Mapping The Soil Organic Matter Changes In Managed Agro-and Forest Ecosystems By Aggregate Carbon Pool Distribution And Molecular Composition

The current increasing climate change affects the carbon cycle and productivity of terrestrial ecosystems,and is regulated by the ecosystem carbon cycle.As the largest carbon pool in terrestrial ecosystems,soil organic matter(SOM)has strong response and feedback to climate change,and anthropogenic management plays an important role in regulating soil carbon pools in response to global climate change.SOM is a key component of soil and plays a pivotal driving role in providing soil ecosystem functions and services.The material nature of SOM is various degrees of residues of organic molecules from life in the soil,and its nature of stabilization is protected by combining with soil aggregates and minerals.Both vegetation changes in forest ecosystems(e.g.,forest restoration)and straw return in agroecosystems strongly influence the rapid evolution of SOM,but there is a lack of systematic understanding of how SOM existence and molecular composition relevant to soil biological processes in this evolution.Therefore,in this thesis,we took forest soil and cropland soil as the objects,analyzed the accumulation characteristics of topsoil organic carbon under vegetation restoration and straw return from two perspectives:the distribution of carbon pools in the aggregates and the retention of organic molecules of life source,in an attempt to map the microscopic patterns of SOM dynamics under the managements,to improve the understanding of SOM nature,and to elucidate the mechanisms of the influence of anthropogenic managements on SOM changes,for further providing scientific basis for combating with climate change and improving soil ecosystem functions.In this study,a chronological sequence representing the natural vegetation restoration of degraded forest(located in Chenqi watershed of karst region in Puding,Anshun,Guizhou,Southwest China;with loam soil,Mollic Inceptisols)and different form sequence representing straw return to the rice paddy(located in Xiaoqiang farm of Taihu plain in Baimao,Changshu,Jiangsu,with loam soil,Gleyic Stagnic Anthrosol)were constructed.The chronological sequence of forest restoration consisted of maize cropland(CL)and forestland restored for 10,20 and 40 years(labeled as RL10,RL20 and RL40,respectively);the different form sequence of maize straw consisted of no straw return(CK),fresh straw return(CS),straw manure return(CM)and straw biochar return(CB)treatments.The field experiment with a one-time straw application of 10 t OC ha-1,was laid out after wheat harvest in June 2015.Only undisturbed topsoil was collected for both sequences,and sampling for the forest restoration sequence was completed in November 2019,while sampling for the straw return sequence was completed after the rice harvest in November 2015,2017 and 2019,respectively.Soil water-stable aggregate size fractions(Macroaggregates,2000-250 μm;Microaggregates,250-53 μm;Silt-clay fractions,<53 μm)were obtained by wet-sieving,and the density/size fractions(light fraction,LF;Particulate organic matter,POM;Mineral-associated organic matter,MAOM)in the aggregates(>53μm)were further separated.The organic carbon content and δ13C(‰,VPDB)values of the bulk soil and aggregate size fractions were determined;The abundance of biomarkers in plant,soil and aggregate fractions were identified by sequential extraction-gas chromatography/mass spectrometry(GC/MS);Combined with FT-ICR-MS and NMR,the labile OM components and functional groups in the straw materials and soil were analyzed.The main results were as follows:1)Vegetation restoration promoted the OC accumulation of topsoil,and SOC evolved from MAOM dominated to POM dominatedIn degraded karst region,compared with maize cropland(CL,OC content of 24.92 g kg-1),the SOC content was increased by 24%,79%and 181%,the mass proportion of macroaggregates increased by 136%,179%and 250%,and POM increased by 13%,108%and 382%,respectively at RL10,RL20 and RL40.SOM of maize cropland derived from C4-plant was dominated by MAOM(60%),following 10-40 years of forest restoration,SOM changed to C3-plant derived(67%-74%)and POM gradually dominated(37%-65%).In addition,topsoil OC content was significantly positively correlated with the mass proportions of macroaggregates and POM,and negatively correlated with the mass proportions of silt-clay fractions and MAOM.With the accumulation of SOC,the ratios of MA-C/SC-C and POM-C/MAOM-C,as well as C pool diversity increased notably.Thus,the changes of these ratios depicted the quantitative accumulation of forest soil organic matter under vegetation restoration as an anthropogenic management.2)Vegetation restoration promoted the protection of plant-derived organic matter by topsoil aggregatesQuantitative analysis of the biomarkers of topsoils and their aggregate fractions,combined with NMR spectra to understand the functional groups of density/size fractions within the aggregates,can investigate the changes of the molecular composition of SOM following the vegetation restoration.Firstly,plant-derived lipids(PL,including long-chain fatty acids,phytosterols,cutin and suberin)were mainly concentrated in the macroaggregates and microaggregates;On the contrary,microbederived lipids(ML,including short-chain fatty acids,ergosterols and microbial hydrolysable lipids),were relatively uniformly distributed in the aggregates and siltclay fractions.The abundance of PL in the forest topsoil and macroaggregates increased with the prolonged restored years,while the abundance of ML was significantly lower in bulk soil and aggregate fractions than in cropland soil.Calculations showed that the OC content of bulk soil and aggregate fractions was significantly and positively correlated with the abundance ratio of PL/ML and the Shannon diversity index(H’)of biomarkers.NMR analysis showed that in the density/size fractions within the aggregates,the relative abundance of O-alkyl C in the restored forestland SOM was higher than that in the cropland,while aromatic C was the opposite.Therefore,SOM of maize cropland with poor OM was mainly derived from microbes,which was retained in the silt-clay fractions as recalcitrant C;while the abundant OM accumulated under vegetation restoration was mainly derived from plants,and most of them were stabilized in aggregates by physical protection.From the molecular perspective,and the PL/ML ratio and H’ index of biomarkers could explain the internal nature of rapid accumulation of forest SOM under vegetation restoration as a management intervention.3)Py