Carbon Sequestration And Its Mechanisms On An Anthrosol Under Long-term Fertilization Regimes In Winter Wheat-summer Maize Cropping System

Soil organic carbon?SOC?pool is the most important and active in the terrestrial ecosystem.Increasing SOC storage is not only beneficial for improving soil properties,increasing crop productivity,but also for mitigating global climate change.SOC sequestration is influenced by tillage practice,cropping system,fertilization,and so on.Of which,the impact of fertilization management practices on SOC is the most direct and significant.In addition,the change of SOC is a long and slow process,long-term fertilization experiment is an effective way to study the characteristic and mechanism of SOC sequestration.The Guanzhong Plain in Shaanxi Province is one of the most important crop production areas in China.Anthrosol is the main arable soil in this area,where the main cropping system is winter wheat-summer maize,double cropping system.This study is based on a 35-year fertilization experiment,to investigate 1)effects of long-term fertilization regimes on crop yields,OC input,SOC storage,SOC sequestration rate,efficiency and potential;2)changes of labile OC fractions?particulate organic carbon,POC;light fraction organic carbon,LFOC;labile organic carbon,LOC;microbial biomass carbon,MBC?and responses of SOC functional groups to long-term fertilization regimes;3)soil aggregation and distribution of OC and total nitrogen?TN?in water stable aggregates and silt+clay fraction;4)ifferences in physical protection of OC by macroaggregate fractions and 5)mineralization characteristics of SOC pool in bulk soils and aggregates.The long-term experiment involved included nine treatments,i.e.three levels of organic manure,three levels of chemical nitrogen combined with phosphorus,and their cross combinations?CK,N₁P₁,N₂P₂,M₁,M₁N₁P₁,M₁N₂P₂,M₂,M₂N₁P₁ and M₂N₂P₂?.The main results obtained were as follows:?1?Characteristics of carbon sequestration of an Anthrosol under long-term fertilization regimesLong-term application of contrasting fertilization regimes significantly improved wheat and maize yield,and the yield increase was most significant under MNP treatments.The yield difference resulted in differences of crop residue input into soil.The amount of OC by wheat residue ranged from 6.89 to 32.18 Mg ha^-1,by maize residue from 16.88 to 37.92 Mg ha^-1,and by manure from 39.65 to 76.51 Mg ha^-1.Fertilized treatments markedly increased OC input over CK by 79.3-515.8%.Consequently,there were significant differences in SOC storage among treatments(20.56 Mg ha^-1 to 31.11 Mg ha^-1)after 35 years,and fertilized treatments increased SOC by 4.9-60.7%than CK.Moreover,the SOC sequestration rate showed an augment tendency with the increase of OC input,as shown by was the lowest rate under CK,and the highest one under M₂N₁P₁.However,SOC sequestration efficiency declined with OC input,as shown by the highest value under CK,and the lowest one under M₂N₂P₂.In addition,the results of Jenny model showed that maximum SOC storage ranged from 15.71 Mg ha^-1 under CK to 36.54 Mg ha^-1 under M₂N₁P₁.If maintaining the current fertilization regimes,SOC storage has reached the equalibrium under CK,while other fertilization treatments still had potential to sequestrate SOC from 6.53 Mg ha^-1 to 18.73 Mg ha^-1,and the highest potential was under M₂N₁P₁.Therefore,fertilization regimes not only promoting SOC sequestration,but also affected SOC sequestration potential.?2?Changes of soil labile OC fractions and composition of SOC functional groupCompared with CK,long-term application of synthetic fertilizers had no effect on different labile OC fractions'concentrations at 0-10 cm and 10-20 cm soil depths.While application of manure alone significantly increased labile OC fractions'concentrations at two depths by 137.6-258.9%,44.5-74.5%,45.5-71.2%and 71.4-194.9%,respectively for POC,LOC,MBC and LFOC after wheat seasonwere,and by 158.6-191.4%,65.5-84.4%and36.5-229.1%,respectively for POC,LOC and MBCafter maize season.Similarly,application of manure combined with synthetic