Effects Of Organic Substiution On Greenhouse Gas Emission And Nitrogen Transformation In The Soil Of Vegetable Field

Reducing mineral fertilizer using agricultural organic materials,and subsequently the effects of this operation on the soil quality and crop yields have been the research emphases of agricultural scientists.Vegetables are cultivated using intensive cropping,frequent irrigation and high nitrogen?N?application rate,compared with general food crop.The N fertilizer applied to vegetable fields is usually 600�1300 kg N ha^�1 per year in China,which is 3 and 4-fold more than that to non-vegetable crops.These management practices easily accelerate of the soil nitrogen cyle processes including soil nitrogen mineralization,nitrification and denitrification processes.As results,greenhouse gas?GHG?emission increases,and more surplus N remians in vegetable filed and subsequently more nitrate N cumulatved.Besides,the conditions of soil acidification,soil sreucture degradation,soil nutrient imbalance and soil microorganisms'community composition change would occur.Input of exogenous organic materials is one major methord for reduding the dosage of mineral fertilizer and improving the quality of soil.There are some reports about the effects of organic materials on GHG?carbon dioxide?CO₂?,methane?CH₄?and nitrous oxide?N₂O??emission,soil N types and microbial action mechanism,but the results remian inconsistent.Meanwhile,the lack of field observation data of GHG emissions from vegetable fields in China and the inconsistency of research results cause the great uncertainty in the estimation of total GHG emissions from vegetable soil in China.Therefore,reducing fertilizer application rate through the use of farmer-friendly residues can mitigate the environmental burden,such as increased GHG emissions,as well as promote resource utilization fo agricultural residues,contribute to the key task of�Fertilizer Use Zero-Growth Action Plan by 2020�in China,provide the technical support for the construction of ecological civilization.In this study,we designed a mesocosm experiment in the purple soil of vegetable field from2016 to 2018.A five-season?lettuce?I?-cabbage-chili-lettuce?II?-pakchoi?field mesocosm vegetable cultivation was conducted to examine the temporal variation characteristics of GHG emission using close-vented chambers under six fertilization treatments:non-fertilization no straw return control?CK?,fully solely mineral fertilization?F?and four mineral fertilization plus maize straw?100%,70%,60%and 50%of the F rate as FS,0.7FS,0.6FS and 0.5FS?.Meawhile,the effects of organic materilas from different agricultural systems?maize straw?SF?,biomass ash?BF?,mushroom dregs?MF?,cow dung?CF?coupled with reduced mineral fertilizer,respectively?on GHG emission were also monitored according to the same nitrogen,phosphorus and potassium ratio additions.The variations of different soil N,soil nutrient,vegetable nutrient and vegetable quality were analysied,as well as agronomic utilization rate of nitrogen,phosphorus and potassium fertilizer with the methord of indoor analysis combining with field experiment.The stability of soil aggregates and the distribution of soil mineral N in soil aggregates were monitered under organic materials coupled with reduced mineral ferilizers.The abundance of soil N related microbial functional genes?amoA,nirS,nirK and nosZ?and microbial community structure were monitored and analyzed with multiple molecular techniques?DNA-based clone library,qPCR and High throughput sequencing?bonded with soil physical-chemical properties,aiming to reveal the microbiological mechanism of N transformation.The generated outcomes could promote our better understanding of coupled management effects of agricultural organic materials and mineral fertilizers on purple soil nutrient management and soil quality improvemnt in intensively cultivated vegetable farmlands.Meanwhile,it is important for promoting our country saving cost to increase efficiency,as well as saving energy to reduce emissions.The main conclusions show below:?1?Fertilizering and irrigating operation stimulated the GHG emissions.The 0.7FS treatment achieved a best trade-off between vegetable yields and CO₂,CH₄ emissions,but no significant effects on mitigating N₂O emission.During the entire sampling periods?from Novermber,2016 to February,2018?,high GHG emission?N₂O,CO₂,CH₄?were recored during April to August.The high emissions were also obtained after fertilizering and irrigating and had close relationship with soil moisture and temperal.The N₂O flux,cumulative emission and emission factor under the maize straw additions treatments?FS,0.7FS,0.6FS,0.5FS?were higher than those under the F treatment?fully solely mineral fertilization?,especially under the FS treatment with the high mean N₂O emission of 65.45 kg�hm^-2.The low emissions of CO₂ and CH₄ were obtained under the 0.7FS,as well as the low GHG global warming potential?GWP?.The valu of GWP under the FS treatment increased by 34.1%relatived to the F treatment.Vegetable yields were similar under all mineral fertilizer plus straw treatments.The greenhouse gas intensity?GHGI?of vegetable production under F and 0.7FS treatments was lower than that under other