Mechanisms Of Soil Dissolved Organic Matter Stabilization In Gray Desert Soil Under Long-term Different Fertilizations

Dissolved organic matter ?DOM? is readily decomposable and can be preferentially utilized by the microorganisms than the recalcitrant fraction of SOC and playan essential role in soil physical, chemical and biological processes. In natural soil, DOMis not exist alone, associations of SOM with soil minerals, soil oxides and soil aggregates have been determined as the main mechanisms by which DOM is protected from microbial decomposition in most soils of the world. The organic-inorganic complex is considered as an important index to reflect the effect of improving soil fertility. Thedissolved organic matter can either be absorbed or coprecipitated in the surface of soil minerals, also be intercalated into the layer of soil minerals. The investigations on the mechanisms of soil dissolved organic matter stabilization and its interaction with minerals in gray soil under long-term fertilizations is an effective way to figure out the soil dissolved organic matter reservationmechanisms in arid and semi-arid regions. In this dissertation, mechanisms of soil dissolved organic matter stabilization of gray soil under 22 year long-term different fertilizations ?CK, NPK, NPKM, M? were investigated by the combination of 2D-FTIR,13C NMR, high resolution transmission electron microscopy ?HRTEM?and near edge X-ray absorption fine structure spectroscopy ?NEXAFS?from four aspects: the effects of long-term fertilizations on the soil organic matter and DOM; the interaction of DOM and Ca²⁺; the interaction of DOM and soil clay minerals; the interaction of DOM and iron oxides. The main results are listed as follows:1. Long-termfertilizations changed the contents of soil organic carbon significantly?P<0.05?. The contents of SOC under NPKM?14.77g/kg? and M?14.77g/kg?was greatly higher than NPK?12.02g/kg?, which was 1.86 and 2.52 times higher than CK.13C NMR results showed that long-term fertilization changed the relative content of the major C-types of SOC. The sequence of the contents followed as: O-alkyl C>aromatic C >alkyl C>carbonyl C. Compared with NPK, NPKM and M, CK showed the highest alkyl C ?18.20%? and O-alkyl C ?18.20%?. Compared with NPKM ?8.98%? and M ?9.97%?, NPK?12.21%? showed higher contents of O-alkyl C. The ratio of alkyl C/ O-alkyl C followed the sequence as: CK>NPK>M>NPKM. The contents of HFOC and DOC had significant positive correlation with TOC, the correlation index ?R? were 0.998 and 0.997, respectively.2. The EEM results showed that long-term fertilization changed the relative contents of humic acid in DOM. In CK, DOM was primarily composed of fulvic acid substances and asmall amount of humic acid and protein substances. Compared with other treatments,the protein substances disappeared in CK. Long-term fertilization changed the metal concentrations of Ca²⁺, Fe3+, Al3+ in DOM, DOC contents had significant positive correlation with concentration of Ca²⁺. Compared with the control, application of chemical fertilizers and manure significantly increased the concentrations of Ca, Mg, Si, humic and fulvic acid-like substances in DOM but decreased the amounts of trivalent metals ?Al and Fe? and protein-like substances. Although crystalline Al/Fe nanoparticles and amorphous or short-range-order Si/Al nanoparticles existed in all DOM samples, crystalline Ca/Si nanoparticles were predominant in the samples treated with goat manure.3.The XRD results confirmed the intercalation of DOM with clay minerals in the long-term fertilization treatments. FTIR results showed NPKM and M treatments improved the contents of polysaccharide that intercalated with soil clay minerals and decreased the contents of aliphatic substances. The contents of the intercalation substances are changed by the different long-term fertilization treatments, the contents of the soil organic carbon and nitrogen followed the sequence of NPKM ?1.40g/kg? > M ?1.12g/kg? > NPK ?0.96 g/kg? > CK ?0.74 g/kg?. Moreover, the results showed that the combination of organic and inorganic fertilizer can significantly improved the intercalation of soil organic matter with soil clay minerals, which was an important mechanism to protect DOM in gray soil.The structure of the intercalation substances under different fertilizations were similar,such as sugar, carbohydrates, aliphatic organic acid,protein. The intercalation of DOM significantly increased the SSA under different fertilizations, the sequence followed as NPKM ?14.24 m2/g? > M ?13.56 m2/g? > NPK ?12.59 m2/g? > CK ?12.34 m2/g??4. In adsorption experiment, the amount of C increased greatly with the increasing C/Fe ratioat lower molar C/Fe ratio?4-12?, while the gradual increase appeared at the higher C/Fe ratio?12-20?. Under C/Fe ?20?, the amount of C reached the highest at 70.1mg/g. The SSA of the pure ferrihydrite was 180.2 m2/g and the N2 micro-porosity accounted over 50%of the TPV. Generally, the SSA, MIV, MEV decreased with the increasing molar C/Fe ratios for adsorption complexes. As for the adsorption complexes,the SSA decreased greatly from 529.0 m2/gto 379.5 m2/gat lower C/Fe ratios while decreased gradually at high C/Fe ratios. FTIR results showed thatthe peak intensity of the absorbance bands at 1130cm-1 significantly decreased after adsorption, which clearly indicates carbohydrates seem to be less important for adsorption of DOM. In addition, adsorption aroused a sharp decrease of the band of asymmetric carboxyl groups at 1610 cm-1 at higher molar C/Fe ratios, the decrease in the asymmetric COO-band at 1610 cm-1 became smaller.This implies that the higher molar C/Fe ratios could result in fewer carboxyl groups per molecule involved in binding reactions, which might be explained by a decrease in bonding sites, as more OM is associated with minerals. Results from C 1s NEXAFS showed that phenolic-C disappeared inadsorption complexes. STXM-NEXAFS showed that the color-coded composite maps of C and Fe show C was highly spatially correlated with Fe for both adsorptioncomplexes. There was no separate phase of C and Fe.5.In coprecipitation experiment, the amount of C for coprecipitates complexes also had a positive correlation with the initial molar C/Fe ratio. The amount of C varied from 379.5mg/g to 776. mg/g. At lower molar C/Fe ratio?4-12?, the amount of C increased greatly with the increasing C/Fe ratio, while the gradual increase appeared at the higher C/Fe ratio? 12-20?. At the same C/Fe ratio, coprecipitates complexes had more amount of C than adsorption.Generally, the SSA, MIV, MEV decreased with the increasing molar C/Fe ratios for both adsorption and coprecipitates complexes. This was due to the blockage of mineral surface sites and pores by OM. Besides, adsorption complexes showed higher value of SSA, MIV, MEV than coprecipitates at comparable C/Fe ratios. Specifically, for the coprecipitates complexes, the SSA decreased sharply from 156.6m2/g to 2.1 m2/g at lower molar C/Fe ratios while leveled off at 2.1m2/g at high molar C/Fe ratios. Besides, the SSA showed an constant of 2.1m2/gwhich is similar to the SSA of freeze-dried OM ?2.1 m2/g?, suggesting that an organic surface of coprecipitates. The XRD and SSA results confirmed the presence of the insoluble Fe???-OM complexes in the coprecipitation complexes, Fe-EXAFS was used to quantity the amount of the insoluble complexes.Fe-EXAFS results implied that the increased molar C/Fe ration induced the formation of insoluble Fe?III?-OM complexes and inhibit the formation of ferrihydrite, the content reached 11.1% at C/Fe ?20?. The coprecipitates complexes showed a more stable structure than adsorption complexes.