| Soil contains the largest carbon(C)pool of any terrestrial ecosystem globally,and soil organic carbon(SOC)is one of the most important C storage pools.The dynamics of SOC stocks and the role that the soil may play in long-term accumulation and sequestration of atmospheric CO2 are of incredible concern because of the acceleration of global warming.Although the SOC storage in agricultural soils accounts for about 10% of the global soil carbon pool,the SOC sequestration in these soils is crucial for soil fertility improvement,crop productivity sustainability,and climate change mitigation.Lots of studies find out SOC sequestration in farmland is affected by agricultural managements such as cropping system,tillage system,and nutrients management including N fertilizer application and crop residue management.At present,straw return is widely conducted meanwhile organic manure use is negligible in Chinese grain crop production in several cropping systems especially in annual winter wheat–summer maize rotation cropping system to improve farmland fertility and maintain sustainable grain crop production;in North China Plains where water and heat regimes are favorable for straw decomposition.However,inappropriate farm practices can increase GHG emissions and reduce SOC sequestration,thereby increasing the carbon footprint(CFP)and affecting soil fertility improvement and climate change.Moreover,the results of various studies about long-term straw return on SOC are controversial,and the methods for increasing SOC stocks in upland soils is challenging.In view of this,the study mainly focuses on crop yields,the SOC sequestration and CFP within the straw return practices combined with N-fertilizer use.Firstly,for the winter wheat monoculture system,spring maize monoculture system and winter wheat–summer maize rotation system,is it more beneficial to SOC sequestration and crop yield with the longer the straw return period? Does SOC and crop yield under straw return combined with different mineral fertilizers application respond differently to straw return durations,soil types,and cropping systems? Secondly,which kind of straw return mode could enhance higher SOC storage and produce lower carbon footprint with optimum yield under winter wheat–summer maize rotation system? And which nitrogen fertilizer rate is suitable to SOC sequestration change and crop yield with low carbon footprint? Based on the above scientific problems,the following hypothesis were proposed,H1: In the three cropping systems of wheat and maize,straw return increases SOC and crop yield in longer period;Different factors;fertilizer treatments,soil types,experiment durations,and cropping system affects the response of SOC and crop yield to straw return.H2: Different straw return modes have different effect on SOC,crop yield and CFP under winter wheat–summer maize rotation system,and there is no significant difference of SOC and yield with reduced nitrogen fertilizer rates.The first hypothesis uses meta-analysis to study,and another hypothesis uses field experiment data for empirical research.The main methods and results obtained in this study are as follows:(1)A meta-analysis study in North China upland crops,quantified the effects of several fertilizer and straw return treatments on SOC storage changes and crop yields,considering different cropping duration periods,soil types,and cropping rotations to establish the relationships of SOC sequestration rates with initial SOC stocks and annual straw C inputs.We used keywords related to SOC stock or SOC content in farmlands using different fertilizer types and straw return to search peer-reviewed,English research articles available on internet data bases at the Northwest A&F University online library(http://www.sciencedirect.com,http://link.springer.com,http://apps.web ofknowledge.com).To be included in the meta-analysis,a study had to meet the following criteria:(a)Initial and final SOC content or stock with known years the study started and ended must be provided in the publication.(b)The work must have been published in peer reviewed publications and not conference proceedings or books.(c)The study must have been a field experiment and not a pot experiment or survey study.(d)The soil sampling depth must have been 0-20 cm but,in order to increase the study count,we included a few papers examining soil depths of 0-30 cm.