| The Intergovernmental Panel on Climate Change（IPCC）launched"the Special Report on Global Warming of 1.5℃"in its Sixth Assessment Report,which emphasized"limiting global warming at 1.5°C instead of 2°C or any higher temperature".Excessive emission of Greenhouse Gases such as carbon dioxide caused by human activities is one of the main causes of global warming.Farmland soil carbon pool is the most active part of the global carbon pool,and fertilization measures have a great impact on it,so it is of great significance to clarify the impact of different fertilization measures on the carbon cycle of the farmland ecosystem.Dry farming in the Loess Plateau plays a very important strategic role in China’s agriculture.At present,the main fertilization measures for dry farming are to increase nitrogen application and organic fertilization.However,the effects and their formation mechanism of different fertilization measures on carbon balance and the stability of soil carbon pool in dry-farming cornfields has been seldom analyzed thoroughly,and the effect of organic fertilization mode on soil carbon emissions at regional scale has been still unclear,and how to comprehensively evaluate the effect of fertilization measures from the perspective of environmental and economic benefits has not been studied completely.Based on above scientific issues,firstly,the effect and mechanism of organic fertilization measures on soil CO2 emissions at regional scale were discussed with Meta-analysis method.Secondly,relying on two long-term positioning experiments including the fertilization experiment of organic materials with equal amount of nitrogen input in corn farmland[no fertilizer（CK）,nitrogen fertilizer(NF,200 kg N·m-2),organic fertilizer(OM,6000 kg·hm-2),straw(ST,28500 kg·hm-2),and organic fertilizer combined with inorganic fertilizer(OMNF,3000 kg·hm-2+100 kg N·hm-2)]and the split-plot experiment of different levels of nitrogen fertilizer[principal factor,N0(0 kg N·hm-2),N1(100 kg N·hm-2),N2(200 kg N·hm-2),N3(300kg N·hm-2)]combined with different nitrogen application methods[side factor,T1（1/3 base fertilizer+2/3 jointing stage fertilizer）,T2（1/3 base fertilizer+1/3 jointing stage fertilizer+1/3large trumpet stage fertilizer）)in the dry farming area of the Loess Plateau in central Gansu in2012,and by measuring the relevant indicators in 2017 and 2018,combining with the yield data from 2014 to 2018,this study researched the effects of different fertilization measures on soil CO2 emissions and net carbon balance of farmland ecosystem,and analysed the response of soil carbon fractions,soil carbon pool stability and soil carbon pool management index to different fertilization measures.The study also expounded the mechanism of soil carbon pool stability and soil CO2emissions,and clarified the yield stability and ecological service value of different fertilization measures.The research results of the study can provide data supports for carbon sequestration and emission reduction in the farmland ecosystem and the formulation of ecological compensation policies,and provide a theoretical basis for the promotion of"chemical fertilizer reduction and organic fertilizer replacement"environmentally friendly agriculture.The main conclusions are as follows:1.Meta-analysis showed that organic fertilization measures increased soil CO2 emissions,so it was suggested to apply organic-inorganic fertilizer combination or the combined application of nitrogen fertilizer,organic fertilizer and slow-release fertilizer in Northern China.Organic fertilizer application significantly increased the CO2 emissions in farmland soil than the non-fertilizer and inorganic fertilizer application during the growth period,and there had no significant difference in soil CO2 emissions among northeast,north and northwest regions（P>0.05）.The application of chicken manure could increase the soil CO2emissions than other organic fertilizers,and it was not recommended to apply chicken manure alone in large quantities.The soil CO2emissions of farmland in the gray desert were relatively high when organic fertilizer was applied.The soil CO2emissions of farmland were positively correlated with the annual average temperature and negatively correlated with the annual average precipitation.Compared with a single application of organic fertilizer,organic-inorganic fertilizer combination and combined application of nitrogen fertilizer,organic fertilizer and slow-release fertilizer had decreased soil CO2 emission from farmland.However,there was no significant difference between the combined application of organic fertilizer,nitrogen fertilizer and slow-release fertilizer and no fertilization（P>0.05）.2.Organic fertilization and the added amount of nitrogen application increased soil CO2emissions to some degrees,but they also reduced the carbon emission efficiency of crops,enhanced carbon sink function of farmland.With different fertilization measures,the soil CO2 emission flux increased first and then decreased along time in the whole year and growth period,but there was no obvious difference among treatments in the leisure period.With different fertilization measures,soil CO2 emissions and crop carbon emission efficiency of NF and OMNF treatments were significantly lower than those of ST and OM treatments（P<0.05）.The 2-year average value of net carbon emission from farmland showed that ST,NF,OMNF and OM treatments were all carbon sinks,and among them ST treatment was the most efficient one,OMNF was the second,and CK treatment was the carbon source.The nitrogen application period,interaction of nitrogen application period and nitrogen application rate had no significant effects on the emission average