Mineralization Of Soil Organic Matter In The Jiugong Mountain Along Its Altitude Distribution And Mechanisms Related To Organic Matter Stability

Soil organic matter(SOM)comprises one of the largest terrestrial carbon reservoirs and it closely correlates with soil fertility and global carbon cycles.The stability of SOM plays a central role in soil ecosystem responding to environmental changes.SOM stability is regulated by various biotic and abiotic factors for its intensive interactions with other soil constituents.Soil samples at different altitudes in the Jiugong Mountain were collected,fluorescence spectroscopy and acid hydrolysis method were adopted to analyze the properties of SOM before and after soil incubation,which helped to elucidate the differences in SOM mineralization of these soil samples.Modern techniques,such as elemental analysis,Fourier transform infrared spectroscopy(FTIR)and 13C-nuclear magnetic resonance(NMR)were used to investigate structural characters of humic acid(HA)isolated from soils studied.Meanwhile,particle fractionation of soils,X-ray diffraction(XRD)and FTIR techniques were used to investigate the physical,chemical and biological mechanisms involved in SOM stability.Based on these researches,the adsorption of HA on coprecipitated iron-aluminum hydroxides and Gram-negative/ positive bacteria,and the underlying mechanisms were studied by combining batch adsorption techniques,Zeta potential measurement and fluorescence spectroscopy.The results were as follows:1)The contents of biologically active carbon and the amount and rate of soil organic carbon(SOC)mineralized decreased with decreasing altitude.Fluorescence spectra of WEOC showed an increase in average molecular weight,humification degree,aromatic condensation and molecular structure complexity as soil depth increased,which contributed to lower SOC mineralization rate in the subsoils.The proportion of recalcitrant nitrogen in total nitrogen(RN)increased with soil profile before and after incubation while it was not obvious for the proportion of recalcitrant C in SOC(RC).Furthermore,a negative correlation was observed between SOC mineralization and RN(P < 0.05)rather than RC,implying that the availability of N was a limiting factor in SOC decomposition in the subsoils when compared with C.Generally,the amount of SOC mineralized mainly depended on soil organic matter quantity while SOC mineralization efficiency was closely related with bio-availability of organic components.After a short-term of incubation,the molecular weight,conjugated structure and aromaticity and humification degree decreased,resulting in a shift to less complicated structures of WEOC.2)The contents and structural characteristics of HA isolated from soils at different altitudes in the Jiugong Mountain were significantly different.The amounts of HA and its relative proportion in humic substance both increased with altitude and HA tended to remain in the top layer of acid soils.Besides,HA mainly originated from plant litter,and microbial metabolism products were also included.On average,alkyl C(25-40%)and O-alkyl C(21-44%)were the dominant C constituents in all HA samples.Specifically,the contents of alkyl C and O-alkyl C were similar at the altitude 1500 m;at the altitude 1200 m and 600 m,the more dominant C structure was alkyl C and O-alkyl C,respectively.The aromaticity of HA increased with increasing altitudes and HA was less aromatic in the subsoils.The humification degree of HA was highest at the altitude 1500 m and lowest at the altitude 600 m,which was in line with the amount of SOC mineralized.Additionally,the aromatic condensation,structural complexity and hydrophobicity of HA decreased in the order of altitude 1200 m > 1500 m > 600 m,which was contrary to SOC mineralization efficiency.3)The contents of mineral stabilized organic carbon(MOC)were related to organic carbon forms and iron oxides at an altitude of 600 m and 1500 m,while that of MOC was positively related to aluminum oxides at an altitude of 1200 m.Generally,iron and aluminum oxides were more effective in SOM protection in comparison to clay minerals,among which aluminum oxides were more prominent than iron oxides.Furthermore,armorphous minerals were more important than crystalline ones in SOM stabilization.Micro-aggregates were primary contributors to SOC conent.Phenols,polysaccharides and aromatic-C were inclined to remain in the soils with smaller particles,whereas,aliphatic-C and alchols were rich in the soils with larger particles.Additionally,macro-aggregates were more effective in the protection of easily oxidized organic carbon than micro-aggregates,while the latter showed stronger potentials in aromatic-C stability.The formation of organo-mineral complex was a major approach to aromatic-C stability,while inherent recalcitrance was responsible for the comparatively lower decomposition rate of phenols,polysaccharides and alkyl-C structures.MOC was the main source of recalcitrant organic carbon(ROC),meanwhile,HA also contributed to the accumulation of ROC.SOM could be stabilized against biodegradation by association with minerals,by occlusion in aggregates and by its structural recalcitrance.4)The interactions between multi-component Fe-Al hydroxides and HA was influenced by properties of mineral surface,pH and ionic strength of solution.At pH 5.0,the maximum HA adsorption on Fe was more than that on Al;in the Fe-Al hydroxide complexes,HA adsorbed increased with Fe content.The adsorption affinity increased with Al content.Aluminum oxides showed stronger coordination capability than iron oxides in the acid medium.Both iron and aluminum oxides showed preferential adsorption towards aromatic fractions of HA.HA adsorbed by aluminum oxides were more aromatic than that by iron oxides.Carboxyl,phenolic hydroxyl groups and aliphatic structures were involved in the sorption and amino groups were more inclined to bind with Fe than Al.HA adsorption capacity increased with ionic strength.In the single-component syetem HA adsorbed by Al was more than that by Fe;in the multi-component system the content of HA adsorbed decreased with Al content.The increase in pH inhibited HA adsorption on oxdies.The adsorption affinity was highest at pH 7.0 and lowest at pH 5.0 for all the oxides except Al,the affinity for Al decreased with increasing pH.At pH 7.0 and 9.0,iron and aluminum oxides presented preferential sorption towards aliphatic component in HA.5)HA adsorbed by ferrihydrite and B.subtilis was more than that by hematite and P.putida,respectively.The interaction between Fe oxides and bacteria reduced the sorption of humic acid;moreover,the reduction was greater by the interaction of bacteria with ferrihydrite than that with hematite.Phosphate exerted negligible and inhibiting influence on the sorption of humic acid by bacteria and Fe oxides,respectively.On Fe oxide-bacteria composites,humic acid sorption was initially inhibited and then promoted or weakly inhibited by phosphate with increasing concentration.The relative contribution of aggregation,adsorption and inherent recalcitrance to SOM stabilization was analyzed,which may benefit the understanding of turnover and accumulation of SOM in the natural environment.