Long-term Fertilization Impact On The Distribution And Stabilization Of Organic Carbon And Nitrogen In Black Soil Of China

Increasing soil organic carbon and total nitrogen(TN)storage is crucial to mitigating climate change and ensuring food security.This study aimed to quantitatively evaluate the influence of longterm application of different mineral fertilizers,manure and combination of manure and mineral fertilizers on SOC and TN storage.The results indicated that the long-term application of organic manure(M),combined application of mineral and organic fertilizers(MNPK)significantly enhanced the SOC and TN storage.Results indicated that MNPK significantly enhanced the SOC and TN densities in 0-20 cm layer whereas in the subsoil M significantly increased SOC and TN densities SOC stocks in 0-20 cm by 16% while in 20-40 cm layers by 12%(p < 0.05).Similarly,the highest SOC and TN stocks were found in MNPK in both topsoil layers(0-40 cm).While compared with CK,M significantly increased SOC stocks by(56%,69% and 70%)and increased TN stocks(75%,101% and 202%)in 40-60 cm,60-80 cm and 80-100 cm,respectively(p < 0.05).Soil organic carbon as a complex continuum of distinctively stabilized fractions represents a key soil quality indicator in optimizing crop yield.However,little information exists regarding the effects of long-term fertilization on the sensitivity of SOC protection mechanisms along with the whole profile.The present study investigated the contents and profile distribution of distinct SOC protection mechanisms after 35 years of mineral and manure fertilization in the black soil of Northeast China.The soil was fractionated using a combination of three methods,i.e.,physical,chemical,and density fractionation methods.In the topsoil(0-20,20-40 cm)mineral plus manure fertilization(MNPK)was prominent in enhancing the total SOC content accounting for 16.15% and 12.34%,respectively.In contrast,sole application of manure(M)dominantly increased total SOC by 56.14,48.73 and 27.73% in the subsoil(40-60,60-80 and 80-100 cm,respectively).Moreover,in the topsoil MNPK and subsoil M increased unprotected coarse particulate organic carbon(cPOC)(by 48 and 26%,respectively)and physically protected micro-aggregate(?agg)(by 20 and 18%)and occluded particulate organic carbon(iPOC)contents(by 279 and 93% respectively)compared to CK.Regression analysis showed a positive linear correlation between total SOC and the cPOC,iPOC,physico-biochemically protected NH-?Silt and physico-chemically protected H-?Silt(p < 0.01)across the whole profile.The unprotected cPOC followed by the physically protected iPOC were the most sensitive fractions under the influence of variable fertilization.Overall,physical,physicobiochemical,and physico-chemical protection were the predominant mechanisms to sequester SOC in the whole profile,whereas the biochemical protection mechanisms were only relevant in the topsoil.Organic manure and chemical fertilizer application caused accretion of SOC content in top-and subsoil with a clear difference in the stabilization mechanisms involving C accumulation in top-and subsoil.Most of the SOC was stabilized in micro-aggregate derived physically protected fractions.Our study confirmed that the application of MNPK notably increased the POC in the topsoil(by 128% and 93% in the 0–20 cm and 20–40 cm,respectively)while M significantly enhanced the SOC content of the iPOC in the subsoil(by 147%,85% and 84% in 40–60 cm,60–80 cm,and 80–100 cm,respectively)Overall,contrasting sensitivity feedbacks by the SOC fractions towards the long-term fertilization depict distinct mechanisms relevant to the SOC sequestration across whole soil profile.The unprotected cPOC and physically protected iPOC fractions were the key fractions for the major portion of C accumulation.Also,physical protection implicates the importance of aggregation for C sequestration not only in the top-but also in the subsoil.Based on our findings,we examined that SOC in the H-dSilt,NH-dSilt,NH-dClay,and NH-?Clay had no significant treatment effects at least in the last two subsoil layers(60-80 and 80-100 cm),implicating that the micro-aggregated silt C-fractions may have attained saturation or there is not enough C input that can cause significant effects in these fractions.Least tendency in case of physico-chemical protection mechanisms and no treatment effect in biochemical and physico-biochemical fractions in subsoil layers(60-80 and 80-100),indicates the near-or complete saturation of these fractions.These results demonstrate the effectiveness of manure application to sequester SOC,especially in subsoils for an extended period in black soils of China.Future studies should focus on the impact assessment of variable amounts of manure on the protection mechanisms and explore the drivers behind the SOC sequestration.Organic manure and chemical fertilizer application caused accretion of TN content associated with soil fractions in top-and subsoil with a clear difference in the stabilization mechanisms involving N accumulation in top-and subsoil.Most of the SON was stabilized in micro-aggregate derived physically protected fractions.Overall,contrasting sensitivity feedbacks by the SON fractions to the long-term fertilization depict distinct mechanisms relevant to the N storage across the whole soil profile.The unprotected cPON and physically protected iPON fractions were the key fractions for the major portion of C accumulation.Also,physical protection implicates the importance of aggregation for N sequestration not only in top-but also in the subsoilSoil organic carbon is an important parameter determining soil fertility and sustaining soil health.Understanding the driving factors of SOC stabilization along with the whole soil profile,is crucial.Moreover,how C,N,and P contents and their stoichiometric ratios(C:N:P)regulate the nutrient availability,and SOC stabilization mechanisms have not been comprehensively explored,especially in response to long-term fertilization.The present study aimed to determine how the long-term mineral and manure fertilization influenced soil C:N:P ratios,and how this elemental stoichiometry influences the various protection mechanisms underlying the stabilization of OC along with profile.The C,N,and P stoichiometry and their contents were measured along with the whole profile(0-100 cm).Results revealed that the soil C,N,P stoichiometry and their contents in topsoil depths(0–20 and 20–40 cm)and subsoil depths(40-60,60-80 and 80-100 cm)varied significantly(p < 0.01)among the soil layers and various treatments affected the contents and ratios of C:N:P.Compared with control(CK),the C,N and P contents were significantly higher(p < 0.05)in NPKM in the topsoil layers,while M alone increased these contents throughout the subsoil.Overall,the C,N,P contents and their stoichiometry decreased with the increase in depth.Regression analysis showed that C:N,C:P,and N:P ratios associated significantly with the OC fractions in the topsoil layers only.These negative correlations indicated that these ratios significantly influence the C stabilization in the surface layers.However,the results warrant further investigations to study the relationship between soil and microbial stoichiometry and SOC at various depths.Long-term manure applications improved the C sequestration both in topthe deep layers;hence these facts can be considered relevant for fertilizer recommendations in cropping systems across China.In Summary,Organic manure and chemical fertilizer application caused accretion of SOC,TN densities,stocks and contents in top-and subsoil.Moreover,application of manure alone and/or in combination with mineral fertilizers enhanced SOC and TN contents associated with fractions based on protection mechanisms.The results implicate the importance of soil aggregation that plays a major role in SOC and TN sequestration throughout profile and is crucial for ensuring the soil quality to carry out the ultimate goals of food security and climate change mitigation.