Molecular Evidence Of SOM Turnover In Mollisol Profile And Its Control Mechanisms

Land use and climate changes present an opportunity to enhance carbon storage to deep soils through increasing root exudates. Increased exudates inputs may cause loss of soil organic matter (SOM) through accelerated decomposition or priming. Little is known, however, about the differences in priming effect between topsoil and subsoil horizons and the controlling factors. Therefore, Incubation experiments were conducted with the application of advanced solid-state NMR techniques and isotopic technique. The research contents were:1) determine the effect of glucose input on the priming effect of deep soil and the chemical composition of SOM; 2) distinguish the formation of new SOM and decomposition of native SOM after glucose addition and elucidate the regulation mechanisms of priming effect by soil horizons; 3) study the effect of soil type and cropping time on the chemical composition of SOM in soil profiles and the humification process. The objective of this study is to reveal the molecular evidence of SOM turnover in Mollisoil profile and its control mechanisms. The results were showed as following:1. Glucose addition (13C-G0.4) caused a positive priming effect of the decomposition of SOM in the deep soil (1.00-1.20 m) of a Mollisol and an increase in SOM due to transformation of glucose (52%) into SOM after mineralization. The CP/TOSS 13C NMR spectra demonstrated that both labile and recalcitrant organic compounds in SOM changed during the incubation, but in different manners in Go and 13C-G0.4. During the incubation, the relative abundance of (CH2)n in Go did not change over time, but in 13C-Go.4 it decreased and then increased, suggesting shifts of soil microbial communities only in G0.4. After the incubation, in Go the relative abundances of nonpolar alkyl C and ketones/aldehydes increased, but those of aromatic C-C and protonated O-alkyl C decreased; In13C-G0.4,the abundances of NCH and protonated O-alkyl C increased, but those of aromatic C-O and ketones/aldehydes decreased. The results indicated that glucose addition likely primed the decomposition of aromatic C-O and suppressed the formation of ketones/aldehydes.2. Because 13C NMR can only detect 13C nucleus, but not 12C nucleus, we used 13C enriched and 13C depleted (12C) at same time to distinguish the 13C signals between native SOM and glucose, and applied multiCP 13C NMR to study the changes of chemical structure of SOM after incubation. This study determined the priming effect due to addition of 13C-enriched and 13C depleted (12C) glucose at 0, 0.05,0.5 and 2.0g C kg-1 soil (Go,13C/12C-G005,13C/12C-G0.5,13C/12C-G2) to a Mollisol profile (A, B and C) and indicated that the priming effects increased with increasing glucose concentration for all the soils. For the first time, we observed loss of aromatic C in A and B horiozns and loss of nonpolar alkyl and COO/NC=O only in A horizon. We demonstrated that the priming effect was input limited from topsoil to subsoils and the differences in different soil horizons were regulated by glucose use efficiency, SOM accessibility and SOM recalcitrance.3. The dominant functional group in Phaeozems and Chernozems were different wthin the soil profiles, NCH/OCH3 and O-alkyl C were most abundant in A horizons, while aromatic C was more abundant in the horizons below A horizon than in the A horizons, demonstrating interaction effects of soil type and soil depth. With increasing soil depth, (CH2)nand aromatic C-C increased in all soil profiles, while NCH, O-CH and aromatic C-H decreased; with increasing cropping time, the abundance of (CH2)n decreased in all horizons of the Phaeozems and increased in the B horizons of the Chernozems; demonstrating interaction effects of soil type, soil depth and cropping time on some structures. The principal component analysis demonstrated that SOM in surface soil contained more proton functional groups due to fresh carbon input, but subsoil contained more nonproton functional groups; cropping time significantly affect the chemical composition of SOM; soil type affect the humification of SOM, that is oxidation of leaching biochar in Phaeozems and oxidative dehydrogenation process in Chernozem.We used glucoseto simulate root exudate, and first found the changes of chemical structure of SOM during the process of priming effect, and distinguished the formation and decomposition of SOM at the same time, which elucidated that glucose use efficiency, SOM accessibility and SOM recalcitrance were the main regulate mechanisms in different soil horizons. With the increase of cropping time, we found small changes of chemical structure of SOM in surface soil, but the oxidation of chemical structure of SOM in subsoil increased, and affected by decrease of organic matter input. We confirmed that increasing the amount of root exudates into deep soil can increasecarbon sequestration efficiency. In future study, we will simulate many kinds of root exudates, providing more scientific evidence for SOM turnover and its regulate mechanisms.