| The increasing application rate of fertilizer,which is one of the most important reason for greenhouse effect,could be a risk of environmental pollutions and cause ammonia losses and greenhouse gas emission.In order to solve all these problems,lots of studies have been applied such as slow-release fertilizer,urease and nitrification inhibitors in wheat,corn,rice and other crop productions and the effect were significant.But only a few studies contain application effects of coated urea and urease and nitrification inhibitors on ammonia and greenhouse gas emissions and cotton yield from a subtropical cotton field.As an important economic crop,cotton has been cultivated in a huge area around the world(up to 31.2 million hm2).The environmental issue which caused by fertilizer application from cotton production is no longer be ignored.In addition,as the world's major carbon stock and carbon sources,the carbon cycle and carbon balance of agricultural field directly affects the total global carbon dioxide emissions.Agricultural management such as crop rotation,cover crop and fertilization can affect the input of soil organic carbon directly,then effect the basic structure of soil carbon pool by altering the formation and transformation of aggregates and then effects SOC sequestration,Greenhouse Gas emission and the global climate.Unfortunately,cotton monoculture was the most pervasive cultivation because of high profitability of cotton.So the scientific literature about the impact of crop rotation,cover crops and fertilization on SOC of cotton field was rather scanty.In order to elucidate the influence of coated urea,urea plus urease inhibitor(NBPT),urea plus nitrification inhibitor(DCD)on ammonia,greenhouse gas emissions,cotton yield,structure of soil microbial community and the effect of long-term crop rotation,winter cover crops,fertilization on content change of SOC in bulk soil,aggregates and the molecular information on the composition of SOC in American cotton production,two typical cotton field were selected which located in the state of Louisiana and Alabama(traditional cotton-producing area of USA): the first experimental field of greenhouse gas emission located at the Central Experiment Center of Louisiana State University,LA and the other long-term cotton rotation experiment established in the boundary between Coastal Plain and Southern Piedmont Plateau Physiographic regions of the eastern part of Alabama,Auburn University.In 2013-2014,a two-years field experiment consisting of five treatments including Check(unfertilized),Urea,Polymer-Coated Urea(ESN),Urea+NBPT and Urea+DCD was conducted to investigate the effects of these treatments on ammonia and greenhouse gas emissions from a subtropical cotton production system and to measure the microbial community richness by using PLFA method.The main results were as fellows:1.The influence of different fertilization treatments on ammonia and greenhouse gas emissionsIn 2013-2014,different treatments showed similar trend of NH3 and N2 O emission,the major emissions of NH3 and N2 O were detected on Day 4 after fertilizer application and all volatilization dropped to background level in 40 days.It showed higher emission of N2O-N than NH3-N and the emission factors(EFs)for Urea,ESN,Urea+NBPT,Urea+DCD were 1.9%,1.0%,0.2%,0.8% for NH3-N,and 8.3%,3.4%,3.9%,1.0% for N2O-N,respectively.Compared to regular urea,NBPT was the most effective in reducing NH3-N volatilization(by 52% and 62%,respectively in 2013 and 2014);whereas DCD was the most significant treatment in mitigating N2O-N emissions and its two-years average reduction was 75%;Polymer-coated urea(ESN)and NBPT also significantly reduced N2O-N losses(both by 52%)over urea.However,there were no significant effects of different N treatments on CO2-C fluxes.For,among all N treatments,only treatment urea showed a significant increase of CH4-C fluxes than treatment check.2.The influence of different fertilization treatments on cotton yield,Global Warming Potential(GWP)and Greenhouse Gas Intensity(GHGI)In 2013,Polymer-Coated Urea(ESN)could increase the cotton yield significantly.Compared with treatment urea,application of coated urea increased the yield of seed cotton and lint by 10% and 15.4%,respectively.On the contrary,the seed cotton and lint of this treatment in 2014 decreased by 18% and 14%.Application of urease inhibitor(NBPT)and nitrification inhibitor(DCD)did not show a positive effect on cotton yield,the seed cotton of these two treatments reduced 8% and 12% in 2013 and 2014,respectively.In 2013,the Global Warming Potential(GWP)of treatment Urea,treatment Coated Urea and treatment Urea+NBPT increased 144,26.6 and 122 times more than treatment Check.However,there was no significant difference between treatment Urea+DCD and treatment Check.In 2014,only the GWP of treatment Urea increased 4.5 times than treatment Check(p < 0.05).For the Greenhouse Gas Intensity(GHGI),the permutation of these treatments was treatment Urea>treatment Urea+NBPT>treatment Coated Urea>treatment Urea+DCD>treatment Check.Compared with treatment Check,the GHGI of treatment Urea,treatment Coated Urea,treatment Urea+NBPT and treatment Urea+DCD increased 76,31,71 and 9 times more.On the contrary,there was no significant difference between the GHGI of all these treatments in 2014.3.The influence of different fertilization treatments on soil microbial communityBoth of these fertilization treatments could increase the microbial biomass except treatment Urea+NBPT.Compared with treatment Check,treatment Urea,treatment Coated Urea and treatment Urea+DCD increased the total microbial biomass by 15.7%,14.9% and 35.2%.Bacteria population of treatment Urea rised 30.5% than treatment Check,meanwhile,bacteria population of treatment Coated Urea,treatment Urea+NBPT and treatment