treatments?P<0.05?,while the highest GHGI value was obtained under the FS treatment with increasement of 0.117 kg�kg^-1 relative to F and 0.7FS treatments.?2?According to the same nitrogen,phosphorus and potassium ratio additions,biomass ash and maize straw coupled with reduced mineral fertilizer mitigated the GWP compared with other treatments.Mushroom dregs addition has better effects on mitigating soil CH₄ emission,while cow dung addition increase the GHG emission.Biomass ash coupled with reduced mineral fertilizer?BF?reduced the N₂O emission compared with the F treatment in the condition of same nitrogen,phosphorus and potassium ratio additions.While no significant difference in the mean N₂O emission between BF and F treatments.Cow dung coupled with reduced mineral fertilizer?CF?significantly increased the N₂O emission with the mean N₂O emission of 48.99 kg�hm^-2.For the soil CO₂,four typles of organic materials combined with mineral fertilizer increased the CO₂ emissions with the values of 76488-89787 kg�hm^-2.The peak of CO₂ emission under the BF treatment delayed.In the condition of same nitrogen,phosphorus and potassium ratio additions,the cumulative CH₄ emission under SF and MF treatments was-0.45 and-0.91 kg�hm^-2,respectively,indicating that biomash ash and mushroom dregs promoted the absorption of soil CH₄,and subsequently reducing CH₄ emission.However cow dung significantly increased the mean CH₄ emission.The values of GWP under organic materials added treatmetns were higher than those under CK and F treatments,especially under the CF treatment with the increase of 34.4%relative to the F treatment.While,compared with CK,the lower GHGI values were obtained in the purple soil treated with organic materials coupled with reduced mineral fertilizer,but not affect the vegetable yields significantly.?3?Compared with sole mineral fertilizer,organic materials coupled with miner fertilizer reduced the risk of N₂O emission and nitrate leaching,and increased the the agronomic utilization rate of nitrogen,phosphorus and potassium fertilize to some extent.In the vegetable fields,responses of organic materials coupled with reduced mineral fertilizer on the contents of different soil N types were different.The operations of fertilization and irrigation increased the soil mineral N?NH₄⁺,NO₃^-and NO₂^-?contents,but the peak of NO₃^-content occured behind the peak of NH₄⁺content.There is a reciprocal relationship between soil NH₄⁺and NO₃^-contents.Genrally,the soil mineral nitrogen was dominat with NO₃^-in the condition of high temperature and low moisture content.The soil NH₄⁺,NO₃^-and NO₂^-contents under the F treatment were not low and even higher than those under organic materials added treatments,indicating the increase the risk of N₂O emission and nitrate leaching.The reciprocal relationship between soil DON and MBN contents was found in this study and the soil temperal played a key role in the variations of DON and MBN content.Biomass ash coupled with reduced mineral fertilizer increased the soil DON content,and maize straw coupled with 60%-100%dosage of mineral fertilizer increased the soil MBN content.Compared with maize straw added treatments,the hiher SOC contents were obtained under BF,MF and CF treatments.In the condition of not affecting the vegetable yields significantly,the contents of vegetable nitrate under treatments of organic materials coupled with reduced mineral fertilizer were lower than that in the state specified standard.Meanwhile,the agronomic utilization rate of nitrogen,phosphorus and potassium fertilizer under treatments of organic materials coupled with reduced mineral fertilizer was mostly higher than those under the F treatment,sometimes showed lower.These difference may be caused by the different typles of organic materials with different composition,the C/N ratio,decomposition rate,needing further studing.?4?Cow dung coupled with reduced mineral fertilizer incaresed the stability of soil aggregates in vegetable field.Fertilizeing operation incaresed the the NH₄⁺contribution rate in silt+clay and larger aggregate fractions,and straw coupled with reduced mineral fertilizer increased the NO₃^-contribution rate in larger aggregate.Compared with CK,the amount of 0.25-2 mm aggregate under fertilizer treatments was higher,but the amout of<0.053 mm aggregate lower.The high amount of>2 mm aggregate and low amount of<0.053 mm aggregate was obtained under the CF treatment,as well was the more stability of soil aggregates.The soil NH₄⁺and NO₃^-were mainly distributed in the fraction of<0.053 mm aggregate.Sole mineral fertilizer additions was beneficial to the enrichment of NH₄⁺and NO₃^-in the fraction of>2 mm aggregate with the content of 23.86 and24.73 mg�kg^-1,respectively.For the NH₄⁺contribution rates of different aggregates,mineral fertilizer additions increased the NH₄⁺contribution rates of<0.053 and 0.25-2 mm aggreagtes.The NH₄⁺contribution rates of<0.053 mm aggreagte were above 75%under mineral fertilizer added treatments,and the highest NH₄⁺contribution rate of<0.053 mm aggreagte was under the MF treatment with the contribution rate of 93.81%.For the NO₃^-contribution rates of different aggregates,maize straw additions?except for FS treatment?increased the