fertilizers also substantially increased labile OC fractions'concentrations at two depths.Additionally,long-term application of fertilizers also influenced the sensitivity index?SI?of labile OC fractions,which was the highest for POC.Thus,POCcan be used as a sensitivity indicator for SOC changes in Anthrosol.The SOC mainly contains carboxylic acid,alcohols,phenol,polysaccharides and other substances.However,functional groups'absorbing intensities of SOC at 0-10 cm depth was stronger than those at 10-20 cm depth.Compared with CK,application of synthetic fertilizers did not affect functional groups absorbing intensities,but application of manure alone or combined with synthetic fertilizers markedly increased functional groups absorbing intensities after wheat and maize seasons by 64.3-200.0%and 22.2-209.0%for phenols and alcohols and other aliphatic compounds,by 33.3-83.3%and 26.1-82.6%for alkane compounds,by12.8-96.6%and 7.8-56.3%for carbohydrate,and 28.6-92.9%and 20.0-66.7%for aromatic compounds,respectively.The increments of carbohydrate were lower than the other compounds,which implied that the SOC stability of chemical composition was more after long-term application of organic manure.?3?Distribution of aggregate and its associated OC and TN under long-termfertilization regimes of an AnthrosolAggregates in size of 0.25-2 mm accounted for more than 50%,the microaggregates?0.053-0.25 mm?constituted about 30-40%,the large macroaggregates?>2 mm?and silt+clay fractions?<0.053 mm?constituted the lowest percentage?less than 10%?at two soil depths.Long-term fertilization regimes and different crops affected distribution of aggregates to various degree.After wheat season,application of synthetic fertilizers markedly decreased percentages of<0.053 mm at 0-10 cm soil depth,while application of manure alone significantly decreased percentages of>2 mm aggregates and<0.053 mm.Application of manure combined with synthetic fertilizers significantly decreased percentages of<0.053mm.After maize season,application of synthetic fertilizers significantly increased percentages of>2 mm aggregates at two depths.Application of manure alone significantly increased percentages of>2 mm aggregates at 0-10 cm depth.Application of manure combined with synthetic fertilizers significantly decreased percentages of 0.25-2 mm aggregates at two depths.Although fertilization regimes changed distribution of aggregates,they did not affect aggregates'stability?MWD value?.The OC and TN concentrations of aggregates and silt+clay fractions at 0-10 cm depth were significantly higher than 10-20 cm depth?20%averagely?.Additionally,the OC and TN concentrations in the aggregates of>2 mm,0.25-2 mm and 0.053-0.25 mm were similar,but higher than those in the<0.053 mm.Generally,all fertilizer treatments had no effect on the OC and TN concentrations in<0.053 mm relative to CK.Long-term application of chemical fertilizers did not affect OC and TN concentrations but applying manure alone or combined with synthetic fertilizers significantly increased OC concentrations in aggregates by.About over 40-50%of SOC and TN concentrations were distributed in 0.25-2 mm aggregates,less than 10%in<0.053 mm size.?4?Differences in physical protection of organic carbon by macroaggregate under long-term fertilization regimes in an AnthrosolLong-term application of synthetic fertilizers had no effect on the OC contents of any macroaggregate size fractions?coarse particulate organic carbon cPOC,fine particulate organic carbon fPOC,intra-microaggregate particulate organic carbon iPOC and mineral associated organic carbon MOC?.However,incorporation of manure alone or combined with synthetic fertilizers greatly enhanced cPOC and iPOC contents by 140.6-193.8%?0-10 cm?and 47.8-85.5%?10-20 cm?and by 166.3-464.4%?0-10 cm?and 94.8-144.5%?10-20 cm?after wheat season,by 173.7-338.0%?0-10 cm?and 215.1-244.9%?10-20 cm?,and by127.7-240.7%?0-10 cm?and 106.0-129.8%?10-20 cm?after maize season respectively.Application of high level of manure significantly increased fPOC contents at 0-10 cm depth by 253.8-350.9%?after wheat?and 482.1%?after