| Soil is an important reservoir of carbon(C),and a component of the global C cycle(Davidson,Trumbore,and Amundson 2000).Maintenance of soil organic carbon(SOC)is essential to sustain soil fertility,productivity,and quality(Katyal,Rao,and Reddy 2001).Soil organic carbon(SOC)and inorganic carbon(SIC)are important reservoirs of carbon on the global lands.Soil organic carbon,as a key index for soil fertility and carbon sequestration,has gained recognition(Lal 2001,2004).Soil organic carbon(SOC)and its fractions,being at the core of soil fertility,play a key role in optimizing crop yield and improving soil quality.However,the responses of SOC protection mechanisms under long-term fertilization are still poorly understood,especially in subsoils.The present study investigated the content and profile distribution of distinct SOC protection mechanisms and contents of SOC,TN and TP and the stochiometric relation of C:N:P on a complete profile(0-100 cm)of Eumorthic Anthrosols in Northwest China after chemical and manure application using combined physical and chemical fractionation methods.Five tretaments were used in the experiment un-fertilized control(CK);recommended applied dose of N(N);applied dose of N with P(NP):NPK+manure(MNPK)and NPK+straw(SNPK).The soil was collected from 0-100 cm in five splits as(0-20,20-40?40-60,60-80 and 80-100 cm)and was fractionated into six constituent fractions by means of a combination of physical and chemical fractionation methods.The first part of the research focuses on the total SOC,TN and TP content as well as the C/N/P stichometry across the whole soil profile.After the analysis of all the content and stichometry results revealed that the combined application of manure(M)with mineral fertilizer NPK(MNPK)significantly(P<0.05)increased the total SOC content,total nitrogen and total phosphorus of bulk soil in both topsoil and subsoil compared to the unfertilized control(CK).The total SOC content under MNPK in the topsoil layers(0-20 and 20-40 cm)were greatest,20.79 and 16.34 g C kg-1,respectively.In the three subsoil layers(40-60,60-80 and 80-100 cm)total SOC was 15.25,8.77 and 6.19 g C kg-1,respectively(Fig.31).The greatest increase under MNPK compared with the unfertilized control(CK)was 38.5%,found in the second layer of the subsoil(60-80 cm).The total nitrogen(TN)content under MNPK was increased 70%in the topsoil layers(0-20 and 20-40 cm)compared to unfertilized control(CK).It can be observed clearly that the total nitrogen(TN)was significantly higher in the whole soil profile compared to CK and other treatments lacking manure averaging between(1.31-0.56 g N kg-1).The highest increase of TP in the top soil layers(0-20 cm and 20-40 cm)under MNPK was(174 and 160%)respectively.However,the increase in the TP content under MNPK compared with CK in the three layers of sub soil was 139,98 and 96%.Furthermore,it was noted that the addition of straw with mineral fertilizer(SNPK)followed the increasing trend of MNPK in the whole soil profile for TC,TN and TP contents.Furthermore,all treatments showed a decreasing trend descending through the topsoil for TC,TN and TP contents.The following order for TC,TN and TP content under different fertilizer treatments was observed in both the topsoil and subsoil across the whole soil profile:MNPK>SNPK>NP>N>CK.There were significant variations found throughout the profile among the various fractions and protection mechanisms.Results revealed that MNPK increased the SOC content of the unprotected cPOC fraction by 60.88%in the 0-20 cm layer and 61.54%in the 20-40 cm layer,while SOC content was low,13.20%,16.79%and 22.39%,in the subsoil layers(40-60,60-80 and 80-100 cm,respectively)compared with the control(CK).Higher OC content,6.36 g C kg-1 and 6.23 g C kg-1,was found for MNPK and SNPK,respectively,in the topsoil layers of physically protected micro-aggregates(?agg).Higher SOC content,1.41 g C kg-1 and 1.31g C kg-1,was found in the occluded particulate organic carbon(iPOC)compared to the control(CK).This results indicated that the long-term application of chemical fertilizer mixed with manure was the most important factor that bossted the SOC level and manure contains high amount of carbon in itself which increase the soc level by the direct input of C to the soil.The third part of the research aimed to explore the complete continuum of distribution of SOC fractions across the soil profile.Furthermore,to examine the relationship between total SOC and its constituents protected fractions across the soil profile in order to give the scientific support to the most feasible management practices and chief C accumulating fractions for improved soil fertility.Results reveled that that in the topsoil layers(0-20 and 20-40 cm),mineral plus manure fertilization(MNPK)increased the SOC proportion of cPOC fraction in the top layers 14.7%and 12.1%than CK.The proportion of fPOC was greater under MNPK in the topsoil layer(0-20 cm)while ?agg and iPOC fractions under MNPK was significantly higher than in CK in the topsoil layers.The greatest increase 50.2%,under MNPK was found for NH-?Clay in the subsoil(40-60 cm)layer.The physicochemically protected H?Silt under MNPK increased in the topsoil layers,18.4%and 15.7%,respectively,while proportions decreased to 11.6%,7.29%and 6.89%in the subsoil compared to the unfertilized control(CK)which indicated that the proportion of these fractions increased due to the direct effect of long term manure addition.Furthermore the regression analysis revealed a positive linear correlation(P<0.01)between total SOC and unprotected(cPOC)fractions,physically protected micro-aggregate(?agg),occluded particulate organic carbon(iPOC),physicochemically protected(H-?Silt)fractions,and chemically protected(H-dSilt)fractions across the soil profile,while a negative correlation was found for lightfraction fPOC in the topsoil,and no relationships with SOC content were found for chemically(H-dClay),biochemically(NH-dClay)and physicochemically(H-?Clay)protected fractions in the topsoil layer.In short it has been noted from the results that the unprotected cPOC fraction was the greatest C accumulating fraction in the topsoil layers,followed by ?agg and physicochemically protected(H-?Silt)fractions in the soil profile,which implies that these fractions were the most sensitive to the fertilization treatments.Overall,the unprotected,physically protected and physicochemically protected fractions were the dominant fractions for sequestration of carbon under all fertilization treatments across the soil profile.The overall results suggested and proposed that the effect of long-term mixed fertilization of chemical and manure siginificantly affected the SOC content in bulk soil as well as in fractions.Optimum SOC level is needed for improved crop production and soil health which will ultimately improved the physical and chemical condition of soil. |
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