Responses Of Soil Organic Matter Components And Structural Characteristics To Climate Warming And Management In Mollisols

Mollisols are the most productive and fertile soils in the world.Soil organic matter（SO M）and its components are crucial importance to soil health and food security.However,whether climate warming and management influences the SOC components and soil structural characteristics remains unclear,mainly due to the scarcity of long-term experimental data.In view of this,we studied the changes of quantity and internal molecular structure in Mollisols SOM and its components（labile organic matter,slow organic matter,and recalcitrant organic matter）in in situ and transplanted soils under long-term（15 years）climate warming.The mid-term（7 years）organic fertilizer application（dairy manure）and long-term（15 years）conservation tillage（reduced tillage and no tillage）on the changes in the quantity and internal molecular structure of Molliso ls SOM and its components were also examined.The physical,chemical,biological,and physical-chemical combined grouping of SOM methodology were used to carry out a systematic and in-depth study on three organic matter components（labile,slow,and recalc itrant organic matter）with different decomposition capabilities.Simultaneously,the structural characteristics of each organic matter component were analyzed by Fourier Transform Infrared Spectroscopy（FTIR）and solid-state nuclear magnetic resonance carbon spectroscopy(¹³C-NMR).In addition,the optimal fertilization practice and tillage method conducive to soil carbon sequestration and soil productivity improvement in Mollisols were explored using correlation analysis,stepwise regression analysis,and principal component analysis.The results of this research were as follows.（1）Fifteen years of a warmer climate（natural warming-2019 in situ or simulated warming-2019 transplanted）decreased the total organic carbon（TOC）content by 25.6%-44%in higher initial SOM Mollisols through decreasing O-alkyl C,but no significant effect was found in TOC content for lower initial SOM Mollisols.Warming noticeably increased dissolved organic carbon（DOC）content by 20.1%-47.7%in transplanted soils of the SOM5,SOM6,and SOM11treatments,but reduced organic carbon oxidized by potassium permanganate（PPOC）and microbial biomass carbon（MBC）contents by 21.3%-33.6%,and decreased the soil organic carbon（SOC）content of“light fraction organic matter”（LFOM）by 19.1%-20.6%through declining polysaccharide materials.（2）Fifteen-year climate warming significantly decreased the SOC contents of“coarse particulate organic matter inside macroaggregates and outside microaggregates”[M（c）POM]by25.8%-40%through reducing alcohol and phenol substances.The SOC contents of“non-occluded fine POM outside of aggregates”[Free（f）POM]and“POM inside free microaggregates”（Fm-POM）components in the slow organic matter,and“silt-and clay-sized organic matter inside microaggregates-within-macroaggregates”（m M-MOM）component in the recalcitrant organic matter were also decreased by 11.9%-21.5%under long-term climate warming.Whereas,the SOC contents of“mineral-associated organic matter”（MOM）via decreasing organiosilicone materials and“non-aggregated silt-and clay-sized organic matter”（n A-MOM）via increasing phenol C and decreasing carboxyl C were both significantly enhanced by 24.6%-47.4%and 0.91-1.9 fold,respectively.Warming only increased the SOC content of“silt-and clay-sized organic matter inside free microaggregates”（Fm-MOM）in the SOM11 treatment by 14%.（3）After 7 years of 30 000 kg/ha dairy manure application with commercial fertilizers,the contents of TOC,PPOC,AHOC,DOC and MBC in Mollisols,as well as the SOC contents of various organic matter components reached a saturated state.Compared with the application of commercial fertilizers alone,the combined application of commercial fertilizers with dairy manure for 7 years increased the TOC content in the 0-20 cm soil depth by 7.3%-22.1%,enhanced the content of labile organic carbon（PPOC and DOC）in the 0-30 cm soil depth by 12.5%-37.7%,and increased the SOC content of LFOM,“large macroaggregates”（LM）and“macroaggregates”（M）components in the 0-30 cm soil depth by 19.6%-53.9%,and also enhanced the labile organic carbon（AHOC and MBC）content in the 0-10 cm soil depth by 14.2%-23.5%.At the same time,the 7-year combined application of commercial fertilizers and dairy manure compared to commercial fertilizers alone increased SOC contents of slow organic matter in“coarse organic matter”（c POM）,“fine organic matter”（f POM）,and Free（f）POM in the 0-30 cm soil depth,as well as enhanced the SOC contents of M（c）POM in the 0-10 cm soil depth,and“non-occluded f POM inside macroaggregates but outside microaggregates”（M（f）POM）in the 0-20 cm soil depth,the increase amplitude was 2.9%-83.5%.But they only increased the SOC content of recalcitrant organic matter（MOM）in the 0-10 cm soil depth by 17.4%-20.9%.