above results indicated that different upland cropping could affect the N2O emission in the following rice season.The main reason was that planting different crops in winter not only affected the changes of active carbon and nitrogen contents and their ratios,but also influenced the microbial activities and the abundance of functional genes related to N2O production in rice seasons.However,the cumulative N2O emission for two rapeseed and wheat-growing seasons were 1.24±0.20 and0.81±0.10 and 0.98±0.25 and 0.70±0.04 kg N ha-1,respectively,they were significantly higher than those during the fallow periods(0.12±0.05 and 0.13±0.04 kg N ha-1).Because N fertilization increased substrate needed for microbial activities and promoted soil nitrification and denitrification and the expression of related functional genes,thereby increasing N2O production and emission.2.Cropping in rice-growing season under both upland rice-rapeseed(UR)and RR system could reduce soil N2O emissions.For UR pattern,the cumulative N2O emission for upland rice-rapeseed with crop(UR-CC)treatment in two rice-growing seasons were 1.54±0.16 and 2.57±0.28 kg N ha-1,and that were 2.45±0.07 and 3.74±0.37 kg N ha-1for upland rice-rapeseed without crop(UR-NC)treatment,respectively.For RR pattern,the cumulative N2O emission for rice-rapeseed with crop(RR-CC)treatment in two rice-growing seasons were 0.71±0.20 and 0.76±0.04 kg N ha-1,and that were 1.43±0.35 and 1.16±0.08 kg N ha-1,for rice-rapeseed without crop(RR-NC)treatment,respectively.Bare soils emitted more N2O,which was mainly due to the higher inorganic nitrogen content after N fertilization that provided substrates for nitrification and denitrification processes.For UR-NC treatment,crop-free could increase AOA-amo A and reduce the copy number of nos Z gene,thus promoting N2O production and emission.In RR-NC treatmen,crop-free could reduce the copy number of nos Z gene and accelerate N2O emission.With or without crop planting,N2O emission in rice-growing seasons for UR system was significantly higher than that for RR system,mainly because of suitable soil water filled pore space(WFPS)and higher soil inorganic nitrogen(SIN)content.This indicates that suitability of both WFPS and SIN under UR system were conducive to microbial nitrification and promoted soil N2O emission.Compared with UR,flooding in rice-growing season under RR affected the nos Z gene and promoted the N2O reduction.3.N application in rice season had no significant effect on soil N2O emission in subsequent rapeseed season,while N applied in rapeseed season accelerated N2O emission in subsequent rice growing season.During the rapeseed growing season,higher soil DIN concentration caused by N application resulted in higher N2O emission compared with no N application.The N2O emission in the rice growing season was related to the higher DOC content generated by the decomposition of soil organic matter.In addition to active C and N,the ratio of soil DOC to NO3-played a key role in controlling soil N2O emission.Therefore,when reducing soil N2O emission under paddy field,the effect of prior N fertilizer and the active C and N concentration and their ratio should be considered.4.The N uptake of wheat in the medium rice-wheat cropping pattern was the highest,reaching 344.11 kg N ha-1,which was significantly higher than that of other paddy cropping patterns,mainly due to the N uptake of wheat in the dry cropping season.In addition,leaching and N2O emission loss was the lowest under medium rice-wheat planting mode.Therefore,compared with other paddy planting modes,the medium rice-wheat planting mode has greater environmental friendly advantages.In summary,we suggested that different upland crop planting in winter,cropping in rice-growing season and prior N fertilization could affect N2O emission in paddy field,especially in rice-growing season by changing soil moisture,active C and N content and their ratios and the abundance of N2O production related functional genes,consequently affecting N2O emission.This study suggested that N2O emissions from paddy fields could be regulated by changing crop planting,adjusting soil moisture and available C and N content and their ratios. |
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