| Under the background of climate change and straw has not been used effectively,how to implement the carbon sequestration of farmland ecosystem and straw utilization that is a challenge to the current low carbon agriculture.The evaluation of carbon sink/emission reduction potential of farmland ecosystem with straw and biochar returning,is helpful to reveal the mechanism of carbon cycle in farmland ecosystem and to seek regulation pathway.For the study of the response of farmland ecosystem carbon balance to straw and biochar returning,the field experiment conducted on a long-term field experiment in the national monitor station of soil fertility and fertilizer efficiency of purple soils with rape-maize rotation systems.There were five treatments including CK(no organic material),CS(straw),CSD(straw+microorganism),BC(biochar),CSBC(50%straw+50%biochar),which were replicated three times.The soil respiration,soil organic carbon and,active carbon fractions,soil aggregate carbon and field survey data were measured during the growth period of rape and maize in rape-maize rotation systems.Based on the life cycle from the perspective of soil respiration,soil carbon pool and crop carbon pool,and also considering the cost of agricultural inputs,to analyze the farmland ecosystem carbon emissions,carbon sequestration,carbon sink effect and economic and environmental benefits in cropland ecosystem with straw and biochar returning.The main results showed as follows:(1)Straw and biochar returning significantly affected the soil respiration and seasonal variations and the peak.In addition to BC treatment,other treatments promoted soil respiration and cumulative emissions of soil CO2.Soil respiration rate was significantly different under different treatments,the changes in soil respiration rates a single peak curve under all treatments,the seasonal variations in soil respiration rates under rape was 0.12-2.29?mol·(m2·s)-1,displaying an order of CS>CSD>CSBC>CK>BC.Soil respiration was pretty complex in maize season,the seasonal variations in soil respiration rates under rape was 1.02-15.32?mol·(m2·s)-1,the contribution of root respiration(26.49%-32.86%)was significantly lower than CK treatment(53.65%).Heterotrophic respiration could explain 86.50%-93.94% of seasonal variations in the soil total respiration,Straw and biochar returning did not change soil temperature and soil moisture.Soil temperature at 5 cm depth had significant effects on the change dynamics of soil respiration rates,but soil moisture did not.Soil temperature at 5cm depth could explain 82%-94% of the variations in soil respiration.There was no synergistic effect between soil temperature and soil moisture on soil respiration,the dynamic changes of soil respiration rate could be simulated by single factor index function of soil temperature.Overall,soil respiration was promoted significantly by returning of straw,straw + microorganism,straw + biochar,while it was inhibited by returning of biochar.The MBC/SOC,DOC/SOC and ROC/SOC of straw and straw + microorganism were significantly higher than that of biochar and straw + biochar,while soil organic carbon and metabolic entropy(Mq)were lower than that of biochar and straw + biochar.(2)All treatments significantly increased the soil total organic carbon content increased from 31.26% to 107.82%,among which BC is the most significant,and BC and CSBC was significantly higher than that of CS and CSD,biochar is more conducive to soil carbon sequestration.biochar is more advantageous to soil carbon sequestration.The soil active organic carbon content is highest in the pod stage of rape pod and filling stage of maize,besides soil dissolved organic carbon of maize season,each dealing with microbial biomass carbon(MBC),dissolved organic carbon(DOC)and readily oxidizable carbon(ROC)content of the sequence are: CSD> CS>CSBC>BC>CK.The treatment of straw and biochar to soil MBC,DOC and ROC were 125.53-172.02 mg·kg-1?102.73-79.07mg·kg-1 and 2.00-6.01g·kg-1,than control increased by 11.59%-52.80%,50.35%-95.34%,17.76%-56.37%,respectively.In conclusion: straw,straw + microorganism can promote microbial transformation of activated carbon,improve the activity of soil organic carbon,and accelerate the soil microbial biomass carbon turnover rate,significantly reduce the soil microbial metabolism entropy,is beneficial for microorganisms fixing carbon,improve the utilization efficiency of the microbial carbon and soil biological fertility.Despite the microbial activity of biochar was lower,but the soil organic carbon was extremely stable,which is advantageous to the soil organic carbon accumulation,enhance soil carbon sequestration.Soil respiration and metabolic entropy were significantly correlated with soil microbial biomass carbon,dissolved organic carbon,microbial entropy and dissolved organic carbon allocation,but not significantly correlated with soil organic carbon.The soil active carbon was sensitive to straw and biochar returning,and there were strong correlation among MBC,DOC and ROC.Therefore,the dynamic changes of MBC,DOC and ROC can be as a indicator the quality of organic carbon in the short term,it is important to keep the soil biological fertility.