（4）In the 7-year experiment of organic fertilizer application,the bulk soils developed in th e direction of aliphaticization and rejuvenation in the 0-10 cm soil depth in Mollisols.The LFOM component showed the trend of aliphaticization in the 0-20 cm depth and the trend of aromatization in the 20-30 cm soil depth.Organic fertilizer application increased the stability of c POM component,but decreased the hydrophobicity and physical protection of f POM component in the 0-30 cm soil depth.The internal molecular structural substances in organic matter components from LM and M components transformed mutually.The“free microaggregates”（Fm）component in the 0-30 cm soil depth developed towards stabilization and complexity.Additionally,the M（c）POM component in macroaggregates developed in the direction of aromatization and complexity in the 0-10 cm soil depth.The Fm-POM component in the free microaggregates developed towards aliphaticization and rejuvenation,while the Fm-MOM component tended to be aromatized and complicated.（5）Compared with conventional tillage,15-year reduced tillage significantly increased the TOC content by 11.4%in the 0-10 cm soil depth in Mollisols,while 15-year reduced tillage and no tillage significantly enhanced the labile organic carbon（PPOC,AHOC,DOC and MBC）and the SOC content of LM component by 7.3%-72.9%in the 0-30 cm soil depth,but reduced the SOC content of the LFOM component by 10.3%-50.4%in the 0-30 cm soil depth.15 years of reduced tillage and no tillage compared to conventional tillage increased the SOC content of slow organic matter components（c POM,f POM,M（c）POM,M（f）POM and Free（f）POM）by 0.21-2.3 fold in the0-30 cm soil depth,and also increased the SOC content of“microaggregates within macroaggregates”（m M）component by 8%-9.6%in the 0-10 cm soil depth,but reduced the SOC content of Fm component by 19.5%-43%in the 0-30 cm soil depth.Simultaneously,compared with conventional tillage,15 years of reduced tillage and no tillage increased the SOC content of recalcitrant organic matter component[“silt-and clay-sized organic matter inside macroaggregates”（M-MOM）]by 19.1%-44.7%,but decreased the SOC content of Fm-MOM component by 30.5%-66%in the 0-30 cm soil depth.Only reduced tillage decreased the SOC content of n A-MOM component by 44.7%-61.9%in the 0-30 cm soil depth.（6）In the 15-year conservation tillage experiment,the mutual transformation of aromatics,polysaccharides and carbohydrates jointly controlled the SOC content of LFOM component.The increase of SOC content in the c POM and f POM components was codetermined by the decrease of carbohydrates,polysaccharides,and alcohols and phenols materials.The increase in the SOC content of LM component was the result of the increase in aromatic substances and was affected by easily decomposed aliphatics and carbohydrates.The increase in the S OC content of m M component was a complex process involving multiple substances.Simultaneously,the decrease of SOC content in n A-MOM component and the increase of SOC content in M（c）POM component were the results of the interaction among polysaccharides,aliphatics,carbohydrates and organiosilicone materials.The significant reduction in the SOC content of Fm component was the codetermined result by aliphatics and organiosilicone substances.Furthermore,the significant increase in the SOC content of M-MOM component was attributed to the decrease of organiosilicone substances,and the significant decrease in the SOC content of Fm-MOM component was attributed to the decrease of alcohol and phenol substances.（7）The higher initial SOM was more sensitive to the long-term climate warming than the lower initial SOM in Mollisols,indicating that lower SOM Mollisols may not experience net SOC losses to the atmosphere with climate warming.The SOC content of recalcitrant organic matter component of m M-MOM within macroaggregates and slow organic matter component of Fm-POM within free microaggregates were more sensitive to temperature changes than other components in Mollisols.Mid-term application of 15 000 kg/ha dairy manure combined with commercial fertilizers mainly increased the TOC,SOC contents of labile and slow organic matter in the surface（0-20 cm）soil than the application of commercial fertilizers alone,but had no obvious effect on the increase of SOC content in recalcitrant organic matter.This practice also demonstrated the largest increase in corn yield.The application of 15 000 kg/ha dairy manure combined with commercial fertilizers is the best practice for carbon sequestration and soil productivity in Mollisols.Long-term conservation tillage exhibited a carbon sequestration effect not only on the labile and slow organic matter,but also on the recalcitrant organic matter,however,the corn yield was significantly reduced by long-term no tillage than conventional tillage.Therefore,long-term reduced tillage and no tillage practices are more effective for carbon sequestration in Mollisols.Whereas,the improvement of soil productivity needs to be further investigated and verified.