Abstract: The Phospholipid Fatty Acids (PLFAs) fingerprint and humus fraction methods were used to determine the accumulation of soil organic carbon in cultivated farmlands and the interaction mechanisms of soil microbial communities with soil organic carbon. The combined effects of organic manure and chemical fertilizer on soil organic carbon accumulation, soil humus forms and their relationships with soil microbial community structures were studied in a 5-crop rotation mode of 'potato-potato-oil sunflower-potato-oil sunflower' over a period of five years. The experimental treatments were as follows-no fertilization, inorganic fertilizer application, combined application of inorganic fertilizer and cow dung, combined application of inorganic fertilizer and sheep manure, combined application of inorganic fertilizer and biological organic fertilizer, and then combined application of inorganic fertilizer and fulvic acid potassium. The results showed that soil organic carbon fluctuated with increasing tendency over the five-year period. Compared with the control (no fertilization), soil organic carbon increased at annual average rates of 6.61% and 8.97% under the treatments of combined application chemical fertilizer with cow dung or separately with sheep manure. The rate of accumulation of soil organic carbon was influenced by the amount and type of added exogenous organic matter to the soil. The contents of stable or tightly combined humus and ratio of loosely combined humus to tightly combined humus increased following the addition of high quantities of organic manure (chemical fertilizer plus cow dung or chemical fertilizer plus sheep manure) and inorganic fertilizer. Compared with no fertilization, there were significant differences in soil bacteria, fungi, actinomycetes, protozoa and total microbial biomass, marked by phospholipid fatty acids under combined application of inorganic fertilizer and sheep manure or cow dung. The biomass ratio of gram-positive bacteria to gram-negative bacteria (G⁺/G^-) in the treatments with inorganic fertilizer plus organic fertilizers decreased. There was no obviously difference in the ratio of G⁺/G^- among treatments of combined application of inorganic fertilizer and organic fertilizers. Multivariate analysis showed a good correlation between the first ordination axes based on soil microbial biomass marked by PLFAs and the second ordination based on combined soil organic carbon and humus (P₁=0.568, P₂=0.611). The relationship between soil microbial biomass and soil environmental factors was explained by the 98.69% cumulative variation in spatial scale. There was a positive correlation between the content of loosely combined humus and G⁺/G^- of soil microbial community. It was concluded that the tightly combined humus fraction gained higher stability than the loosely combined humus when exogenous organic carbon was applied to the soil. The soil bacteria and fungi biomass marked by PLFAs were promoted with increasing amounts of exogenous organic carbon in the soil. The biomass ratio of fungi to bacteria was not influenced by fertilizer management and amount of exogenous organic carbon. Generally, the change in soil microbial community structure was driven by the change in soil organic carbon content and soil humus fraction. Combined inorganic fertilizer with cow manure and inorganic fertilizer with sheep manure supported the accumulation of soil organic carbon and the formation of loose combined humus, but also promoted an increase in soil microbial biomass. The research provided a reliable scientific basis for soil fertilization in semiarid areas in Ningxia.