(3)Compared with the control,straw significantly increased the content of >2mm aggregates and the increasing rate was up to 94%-117.78%,and straw is significantly higher than biochar.CS and CSD treatment significantly increased the MWD,GMD,R0.25 of water stable aggregates,and decreased the D.The soil organic carbon of< 0.053 mm aggregate was increased highest by straw and biochar returning,and the effect of biochar on soil organic carbon was better than that of straw.CS,CSD increased the relative contributions of coarse particle organic carbon and fine particulate organic carbon within large macro-aggregates(LM),while BC and CSBC increased the relative contributions of different fractions of organic carbon within micro aggregates(MA),it is conducive to improve the stability of soil organic carbon.The results showed that the new carbon of the straw was mainly distributed to the macro-aggregates,and the straw + microorganism also can promote the small macro-aggregates in combination with new carbon,biochar,straw + biochar mainly to enrichment in the microaggregate.Straw and biochar returned to increase the cPOM/fPOM in large macro-aggregates,and decreased the cPOM/fPOM in small macro-aggregates,including the BC treatment was the most significant.The application of straw or biochar into soil accelerated the turnover of large macro-aggregates,which slowed down the turnover of small macro-aggregates and promoted the accumulation of organic carbon storage in the soil.It increased soil carbon sink while improving soil physical properties.(4)Straw and biochar returned could could increase crop yield and crop net primary productivity(NPP),compared with the control treatment increased by 1.49%-3.92%,NPP increased by 4.44%-17.90%,the straw + microorganism is best.Compared with conventional fertilization,straw and straw returning increased carbon inputs(3.14-3.30t·hm-2),reducing carbon losses by burning of straw(5.49-6.12 t·hm-2),while increasing the soil carbon sequestration(0.01-0.18 t·hm-2),therefore,CS,CSD,BC,CSBC system net carbon quantity is positive,is "carbon sink",while CK is negative,as "carbon source",is the net carbon emissions.The net quantity of carbon sequestration with CSD(9.05 t·hm-2)is the highest in rape season.For maize season,despite biochar returning increased the system of carbon emissions in agricultural investment,but it also reduced the soil respiration carbon emission which was major source emissions,the net quantity of carbon sequestration(10.75 t·hm-2)with biochar returning is highest that is conducive to a steady increase in soil carbon sequestration.It steadily increased soil carbon sink while returning biochar.As for the carbon budget items,net primary productivity of farmland ecosystem(NPP)is the most important part of system of carbon sinks,soil respiration carbon emissions is the major carbon emissions source,accounting for up to 44.74% to 92.99%,while the major carbon emissions by burning of straw is as high as 54.25%-84.46% for the control.Nitrogen fertilizer inputs is the main source of carbon emissions from agricultural inputs,which is accounted for 25.81%-42.10%.For BC and CSBC treatment,biochar production equipment power consumption is the main source of agricultural carbon emissions,BC is as high as 53.40%-54.40%.(5)Straw and biochar returning are better economic benefits,the ratio of production to investment is between 1.52 and 2.23,the highest is CS,while BC decreased,indicating that the economic rate of return of biochar is lower than straw.The net profit of rape season between 3012.03 to 3487.01 yuan ·hm-2,the highest is CS(3487.01 yuan ·hm-2),profit rate reached 40.93%,equivalent to the farmland per ton of straw,the net profit of up to 59.08 yuan·t-1,considering environmental benefits,carbon trading income of about 443.09 yuan·hm-2.The total income of each treatment in the maize season(9133.47-9603.01 yuan ·hm-2)is about 3 times of the rape season,the profit margin is 51.92%-54.88%,the CSD economic benefit is supreme.Although economic and ecological benefits of BC and CSBC were significantly lower than other treatments(P < 0.05),but carbon emissions trading is the highest,so environmental benefits is the best;The economic benefits of CK mediated,carbon income is negative,environmental benefits is the lowest.Therefore,the farmland ecosystem of conventional fertilizer for atmospheric carbon is carbon source,it is in a passive position for the global carbon trading market,CS has good economic and environmental benefits,enhancing the utilization of straw returned can improve the quality and production of food,a effective way to reduce the carbon footprint of farmland system and improve the ability of carbon sequestration. |
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