Carbon and phosphorus addition effects on microbial carbon use efficiency, soil organic matter priming, gross nitrogen mineralization and nitrous oxide emission from soil

The quantity and chemical composition of soil organic carbon (C) are primary factors controlling the growth and activity of soil microorganisms. However, availability of phosphorus (P) can also limit microbial activity as it is required for the synthesis of genetic and cellular components, metabolism and energy transfer. Little is known about how P availability influences microbial activity in response to C of varying chemical composition and recalcitrance. A laboratory incubation experiment was conducted to examine the effect of 13C-labeled glucose, oxalic acid and phenol, with and without P, on microbial C use efficiency (CUE), soil organic matter (SOM) priming, gross nitrogen (N) mineralization and nitrous oxide (N2O) emission from a grassland soil. Our results showed that microbes used glucose more efficiently but oxalic acid less efficiently compared to more recalcitrant phenol, and did not rely on P nutrition to partition C into growth and respiration. All three C substrates caused real SOM priming independent of their energy content or chemical structure, while addition of P increased the priming effect. Variability in chemical structures of C substrates affected gross N mineralization and hence N2O emission, while P application directly influenced N2O emission, especially when C substrates were added. In conclusion, our findings emphasize that the coupling of C and P fertilization in soils can have strong effects on terrestrial C stocks by favoring native soil organic C loss, as well as on N2O emission.