Stability Of Soil Organic Carbon In High Latitude Profiles And Its Response To Land Use Change

Soil is a huge organic carbon pool,especially in high latitudes,which exceeds 50%of the global soil carbon pool,and the SOC content below the surface layer accounts for 50%-81%of the total.However,the effects of land use change on the chemical structure of SOC in profile and its impact on soil C loss in high latitudes are not clear.In this study,13C advanced solid-state NMR technologies were applied to study the SOC in profiles of high latitudes.The research contents were:1)to study the chemical structure of SOC and its influencing factors in soil profiles of Siberia regions;2)to study the effect of cultivation time from land-use change on the structure of SOC chemical composition in the northeast of China;3)to clarify the chemical structure changes of SOC induced by glucose addition and elucidate the the importance of microbial mediation,mineral reactivity and chemical structures in controlling the stability of SOC.The results were showed as following:1.Nonpolar alkyls,O-alkyls and aromatics were more abundant in all the Siberian permafrost soil profiles.The decrease of O-alkyls and increase of nonpolar alkyls with soil depth in the SOC indicated their more advanced decomposition states in deep horizons.Eastern CH-D contained more abundant(CH2)n and OCH,but less abundant aromatic C-C/C-O than other sampling sites,which indicated that low temperature and low precipitation led to slow decomposition of plant residues,accumulation of(CH2)n from plants or microorganisms and humification process.Because of the different vegetation types,the organic matter input at the two sampling sites in TZ area is different,which not only affects the SOC content in the profile,but also affects its chemical structure.In addition,cryoturbation promoted the decomposition of labile components(OCH,OCHO)and the accumulation of recalcitrant components(aromatics).The difference of soil organic carbon content and SOC chemical structure between A horizon and Ajj horizon indicated that physical protection might be more important in controlling its stability than its chemical recalcitrance.2.The chemical composition of SOC was differentiated by soil type and depth.The Phaeozem and Chernozem profiles differed mainly in their relative proportions of aromatic C-C and(CH2)n groups:the Phaeozem contained relatively more aromatic C-C,whereas the Chernozem contained relatively more(CH2)n groups.The fused-ring aromatics were probably derived from char-like organic matter from burning of plant litter or from SOC humification,while the(CH2)n groups were likely to be from microbially-or plant-derived residues.The main differences between top-and sub-soils were the occurrence of more protonated C in the topsoils and more nonprotonated C in the subsoils.With increasing length of cropping time,aromatic C-C and C-O groups and COO/N-C=O groups increased,but(CH2)n groups decreased in the Phaeozem subsoils and increased in the Chernozem subsoils.Our findings suggested that leaching and soil moisture might influence the origin,redistribution and transformation of the recalcitrant components of SOC in the soil profile,resulting in changes in SOC composition under different climates and soil types.3.The signal intensities of the most dominant fused-ring aromatics and nonpolar alkyl groups were reduced due to the priming effect following the addition of 12C enriched glucose.Those signal intensities of O-alkyl and nonpolar alkyl groups increased in SOC spectra following the addition of 13C-labeld glucose,demonstrating accumulation of glucose and microbial residues.With the increasing glucose concentration,priming effect estimated using isotopic method and the magnitudes of signal loss estimated using 13C NMR both increased as exemplified for the Ap horizon soil.However,soil organic C content increased only when the added glucose concentration was beyond a previously non-quantified priming saturation threshold.The increase of soil organic C was larger in the subsoils than in the topsoil due to lower microbial biomass and higher mineral reactivity,which were related to the reduced priming effect and enhanced accumulation of microbial and glucose residues in the subsoils.Our findings highlighted the importance of priming saturation threshold,microbial mediation and mineral reactivity,but not SOC recalcitrance,in controlling the dynamics of SOC.