Urea+DCD only increased 16%,15.4% and 17.9% than Check.Compared with treatment Check,all other treatments had significant difference on Gram Positive(G+)/Gram Negative(G-)ratio,except treatment Urea+DCD.The G+/G-ratio of treatment Urea,treatment and Urea+NBPT significantly decreased by 17% and 36.7%,respectively.Only G+/G-ratio of treatment Coated Urea increased 50% than treatment Check.All these treatments had no promotion to the population of fungi,except treatment Urea+NBPT which increased 89% of fungi population significantly.Bacteria/fungi retio of treatment Urea was higher than treatment Check,on the contrary,the Bacteria/fungi retio of treatment Coated Urea and treatment Urea+NBPT were much lower than Check.There was no significant difference between Bacteria/fungi retio of treatment Urea+DCD and treatment Check.Cyc/precursor retio was selected to monitor the stress level,the cyc/precursor retio of treatment Coated Urea,Urea+NBPT and Urea+DCD were 35.3%,42.2% and 58.8% lower than treatment Check.In addition,all treatments had no significant effect to the population of actinomycete.In this study,a long term experiment system was selected which contain five different treatments: 1)Continuous cotton with no winter legume/no N(C);2)Continuous cotton with winter legume(C+WL);3)Cotton-corn rotation with winter legume(C-C+WL);4)Cottoncorn rotation with winter legume and fertilizer N as ammonium nitrate at a rate of 134 kg N ha-1 yr-1(C-C+WL+N);and 5)Continuous cotton only with 134 kg N ha-1 yr-1 ammonium nitrate(C+N)was set.In this part,effects of different field practices on physical,chemical and molecular fractions of SOC were evaluated.Results were as follows:1.The influence of long-term field managements on content of SOC,TN,HC and DOC in bulk soilLong-term field management such as crop rotation,winter cover crops and N fertilization would increase Soil organic carbon(SOC),total nitrogen(TN),and acid-hydrolysis carbon(HC)significantly.Compared with treatment C,treatment C+WL,C-C+WL,C-C+WL+N and C+N increased content of SOC significantly by 83%,164%,149% and 87%.These treatments increase more significantly in TN(by 120%,223%,199% and 99%)and HC(by 153%,315%,300% and 146%).The DOC of these treatments ha no significant difference with treatment C,except treatment C+N which significantly increased DOC by 37%(p<0.05).2.The influence of long-term field managements on soil aggregates distributionAll of these treatments had benefit to soil aggregates distribution in soil macro aggregates(>250 ?m),treatment C+WL,C-C+WL,C-C+WL+N and C+N increased 37.3%,156%,86.8% and 29% macro aggregates than treatment C.However,there was no significant influence in soil micro aggregates(53~250 ?m)and only treatment C+N increased 68.2% of silt and clay.Compared with treatment C,the SOC of different treatments in macro aggregates(>250 ?m)had significant difference.Treatment C-C+WL,C+WL,C-C+WL+N and C+N increased 311%,175%,298% and 153% of SOC,but only treatment C-C+WL had 133.4% TN increase than treatment C.In micro aggregates level,only treatment C-C+WL and C-C+WL+N had 2.95 and 2.36 times more SOC and 2,1.5 times more TN than treatment C(p<0.05).There was no significant difference between DOC of these treatments in all aggregates level.Treatment C+WL,C-C+WL and C-C+WL+N all increased soil HC in macro aggregates and micro aggregates but in silt and clay level,all treatments showed significant increase than treatment C.The SOC percentage of treatment C in macro aggregates,micro aggregates and silt plus clay were 15.6%,18.7% and 65.4%,respectively.Treatment C+WL,C-C+WL,C-C+WL+N and C+N increased the percentage of SOC by 87.8%,94%,84.6% and 69.2% in micro aggregates.In micro aggregates,treatment C+WL,C-C+WL and C-C+WL+N increased 43.3%,86.6% and 54.5% percentage of SOC than treatment C.However,in silt plus clay level the percentage of SOC of all treatments were significant lower than treatment C.3.The influence of long-term field managements on molecular stabilization of organic matter compositionAccording to the result of Py-GC/MS Analysis,all of these field managements could enhance the diversity of SOM composition by enhanced biomass of crops and microbial activities,and then increase the carbon sequestration of soil.Compared with treatment C,application of N fertilizer introduced 14 different compounds which composed basically of 6 kinds of aromatic compounds(Benzene,Toluene,Ethyl benzene and so on),3 kinds of polysaccharides(Furan and so on),3 kinds of N-containing compounds and 5 kinds of aliphatic.Therefore,application of N fertilizer mainly to increase the input of aromatic compounds and aliphatic which came from the biomass of crops and soil microbial biomass.Treatment with winter cover crops showed 22 kinds of compounds than treatment C.The Py-GC/MS result indicated that winter cover crops introduced rhizobium into soil and increased the biomass of crops.Treatment C-C+WL and C-C+WL+N showed the most compounds in Py-GC/MS analysis,but N fertilization had no effect to add new kinds of compounds.In this study,crop rotation,winter cover crops and N fertilization could input different kind of compounds which came from plant and microorganism.This kind of organic matter and microbial compounds were very important to Soil organic carbon sequestration.Overall,both of these N process inhibitors and polymer-coated urea could be used as a mitigation strategy for reducing N2 O or NH3 emission(urease inhibitor NBPT for reducing NH3 emission)of cotton production in the traditional cotton-producing area in the south of United States.And the agricultural managements such as cotton-corn rotation,winter cover crop and N fertilization could be an effective strategy to improve the ability of soil carbon sequestration. |
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