roly zed straw return is more effective in SOM sequestration of paddy than fresh and manured straw returnAfter the one-time equal carbon return of straw in June,2015,the OC content of topsoil was significantly higher(8%-36%)in 2015 and 2017 rice seasons compared to CK(OC content of 28-29 g kg-1),but only CB increased that by 24%in 2019 rice season,and the increased OC was mainly from the input biochar.After 4 years of return,compared to CK,both CS and CB increased the mass proportion of macroaggregates by about 50%and the mean weight diameter(MWD)by about 30%.However,all the CS,CM and CB treatments increased the allocated OC ratio of macroaggregates to silt-clay fractions(MA-C/SC-C),the ratio of particulate to mineral-associated organic carbon(POMC/MAOM-C)and the aggregate carbon pool diversity,ranging from 69%-215%,20%60%and 5%-14%,respectively.It could be seen that SOM sequestration effect under straw biochar application was significantly greater than that under fresh and manured straw amendments.The trend of changes among treatments in topsoil OM after many years of straw return indicated that SOM changed from MAOM dominated to POM dominated,and the carbon distribution pattern of density/size fractions tended to be complex;Indicators such as MA-C/SC-C,POM-C/MAOM-C and carbon pool diversity also reliably indicated that the changes in quantitative existence of paddy soil organic matter under straw return as an anthropogenic management.4)Straw biochar amendment promoted topsoil OC sequestration by enhancing the protection of plant-derived OC in aggregatesThe changes in the molecular composition of SOM after straw return were investigated by quantitatively analyzing the biomarkers of soil and aggregate fractions,and further discerning the labile OM components and functional groups in straw materials and soil after 4-year amendments using FT-ICR-MS and NMR.The abundance of lignin phenols in the topsoil under CS and CM peaked after 2-year amendments,and increased by 66%and 115%relative to CK,respectively.The abundance of plant-derived lipids in the topsoil under all the amendments increased significantly after 4 years,and the NMR results also confirmed that straw return increased the proportion of lipids in SOM;accordingly,the PL/ML ratio and H’ index of biomarkers were also significantly enhanced.However,at the aggregate level,only CB increased the PL/ML ratio and H’index of biomarkers of microaggregates by 33%and 20%relative to CK.Differently,compared to CK,the content,molecular number,and relative abundance of lipids and lignin of the water-extractable OM(WEOM)were increased under CB,but its molecular diversity decreased.In this straw return sequence,plant-derived lipids were mainly enriched in macroaggregates,while microbe-derived lipids were more associated with minerals,stabilized in silt-clay fractions.The range of variation in SOM content following amendments with straw in different forms was small,and the size of water-stable aggregate fractions had a greater effect on SOM quantity and quality than the return forms of straw.It was worth noting that lignin phenols were relatively enriched in SOM under fresh straw return,the labile OM were increased under manured straw amendment,and the higher SOM accumulation under pyrolyzed straw amendment was mainly due to the enhanced protection of plant-derived organic components in microaggregates.In a word,the abundance ratio of PL/ML and the H’index of molecular components still clearly reflected the intrinsic nature of SOM accumulation under straw return as a management intervention,and also suggested the biophysical pathway of SOM retention.Integrating case studies of SOM change sequences in forest and agroecosystems under management interventions,we obtained several mechanistic insights into SOM dynamics:1.In both forest and agricultural ecosystems,changes in the amount of SOM might be manifested essentially as changes in the SOM distribution in the aggregates related to binding status and changes in SOM molecular composition related to selective retention,which in turn had interactions.2.The relative accumulation of SOM was reflected in the relative increase of OC associated in the aggregates and the relevant accumulation of particulate organic carbon.The carbon pool of poor-OM soils tended to be dominated by the OC pool stabilized with minerals in the silt-clay fractions,while the relative increase in the particulate OC pool protected by aggregates with SOM accumulation led to the distribution of the OC pools from the relative single mineral-associated to the multiple distribution in macroaggregates,microaggregates and silt-clay fractions.3.The SOM dynamics were also reflected in the changes of the sources of OM components.SOM accumulation was manifested in the retention of fresh plant-derived OM in the soil,while poor-OM soils were relatively enriched in microbial products due to the decomposition of OM,thus the general accumulation characteristic of OM was the increase of molecular diversity and the relative enrichment of plant-derived molecules compared to microbial molecules,which was why high OM corresponds to high molecular diversity.4.SOM accumulation under management intervention was accompanied by the development of(water-stable)aggregates,where plant-derived molecules were physically protected from microbial decomposition under fresh OM input,leading to a synergistic enhancement of carbon pool diversity and molecular diversity in the soil.The evolution of the SOM-aggregate-microorganism interactions was the nature of SOM dynamics.The development of aggregates was the core of SOM accumulation.This suggested that soil carbon sequestration potential might increase with the enhancing physical conservation of POM in aggregates.5.The allocated OC ratio of macroaggregates to silt-clay fractions(MA-C/SC-C),the ratio of particulate to mineral-associated organic carbon(POM-C/MAOM-C),the abundance ratio of plant-to microbe-derived lipids(PL/ML),and the molecular diversity index(H’)could indicate the existence of SOM,and might be further developed as diagnostic indicators of the SOM accumulation.6.Soil genetic types and use/managements affected the level of SOM accumulation and changes in its chemical composition.The differences in the aggregation might explain the different patterns of OC quantity and quality changes in the two sequences.This suggested that among the different types of soil,differences in SOM-aggregatemicroorganism interactions control the stabilization of SOM,which could be a new frontier in the studies on SOM management for climate change and soil health.