Two pathways, fungal and bacterial, are key to organic matter decomposition and nitrogen mineralization in soil. These have different decomposition rates and efficiencies, and play differing roles across a broad range of soil ecosystems. In bacterial-dominated soils, bacteria increase rates of organic matter decomposition and nutrient mineralization; enhancing nutrient provision. In fungal-dominated soils, fungi lower the conversion rate of nutrients and energy; enhancing organic matter storage and nutrient retention. The fungal-bacterial ratio indicates the structural and functional responses of soil food webs to land use types and soil textural conditions. Four main approaches for the measurement of the fungal-bacterial ratio can be distinguished: 1), direct observation via microscopy; 2), culturing involving selective inhibition of bacteria or fungi; 3), measurement of fungal ergosterol; and 4), measurement of fungal glucosamine and bacterial muramic acid. For the same or similar ecosystems, reported fungal biomass varies widely between different data sets obtained using different approaches for measurement; suggesting that reliable estimation is a key issue. Phospholipid fatty acid (PLFA) analysis provides a new approach to the study of microbial community structure. Although bacterial and fungal PLFA levels in the soil (ug nmol^-1) are a useful quantitative indicator, it is very difficult to convert PLFA ratios into carbon biomass ratios, due to the widely varying sizes of fungal cells. For any given PLFA level, the fungal contribution is higher than the bacterial in terms of biomass. Research into biogeochemical organic carbon cycles, suggests that biological mineralization by soil microorganisms is very important. If the fungal-bacterial ratio obtained from PLFA measurement could be converted into biomass, then fungal and bacterial carbon mineralization rates could be distinguished and calculated to determine the influence of each decomposition pathway on carbon flow. Ratios of fungal biomass C to PLFA concentration vary greatly (42-366 C/PLFA μg/nmol), and require appropriate testing. In an experimental site on the North China Plain spring and autumn soil was sampled under a winter wheat and summer maize rotation agroecosystem. Total microbial biomass (carbon) and PLFA were measured. Initially, we compared microbial biomass with total PLFA concentration; determining that 11.3 was a reasonable multiplier for the conversion of total PLFA to biomass. To convert fungal PLFA to biomass, we used 42 as a multiplier. We considered bacterial biomass the margin between total microbial biomass and fungal biomass. Using this approach, we found that the fungal-bacterial ratio was consistent with that indicated by soil fauna indices, including 1), abundance of bacteria-eating protozoa, 2), ratio of fungus-eating nematodes to bacteria-eating nematodes, and 3), abundance of fungus-eating mites. Further research into the use of this ratio will be summarized in our next paper.