[Contribution of Microbial Necromass Carbon to Soil Organic Carbon Fractions and Its Influencing Factors in Different Soil Layers of Typical Forest Land and Grassland in the Loess Plateau， China].

Abstract

Microbial necromass carbon（MNC） is a significant source of soil organic carbon（SOC）. However， the contributions of microbial necromass to different organic carbon fractions and their influencing factors in various soil layers under different land use types remain unclear. In this study， typical soil samples from 0-20， 20-40， 40-70， and 70-100 cm soil layers were collected from representative forest land and grassland sites in the Loess Plateau. The contents of SOC， mineral-associated organic carbon（MAOC）， and particulate organic carbon（POC） were measured. Furthermore， the MNC， fungal necromass carbon（FNC）， and bacterial necromass carbon（BNC） contents and their contributions to POC and MAOC fractions in each soil layer were determined， and their influencing factors were analyzed. The results follow： ① In the 1 m soil profile， ω（MAOC） and ω（POC） ranged from 2.54 to 11.09 g·kg^-1 and 0.19 to 3.06 g·kg^-1 in grassland and from 3.02 to 8.78 g·kg^-1 and 0.30 to 2.45 g·kg^-1 in forest land， respectively. The SOC content in the 0-20 cm layer was 2.53 and 2.35 times higher than the content in the 20-40 cm layer for grassland and forest land， respectively， while the MAOC content was 2.10 and 2.12 times higher， respectively. The POC content in the 0-20 cm layer of grassland was 81.7%， 42.86%， and 40.63% higher than that in the subsequent three layers， but in forest land， POC content decreased by 77.14% in only the 20-40 cm layer. The contents of SOC， MAOC， and POC in the 0-20 cm layer of grassland were 1.31， 1.26， and 1.25 times those in forest land， respectively. In the 70-100 cm layer， the contents of MAOC and POC in forest land were 1.19 and 1.58 times those in grassland， respectively. ② The ω（MNC） ranged from 0.65 to 5.85 mg·g^-1 in grassland and from 0.80 to 6.42 mg·g^-1 in forest land. The ω（FNC） and ω（BNC） ranged from 0.09 to 4.49 mg·g^-1 and 0.03 to 0.58 mg·g^-1 in grassland and from 0.02 to 4.61 mg·g^-1 and 0.04 to 0.39 mg·g^-1 in forest land， respectively. FNC dominated the POC and MAOC fractions， with ω（FNC）/ω（MAOC） in the 0-20 cm layer being 1.48 and 17.33 times that of the 20-40 cm layer in grassland and forest land， respectively. In the 20-100 cm layer， ω（FNC）/ω（MAOC） in grassland was 12.7 times that in forest land. The ω（BNC）/ω（POC） in the 70-100 cm layer was 2.25 and 16.75 times higher than in the 0-20 cm layer for grassland and forest land， respectively. In the 1 m profile， ω（FNC）/ω（POC） in forest land was 5.95 times that in grassland. ③ Random forest modeling indicated relative importances of AGB， BGB， TN， NO₃^--N， SOC， and DOC for MNC accumulation in the 0-20 cm layer of grassland and forest land， whereas BGB， SOC， DOC， TN， MBC， MBN， NO₃^--N， NH₄?-N， and Clay+Silt were more critical in the 20-100 cm layer. Regression analysis revealed that MNC in the 1 m soil layer of grassland and forest land was significantly positively correlated with dissolved nitrogen（NO?^--N and NH??-N） and dissolved organic carbon（DOC） in the MAOC fraction but significantly negatively correlated in the POC fraction. In the 20-100 cm soil layer， MNC was significantly positively correlated with only microbial biomass carbon（MBC） in the MAOC fraction. In summary， the content of MNC varies across different soil layers and carbon fractions in forest and grassland soils， and soluble carbon and nitrogen are key factors influencing the accumulation of MNC. When the nutrient level is relatively high（0-20 cm soil layer）， fungi-dominated microorganisms in forest soils store necromass carbon in the POC fraction through an "extracellular modification" mechanism， while microorganisms in grassland soils store necromass carbon in the MAOC fraction through mainly an "intracellular turnover" pathway. In nutrient-poor conditions（20-100 cm soil layer）， bacteria-dominated microorganisms in both forest and grassland soils utilize necromass in the POC fraction as a carbon source to produce microbial biomass， thereby promoting the accumulation of necromass carbon in the MAOC fraction.