flux and emission of soil CO2（P>0.05）.Soil CO2 emissions increased with the rise of nitrogen application rate,but there was no significant difference between N2 and N3 treatments（P>0.05）.The carbon emission efficiency of N2 and N3 treatments was significantly lower than that of N1 and CK treatments（P<0.05）.The 2-year average value of net carbon emission from farmland showed that N3,N2 and N1 treatments were all carbon sinks,while CK treatment was a carbon source.3.Organic fertilization and the increase of nitrogen application improved corn farmland organic carbon and its fractions to varying degrees,reduced soil carbon pool stability and increased the management index of soil carbon pool in corn farmland.Soil organic carbon and active organic carbon fractions in 0-30 cm soil layers[soil-free particulate organic carbon（FPOC）,occluded particulate organic carbon（OPOC）,particulate organic carbon（POC）,microbial biomass carbon（MBC）and readily oxidizable organic carbon（ROOC）]could be improved to varying degrees（P<0.05）by the different fertilization measures,and the measures of organic fertilization（OM,ST,OMNF）and the increase of nitrogen application（N2,N3）were significantly higher than CK treatment（P<0.05）.The nitrogen application period,interaction of nitrogen application period and nitrogen application rate had little significant effects on soil organic carbon and its fractions（P>0.05）.The organic carbon fractions of different fertilization measures were mainly mineral-associated organic carbon（MOC）.The carbon pool stability of ST and OM treatments was significantly lower than that of NF and CK treatments（P<0.05）,the OMNF treatment was in the middle,on the contrary,the carbon pool management index of ST,OM and OMNF treatments was significantly higher than that of NF and CK treatments（P<0.05）.The carbon pool stability in N2 and N3 treatments was significantly lower than that in N1 and CK treatments（P<0.05）,while the carbon pool management index of N2 and N3 treatments was significantly higher than that in N1 and CK treatments（P<0.05）.4.The regulation mechanism of soil carbon pool stability and soil CO2 emission was differentThe total explanatory degree of environmental factors on soil carbon pool stability was75%,and the total effect was-0.63 under different fertilization measures,among which mineral-associated organic carbon（MOC）had the greatest influence Mineral-associated organic carbon（MOC）positively affects soil carbon pool stability,POC,ROOC,soil water content and soil temperature,which negatively affected soil carbon pool stability,while soil bulk density and invertase had less influence.Soil temperature affected the stability of soil carbon pool through the affecting ROOC and invertase,while soil water content affected the stability of soil carbon pool through affecting POC,ROOC and invertase.The total explanatory degree of environmental factors on soil CO2 emission flux was 52%,and the total effect was 2.84,among environmental factors,soil temperature had the greatest influence,soil water content and urease had greater influence,while microbial biomass carbon（MBC）,ROOC and invertase had less influence.Soil temperature affected soil CO2emission flux through affecting urease,invertase and ROOC,while soil water content affected soil CO2emission flux through its influence on MBC and ROOC.5.Organic fertilization and the increased nitrogen application could affect grain yield stability and farmland ecological service value to varying degreesAccording to the evaluation results of grain yield for many years,different fertilization measures had significant effects on grain yield and water use efficiency of dry-cultivated corn in Loess Plateau（P<0.05）,among which OMNF and NF treatments had the highest average grain yield,OMNF treatment had the highest water use efficiency,the best stability of grain yield and the best potential of increasing yield.Judging from the average total value of ecological services in two years,the total ecological service value of ST treatment was the largest value,followed by OMNF treatments.Among them,the service value of agricultural products of OMNF treatment was greater than that of ST treatment.The nitrogen application period,interaction of the nitrogen application period and nitrogen application rate had no significant effect on grain yield（P>0.05）.There was a significant difference in average grain yield and water use efficiency among different treatments under different levels of nitrogen fertilizer（P<0.05）.N2 treatment had the best grain yield stability and yield-increasing potential,followed by N3 treatment.There had little difference between total ecological service value of N3 treatments and that of N2 treatments,but the total ecological service values of N3 and N2 treatments were significantly higher than those of N1 and CK treatments（P<0.05）.In conclusion,organic fertilizer combined with nitrogen fertilizer treatment（OMNF）can not only increase significantly crop yield and improve carbon pool management index and the carbon pool stability,but also reduce soil carbon emission efficiency and promote soil carbon sink and ecological service value,So it is a more appropriate fertilization measure of corn farmland in the Loess Plateau of central Gansu.Similarly,with the application mode of nitrogen fertilization alone,the fertilization mode of 200 kg N·hm-2 and 1/3 base fertilizer+2/3jointing stage fertilizer is recommended.Under the background of"reduction of chemical fertilizer and replacement of organic fertilizer",it is more recommended to apply the combined application of nitrogen fertilizer,organic fertilizer and slow-release fertilizer. |
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