Anthropogenic impacts have globally increased element inputs (N > P) and have shifted the stoichiometry (C:N:P) of soils and plants in many parts of the world. We aim to understand the effects of single and combined additions of N and P on element cycling in grasslands. For this purpose, we studied effects of N and P inputs on C and N cycling in a 60-years old fertilization experiment in a grassland in South Africa.
We show that addition of N but not of P changed soil element cycling. N fertilization enhanced the amount of dissolved total nitrogen (DTN), decreased the stoichiometry of microbial substrate (i.e. DOC:DTN ratio), and overall increased available N. While the long-term element inputs affected the microbial community composition, the microbial biomass C:N ratio was not affected. Net Cmin rates decreased with N addition, which was attributed to soil acidification following N fertilization. As a consequence microbial biomass decreased and thus lowered the Cmin rate. It is also likely that the saturation of microbial N demands constrained microbial N mining, and/or even inhibited the production of oxidative enzymes for decomposing complex organic compounds. In contrast, net Nmin increased with N fertilization, presumably due to a saturation of microbial N demands. Low Nmin rates, especially at treatments with low available N indicated predominate N immobilization. This was associated with higher Leucine-aminopeptidase activities at these treatments indicating microbial N acquisition. However, continuous low N2 fixation rates, which were independent of the N and P fertilization level, pointed to a minor relevance of free-living N2 fixation in this grassland soil. Overall, our results suggest that additions of N, but not of P, changed microbial element cycles by increasing Nmin rates and decreasing Cmin rates. We conclude that raising N loads via anthropogenic activities enhance the plant available N pool and promote soil C sequestration in this mesic grassland.