Effects of earthworm and arbuscular mycorrhizal fungal inoculation on carbon component accumulation and allocation in rocky desertification soils

Exploring the responses of carbon component accumulation and allocation to arbuscular mycocorrhizal fungi (AM) and earthworm inoculation can provide reference for improving carbon sequestration potential and bioremediation efficiency in rocky desertification soils. In this study, we chose Fraxinus malacophylla as the host plant to inoculate with Funneliformis mosseae (FM), earthworm (E), and E+FM, using no earthworm and mycorrhizae addition as CK to examine the spatiotemporal variations in soil carbon components (i.e., total organic carbon, microbial biomass carbon, easily oxidized organic carbon, and recalcitrant organic carbon) and their allocation (i.e., microbial biomass carbon/total organic carbon, easily oxidized organic carbon/total organic carbon, and recalcitrant organic carbon/total organic carbon). The results showed that 1) The respective and interactive inoculation of E and AM significantly promoted the accumulation of each carbon component. In contrast with the control, the average carbon component levels under three inoculation treatments were ranked as E+FM>E>FM. The three inoculation treatments significantly promoted soil microbial carbon/total organic carbon (30.5%-68.5%) and easily oxidized carbon/total organic carbon (31.2%-39.2%), but decreased recalcitrant organic carbon/total organic carbon (2.9%-16.2%). 2) The spatiotemporal variation in accumulation and allocation of soil carbon components varied between the inoculation treatments. The maximum value of each carbon component occurred in June. The increase in each carbon component was significantly higher in E+FM (33.0%-122.1%) than that in E (31.2%-95.4%) and FM (9.2%-41.3%). The maximum value of microbial biomass carbon/total organic carbon and easily oxidized organic carbon/total organic carbon was observed in June, while that of recalcitrant organic carbon/total organic carbon was recorded in December. In contrast with CK, the amplitude of variation in the proportion of carbon components in total organic carbon under the three inoculation treatments was ranked as E+FM>E>FM. The accumulation and allocation of all carbon components decreased (9.7%-146.2%) along the soil profile. The level of carbon components in the E treatment decreased the smallest. The microbial biomass carbon/total organic carbon and easily oxidized carbon/total organic carbon decreased the least and the recalcitrant organic carbon/total organic carbon decreased the greatest under the E+FM treatment. 3) Changes in soil physicochemical properties under the three inoculation treatments significantly affected the accumulation and allocation of organic carbon components. Soil pH was negatively correlated with carbon component accumulation and allocation, whereas other soil variables were positively correlated with them. 4) The results of principal component analysis showed that soil water content, total nitrogen, and total phosphorus were the main factors driving carbon component accumulation, while soil water content, total phosphorus, and pH were the main factors controlling carbon component allocation. Therefore, we concluded that the earthworms, AM fungi and their interaction affected the accumulation and allocation of carbon components in Yunnan rocky desertification soils, which would primarily depend on the changes of soil water content, acid-base property, as well as nitrogen and phosphorus conditions.