maize?.However,applying fertilizers did not affect MOC contents.In addition,SOC concentration was significantly and positively correlated with all macroaggreagte fractions associated OC contents,especially for the iPOC fraction,which implied that OC sequestration difference occurred primarily in the physically protected OC fraction contained in macroaggregates.The relationship between crop yields and SOC concentration,cPOC,fPOC and iPOC contents can be well described by quadratic plus plateau models.The critical SOC,cPOC,fPOC and iPOC values required to obtain a high crop yield were around 9 g kg^-1(or 25 Mg ha^-1),1.6 g kg^-1,1.0 g kg^-1 and 4.0 g kg^-1,respectively.To sustain crop productivity and a high level of C sequestration efficiency,C input level and duration should be adjusted based on the critical value at a given soil.?5?Effects of long-term fertilization regimes on stability of SOC pools in AnthrosolThe incubation experiment of bulk soil and aggregates?>0.25 mm,0.25-2 mm,<0.25mm and 0.053-0.25 mm?showed that CO₂-C respiration rates of bulk soils and aggregates decreased with the incubation time.The CO₂-C respiration rates were the highest in 0-7 d,the lowest after 51 d,and the middle from 8 d to 50 d.The manure treatments gave the highest CO₂-C respiration rates,followed by synthetic fertilizers treatments,while the lowest rates occurred in the control treatments across bulk soils and all aggregates.In addition,macroaggregates and microaggregates yielded approximately the same CO₂-C respiration rates,but they all smaller than that from bulk soil samples irrespective of treatments,which implied that aggregates had the similarity protection for OC decomposition.Fertilization regimes also had effect on rates of OC decomposition?i.e.cumulative CO₂ respiration/OC storage?in bulk soils and aggregates.Long-term application of synthetic fertilizers significantly increased rates of OC decomposition in<0.25 mm aggregates at 0-10 cm depth,application of manure alone significantly increased rates of OC decomposition in bulk soils at two depths and in>0.25 mm and<0.25 mm aggregates at 10-20 cm depth,application of manure combined with synthetic fertilizers significantly increased rates of OC respiration in<0.25 mm aggregates at 0-10 cm depth.The rates of OC decomposition in macroaggregates were similar with that in microaggregates,but they were smaller than that in bulk soils.The results of two-component model showed that there were significant differences between active C pool?C_a?and slow C pool?C_s?in bulk soils and aggregates.Compared with CK,application of synthetic fertilizers had no effect on C_a and C_s in bulk soils and aggregates.However,application of manure alone or combined with synthetic fertilizers significantly increased pool sizes of C_a and C_s in bulk soils and aggregates at two depths.The ratio of C_a to potential mineralization pool?C_a+C_s?were ranged from 14%to 34%in bulk soils,and from 11%to 28%in aggregates at two depths,which implied that about 70%OC sequestrated in slow pool.Fertilization treatments had little effect on C_a/?C_a+C_s?in bulk soils.Application of synthetic fertilizers did not affect C_a/?C_a+C_s?in aggregates,application of manure alone significantly increased C_a/?C_a+C_s?in>0.25 mm aggregates at 0-10 cm depth,but it decreased C_a/?C_a+C_s?in 0.25-2 mm aggregates at 10-20 cm depth.Application of manure combined with synthetic fertilizers markedly increased C_a/?C_a+C_s?in all aggregates at0-10 cm depth,the increments were ranged from 38.9%?0.25-2 mm?to 154.5%?>0.25 mm?.Overall,with the increase of C saturation,C_a in macroaggregates was more obviously increased,and this may increase the turnover rate of OC,and improve supplying capacity of soil nutrients.In summary,long-term application of manure combined with synthetic fertilizers increased crop yields,OC input,SOC sequestration rate and potential.These are related to changes in soil aggregation,improving stability of SOC chemical structure,and increasing physical protection of OC in microaggregates occluded in macroaggregates under long-term application of manure.