Effect of Microbial Carbon, Nitrogen, and Phosphorus Stoichiometry on Soil Carbon Fractions under a Black Locust Forest within the Central Loess Plateau of China

Microbial C/N/P stoichiometry ratios are of interest because of important ecosystem fluxes of C, N, and P, such as mineralization and immobilization, and they can help to characterize the cycling of soil elements, especially regarding soil C sequestration. However, it is not clear how soil microbial C/N/P stoichiometry influences C cycling. In this study, the concentrations of microbial biomass C, microbial biomass N, microbial biomass P, and soil C fractions were measured in 45-, 40-, and 25-yr-old black locust (Robinia pseudoacacia L.) forest soils and a sloped cropland. The results showed that soil organic C (SOC) concentrations or stocks and the percentage increment of the SOC stocks were highest under 45-yr-old black locust (RP45a). The concentrations of each C fraction in the black locust forest soils (45a, 40a, and 25a) were higher than that of sloped cropland in the 0- to 30-cm soil profile. The concentrations of microbial C, N, and P in RP45a were also higher by an average of 29.6 to 232.9, 9.2 to 17.9, and 0.6 to 1.1 mg kg(-1), respectively, compared with the same concentrations in sloped cropland at the 0- to 30-cm soil depth. Redundancy analysis indicated that soil C fractions, especially for particulate organic C (53-2000 and > 2000 mm), were significantly correlated with the microbial C/P and N/P ratios, and the "best" model selection indicated that the microbial N/P and C/P ratios significantly changed with soil C fractions with regard to stand age and soil depth in the black locust forests. Therefore, the present study demonstrated that C fractions respond to microbial C, N, and P stoichiometry after farmland-to-forest conversion and hence have the potential to affect C storage in the loess hilly region.