(e)The experiment must have included at least one of the following treatments: Control(no fertilization [CK]);mineral NP or NPK(nitrogen,phosphorus,potassium)fertilizers;mineral fertilizers + straw(NP+S or NPK+S);and,to see the effect of fertilizer types,mineral NPK plus manure(NPK+M)was optional.Only those long-term studies that were conducted on upland fields in North China and that used conventional tillage practices were included in our analyses.Then a total of 58 peer reviewed studies with 268 and 139 observations has been selected to evaluate SOC change and crop yields,respectively.We compared the SOC response to straw and fertilizer management regimens among different cropping systems,soil types,and cropping durations.Cropping systems were categorized as annual double-cropped winter wheat–summer maize(DC),single-spring maize crop(SM),and single-winter wheat crop(SW).Experimental sites were categorized into three soil types according to the general soil classification of China: black soils,fluvo-aquic soils,and loessial soils.Cropping duration was categorized into four intervals: 1-5,6-10,11-20,and greater than 20 years,with the minimum and maximum durations being 1 and 33 years,respectively.Necessary data from experimental sites,SOC content or stock,soil bulk density,soil sampling depth,cropping duration,cropping system,soil type,and fertilization regimens were obtained from texts and tables of the publications.If data were expressed as figures or charts,number values were extracted using Get Data Graph Digitizer Version 2.26.The result from long-term field experiments showed that,compared with the only application of mineral fertilizers,the combination of mineral fertilizers and organic fertilizers(NPK+S,NPK+M)was more beneficial to SOC sequestration.And there was no significant difference in SOC sequestration under NP+S and NPK.Black soils with higher initial SOC contents and stocks also had lower net SOC stock increases than did soils with low initial SOC stocks in fluvo-aquic and loessial soils following applications of NPK and NPK+S fertilizers.With the applications of NPK and NPK+S,the SOC content of wheat–maize double rotation system(DC)showed an increasing trend,and the SOC changes were higher than wheat monoculture system(SW)and maize monoculture system(SM).Meanwhile the SOC sequestration in four cropping duration showed an increasing trend in NPK and NPK+S,while significant SOC stock increases in the NP and NP+S treatment was limited to the period of 11-20 years.Also,the annual soil sequestration rates and annual straw C inputs of the treatments with straw return(NP+S and NPK+S)were significantly positively related.Moreover,the initial SOC stocks and SOC sequestration rates of those treatments were highly negatively correlated.The annual crop yield of DC system was higher than that of SM system in all fertilization treatments,but in the NPK and NPK+S,the annual crop yields of the SW system are higher than SM system.Thus,long-term straw return integrated with mineral fertilization in upland wheat and maize croplands leads to increased crop yields and SOC stocks.However,those effects of straw return are highly dependent on fertilizer management,cropping system,soil type,duration period,and the initial SOC content.(2)A long-term field experiment has been conducted continuously for 10 years(2008-2018)under winter wheat–summer maize rotation system in the Guanzhong Plain.The experiment included seven straw-return modes,namely wheat straw high stubble retention + chopped maize straw return(WH-MC),wheat straw high stubble retention + chopped maize straw return and subsoiling once every two years(WH-MM),wheat straw high stubble retention + no maize straw return(WH-MN),chopped wheat straw return + chopped maize straw return(WC-MC),chopped wheat straw return + chopped maize straw return and sub-soiling once every two years(WC-MM),chopped wheat straw return + no maize straw return(WC-MN),and a control or no wheat straw return + no maize straw return(WN-MN),to evaluate the effects of different straw return modes on SOC sequestration,crop yield,and CFP.The results showed that SOC change,crop yield,and CFP were significantly influenced by the straw-return mode in the annual wheat–maize season.Soil organic C sequestration rate was positively correlated with cumulative plant-derived C input,which ranged from 29.4 Mg C ha-1 in WN-MN to 100.7 Mg C ha-1 in WH-MC,except for in WCMM,whose SOC sequestration rate was lower than that of single-crop straw-return(WH-MN).Of all the studied treatments,WH-MC produced the highest grain yield and lowest CFP,which were 26% higher and 20.5% lower than those of the control,respectively.However,crop yield and CFP in the individual wheat or maize seasons were only slightly affected by straw-return mode;only WC-MN in the wheat season and WH-MC,WH-MM,and WC-MN in the maize season produced a significantly higher grain yield and lower CFP compared to the no straw return(WN-MN)treatment.The total GHG emissions were higher in the maize season than in the wheat season,whereas the GHG emissions due to farm operations(diesel)were higher in the wheat season.Annual GHG emissions were highest in the WC-MM treatment.Therefore,WH-MC was found to be the most suitable straw-return mode for lowering CFP and enhancing crop yield and SOC sequestration in the winter wheat–summer maize system in the Guanzhong Plain.However,to achieve the coordinated development of agriculture and the livestock industry it is necessary to remove some straw from farmland for animal feed,and WH-MN produced optimum yield and maintained SOC stock with reduced GHG emissions.