Mechanism Study On Impact Of Soil Organic Carbon And Minerals By Long-Term Fertilization Application At The Submicron Scale

Globally,soil organic matter(SOM)in the terrestrial ecosystem retains large quantities of organic carbon(OC)which are higher than that in the atmosphere and vegetation systems,and it is sensitive to the climate changes and regional environment.Soil organic carbon(SOC)plays a key role in maintaining soil fertility to ensure agricultural production and microbial communities.Also,SOC has the ability to control the transformation of contaminants and promote the quality of terrestrial and aquatic ecosystems.Since SOC has important effects on the evolutionary of terrestrial ecosystem and management,the exact mechanisms are still unknown about the turnover and sequestration.SOC is mainly from the residules of plants and microorganisms which is from the complex biogeochemical degradation.And they are asscociated with soil clays,iron/aluminium minerals and aggregates.SOC groups,mineral availability,microbial diversity and land utilization type are the most important factors for SOC storage.It is effective to study the proposed mechanisms according to the cooperation of soil minerals-plant rhizosphere-microbial communities.Long-term fertilization is one of the effective methods to improve soil fertility and quality in order to gain harvest and economic value.Mineral binding is a major mechanism for SOC stabilization,and mineral availability for C binding critically affects C storage.Yet,the mechanisms regulating mineral availability are poorly understood.Here,we combined long-term fertilization experiments and lab incubation experiments including the techniques of traditional chemical extraction and submicron characterization to study the factors of influencing SOC storage and the proposed mechanisms.Three long-term field experiments were studied,including the Qiyang Experiment(25 years)in Hunan Province,China,the Park Grass Experiment(159 years)at the Rothamsted Research Station in Hertfordshire,England and the Broadbalk Experiment(172 years),the oldest continuously running field wheat experiment at the Rothamsted Research Station,England,respectively.The results showed that long-term organic amendments significantly increased the amounts and composition of SOC by using C 1s NEXAFS.In addition,long-term organic fertilization treatment could promote mineral availability,particularly short-range-ordered(SRO)phases,like ferrihydrite and allophone by using Fe K-edge XANES and XPS.Furthermore,results from DGT experiments confirmed that long-term organic amendments(M)significantly(Tukey’s HSD post hoc tests;P<0.05)increased bioavailable Fe at the Qiyang Experiment,1.23 times than Control treatment and 2.25 times than NPK treatment.Mineral transformation experiments by using citric acid,a major component of root exudates,showed that the treatment of high concentration(100 mg L-1)promoted more SRO minerals,and SRO minerals acted as "nuclei" for C retention.One microcosm experiment with three replicates was conducted to determine the effect of root and Arbuscular Mycorrhizal Fungi(AMF)exudates on mineral availability.Another microcosm experiment with three replicates was conducted to determine the effect of root and fertilization treatments on mineral availability in a greenhouse at Nanjing Agricultural University.The results of both microcosm experiments demonstrated that the presence of AMF or roots exudates significantly increased mineral availability and promoted the formation of SRO phases.To verify the strong retention capability for C by the mobilized A1 and Fe minerals,we then designed an isotopic labeling experiment(using 13C-labeled amino acid)combined with NanoSIMS observation.Here,13C-labeled amino acids were used as a precursor of newly added C(i.e.,animal manure)in soils.After 24 h of incubation with a 13C amino anid mixture,the composite NanoSIMS image showed a profound enrichment of newly added 13C-on 27Al16O-and 56Fe16O-,which served as "nuclei" for the retention of 13C-.This is further supported by the hue-saturationintensity(HSI)image of 13C/12C-,which clearly showed that colloid particles were surrounded by these 13C-enriched spots.Also,line profiles further indicated that the distribution patterns of 13C-,12C-,27Al16O-and 56Fe16O-were correlated at the spatial scale.Compared to no fertilizer or chemical fertilizer inputs,the total sorption capacity of organic carbon by soil colloids was significantly increased with organic amendments.Carbon fraction,mineral availability,microbial community and land utilization type are the main factors which influence the mechanisms of carbon storage.It is available to further build the proposed mechanism according to the cooperation of soil minerals,soil root interface and microbial community.Here,we combined synchrotron radiation based Fourier transform infrared(SR-FTIR)spectroscopy,synchrotron radiation based micro X-ray fluorescence microscopy(μ-XRF)and two-dimensional correlation spectroscopy(2D-COS)analysis to in situ visualize associations between the mineral clusters and biopolymers from a typical Ferralic Cambisol topsoil in China.Results show that the spatial distribution and correlation between clay clusters and biopolymers are heterogeneous and significant,and also demonstrate that clay clusters are associated as nuclei with a potential capability of binding carbon at the submicron scale.Furthermore,the combination of SR-FTIR mapping and 2D-COS analysis explores for the first time the strategy of identifying overlapped spectra and quantifying the sequestration reactivity.Specifically,carbon retention is correlated as the binding sequence orders:3630 cm-1>3610 cm-1,985 cm-1>898 cm-1,indicating that Fe/Al oxyhydroxides and phyllosilicates could regulate the organic matter sequestration without the influence of spatial perturbations.Together,based on the well-controlled long-term field experiments,microcosm experiments,mineral transformation experiments,and isotopic labeling experiments,we propose that soil organic amendments initialize a positive feedback loop by increasing mineral availability and promoting the formation of SRO minerals for further C binding.Production of root or microbial LMW organic acid exudates following long-term organic amendments may be a critical step in this process.The produced acids act as complexing and reducing agents for mineral mobilization and acquisition,further promoting the formationand stability of SRO minerals via four potential mechanisms.First,the mobilized mineral elements(e.g.,Al and Fe)can act as the precursors for formation of SRO minerals.Second,LMW organic acids,common root exudates,can promote transformation of minerals or oxides from more crystalline to SRO phases,a process known as "rejuvenation" in soil and ecology sciences.Third,after the formation of SRO minerals,LMW organic acids can incorporate,through precipitation from solution,into the network structure of SRO minerals and prevent their growth or transformation to crystalline forms.And finally,some biopolymers with soil particles can also limit the dispersal of SRO minerals that may otherwise be transported away from their source via leaching,surface runoff,or drainage in natural ecosystems adding to carbon storage.