NO₃^-contribution rate of0.25-2 mm aggregate.Organic materials additions except for cow dung mainly increased the NO₃^-contribution rate of<0.053 mm aggregate.?5?Organic materials coupled with reduced mineral feritlizer reduced the amoA gene copy numbers of soil ammonia oxidizing archaea?AOA?and increased the amoA gene copy numbers of ammonia oxidizing bacteria?AOB?.The microbial community structure of AOA and AOB in soil treated with straw were different from other treatments.Resopnses of different treatments on the abundant and microbial community structure of AOA and AOB were different.In our study,the amoA gene copy numbers of AOB were higher than that of AOA,but they showed significant differences in tolerance and niche to different environmental factors.The highest amoA gene copy numbers of AOA(5.09�10⁴ copies�g^-1)and lowest highest amoA gene copy numbers of AOB(1.36�10⁵ copies�g^-1)were obtained under the F treatment.Maize straw coupled with 60%-70%of full dosage of mineral fertilizer increased the amoA gene diversity of AOA and AOB in purple soil.However,maize straw coupled with full dosage of mineral fertilizer reduced the amoA gene diversity.The microbial community structure of AOB was affected by application of mineral fertilizer.The microbial community structures of AOA and AOB under maize straw added treatments were different from those under others.Soil phosphorus and potassium contents,especially soil available nutrient part play key roles in the community structures of ammonia-oxidizing microorganisms.Besides,SOC and soil pH has important for the microbial community structure of AOB.Soil NH₄⁺and C/N ratio play a key role in the microbial community structure of AOA.?6?Inputs of organic materials increased the diversity of nirS and nirK gene.In the condition of no carbon and no nitrogen additions,soil C/N ratio played a key role in the nirS community structure.Soil pH,SOC,soil N content and soil moisture were the important factors affecting the nirS,nirK and nosZ community structure.For the nirS-,nirK-and nosZ-harboring denitrifiers,the nirK and nosZ gene copy numbers under the 0.6FS treatment increased by 24.6%and 12.9%,respectively,compared with F treatment.The diversity of nirS and nirK communities under 0.6FS and BF treatments were increased.The diversity of nosZ communities was mainly affected by the fertilizer operate.The lower OTUs,aromatic index as well as evenness were only found under the SF treatment relative to CK,indicating lower diversity of nosZ communities under the SF treatment.Using high throughput sequencing techonology for anlyzing the nirS community,Alphaproteobacteria was more responsive to fertilization than Betaproteobacteria in this study.Bradyrhizobium was the dominat bacterium in the microbial community of F treatments and even of the entire nirS-harboring denitrifier,playing a key role in the denitrification process.Phylogenetic trees showed that nirK-harboring denitrifiers were homologous with Alphaproteobacteria and Betaproteobacteria,nosZ-harboring denitrifiers were homologous with Alphaproteobacteria,Betaproteobacteria and Gammaproteobacteria.Soil total N?TN?had importance in the distribution of nirS community structure under the BF treatment.Soil DON played a key role in the distribution of nirS community structure under MF and SF treatments.Soil NH₄⁺played a key role in the distribution of nirK community structure under the F treatment.Soil pH played a key role in the distribution of nosZ community structure under the BF treatment.The structures of nirS,nirK and nosZ communities were affected by the soil pH,SOC,soil N content and soil moisture,as well as soil phosphorus and potassium contents.In summary,the GHG emission in vegetable field has the seasonal variation,and the high emissions were obtained druing April to August and the operation of fertilizing and irrigating.The GHG emissions were closely related to hydrothermal conditions.Maize straw coupled with 30%of mineral fertilizer reduced the soil CO₂ and CH₄ emissions in the vegetable field,as well as mitigating the GHG warming potential.However this treatment has no significant in reducing the soil N₂O emission.In the condition of same nitrogen,phosphorus and potassium ratio additions,biomass ash,maize straw coupled with mineral fertilizer both reduced the GHG warming potential.Organic material coupled with reduced mineral fertilizer reduced the greenhouse gas intensity,but increased the the agronomic utilization rate of nitrogen,phosphorus and potassium fertilize to some extent.It is suggesting that organic material inputs achieved a good trade-off between vegetable yields and GHG emissions even in the condition of reducing mineral fertilizer.The responses of organic materials from different agricultural systems on the abundance of soil N related microbial functional genes?amoA,nirS,nirK and nosZ?and microbial community structure were different.The microbial community structure of AOA and AOB in soil treated with straw were different from other treatments.Soil available nutrients has been major factors in the microbial community structure of AOA and AOB,and soil pH,SOC,soil N content and soil moisture were the important factors affecting the nirS,nirK and nosZ community structure.