(3)Similarly,a long term experiment(2008-2018)under the wheat–maize rotation system,a straw return mode of wheat straw high stubble retention + chopped maize straw return(WHMC),and 3 nitrogen fertilizer rates are used(i)farmers practice rate(i.e.187.5 kg ha-1 during maize season,and 150 kg ha-1 during wheat season),(ii)the farmers N rate reduce by 15%(-15% Nrate),and(iii)the farmers N rate reduce by 30%(-30% N-rate),to evaluate their impact on SOC change and CFP,and to determine the optimal N fertilizer rate to maintain or increase crop yield with low CFP.The results show that a higher SOC was sequestered in-15% N-rate and SOC change was directly associated with the accumulative straw C input.While grain yield was not significantly affected by N-fertilizer rates,only slightly higher annual grain yield in-15% N-level.The total GHG emissions were directly related with the amount of N fertilizer.At-15% and-30% N-rate application,CFP was significantly reduced.Therefore,considering the SOC holding capacity,increasing grain yield and reducing CFP,a nitrogen application rate of-15%(about 160 kg ha-1 in maize season,and 130 kg ha-1 in wheat season)could be the optimum N fertilizer level under winter wheat–summer maize cropping system in Guanzhong Plain.Because farmers presently applying excessive N rates as economic productivity against potential N limitation,it will be therefore very important to avoid any yield reductions due to N limitation and respective environmental protection,when encouraging and implementing advanced fertilization strategies.To obtain this,it is recommended to use the optimum N fertilizer rate which improve SOC sequestration with optimum crop yield and lower CFP so as to contribute for climate change mitigation.In conclusion,the following three points can be drawn:(1)In this study,through metaanalysis,it was found that combination of straw return and mineral fertilization could increase SOC sequestration in upland wheat and maize crops.Cropping system,soil types,fertilizer management,cropping duration,initial SOC levels could influence SOC dynamics in relation to straw return.And the soil with low initial SOC content has a higher net SOC sequestration,and the SOC sequestration is significantly increased during the period of greater than 20 years.Although there was a significant positive relationship of straw C input and SOC sequestration,SOC sequestration did not always increase with years of straw return,and it is likely to be restricted by the SOC threshold.In order to achieve a higher SOC sequestration rate,what kind of nutrient management are still to be further studied.(2)A long-term field position experiment has been conducted that straw return mode means the different straw return amount under the wheat–maize double rotation system,which will have different effects on SOC,crop yield and CFP.Meanwhile the straw return mode also means different straw return patterns and tillage system,whether straw return in both seasons(e.g.WH-MC,WC-MC)or straw return in one season(e.g.WH-MN,WCMN),it has effects on SOC,crop yield and CFP,too.In all straw return mode,wheat straw high stubble + maize straw chopping(WH-MC)is the best way to improve crop yield and SOC sequestration.But this model is a full straw return mode,which means no extra straw is used for other purposes.(3)The wheat straw high stubble + maize straw chopping(WH-MC)is adopted in the wheat–maize double rotation system,even if the application of nitrogen fertilizer rate is reduced by 15% on the basis of the recommended nitrogen fertilizer rate,it can maintain the grain yield and increase SOC sequestration,and also reduced CFP.At the same time,the total amount of nitrogen fertilizer in the whole year is less than 300 kg ha-1yr-1,which reduces the environmental pressure.Therefore,this mode provides advantage for reducing the nitrogen fertilizer rate,and reduces the potential for GHG emission. |
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