Responses Of Red Soil Organic Matter Structure And Microbial Community To Land Use Change In East Hunan Province

Soils in subtropical China have great potential for agricultural production. Recently, the local land use changes strongly affect the structure of soil organic matter and microbial communities, which may further affect soil C cycling and balance. In this study, four typical land-use systems adjacently distributed in a watershed in eastern Hunan Province were selected, including natural forest (NF), Chinese fir plantation (CF), Chinese chestnut orchard (CO), and sloping tillage (ST). Soil samples were collected in the field from 1 m soil profile at 20 cm depth interval by self-made soil auger boring. Soil organic matter structure and microbial community were tested in laboratory to examine the effect of land use change on soil organic matter structure and microbial community. The main results were as follows:(1)The Py-GC/MS spectra of soil samples showed that chromatograms revealed peaks related to compounds derived from protein derivatives, esters, lignin, and aliphatic compounds. After NF converted to other land uses, the lignin content decreased by 32%-83%, but the content of aliphatic compounds increased 2-19 times. Lignin and aliphatic components of NF soil were single, but other soil had syringyl and p-hydmxyphenyl (P) structures and non fatty acid methyl aliphatic ester compound, and alcohol esters. This is because of changed soil organic matter sources.(2)FTIR showed that topsoil and subsoil organic matter structure in response to land use change differently. After NF converted to other land uses, averagly, the aromatic, alkanes, and carbohydrate content in topsoil organic matter were reduced by 30%. The carbohydrate and olefin compounds in coarse particles organic matter (CPOM) was increased by 0.9%-6% and 3%-50%, respectively. While, phenolic alcohols, aromatic, alkanes, alkenes, carbohydrates, and organic silicide in light fraction organic matter (LFOM) increased by 16%～72%,12%～84%,25%～116%, 26%～35%,9%～26%, and 9%～28%, respectively. And in subsoil, organic aromatic, olefin, and organic silicide content was increased by 17-45%,14%～48%, and 16%～44%, respectively. In CPOM, carbohydrates increased by 0.3%～15%. In LFOM, only phenolic alcohols, aromatic, alkane was rised by 9%～24%,2%～14%,7%～27%, 26%～35%,9%～26%, and 9%～28%, respectively.(3)Land use changes significantly affected chemical structure of soil active organic matter. FTIR showed that the relative proportion of each functional group in LFOM was increased, and topsoil DOM characteristic peaks of absorbance was decreased after NF change to other land uses. The greatest decline was found in Chinese fir plantations (38%), while the smallest decline in aromatic substances (19%) and the largest decline in carbohydrate and phenolic alcohols (29% and 28%). But the maximum decline was alcohol phenols (37%～62%), and the largest increase was olefins (22%～101%) in subsoil. Three main regions were indentified in all EEM:protein-like region, fulvic-like region, and humic-like region. The EEM spectra qualitatively showed that topsoil DOM protein-like region, fulvic-like region, and humic-like region was decreased 1%～60%, respectively; but in subsoil of CO and ST, three fluorescence peaks volume decreased by 21%～41%. After NF converted to other land uses, DOM structure became more complicated in CF, but had simplistic trend in CO and ST.(4)Land use change also affected the soil microbial biomass and microbial community composition. After NF converted to other land uses, topsoil MBC contents were decreased by 46%,66%, and 82%, respectively, and soil microbial population were also significantly changed, with decrease of fungal by 40%～73%, bacterial by 21%-67%, and put actinomycetes by 7%～73%). PLFA showed that total fatty acids in soil, gram positive bacteria, gram negative bacteria, bacteria, actinomycetes, and fungi characteristic fatty acids were decreased by 26%～68%,24%～80%,37%～65%, 30%～38%,23%～78%, and 5%～22%, respectively. Soil microbial community composition dominated by bacteria (22%～69%). The biomass of gram negative bacteria slightly greater than gram positive bacteria, but the ratio is opposite in Chestnut orchard.