Influence of nutrient availability on soil respiration and microbial activity in a tree-grass ecosystem

Increased availability of nitrogen (N) and phosphorus (P) due to human activity has potential to increase carbon (C) stocks if it leads to added plant productivity. Whether increased productivity leads to changes in ecosystem C stocks is dependent on if processing of C by soil microbes is also increased by nutrient addition or its stoichiometry. Likely, the response of soil C cycling to changes in N:P depends on site fertility. In tree-grass ecosystems, like oak-savannas, there is variation in soil organic C (SOC) due to in situ decomposition of tree litter. The aim of this work is to quantify how N:P availability affects C cycling in oak-savanna soil habitats. We found that soil respiration and its response to nutrient addition differed between habitats. Open grassland had higher respiration with N addition while soil under canopies had the opposite, confirming that SOC content modifies soil respiration response to nutrient addition. A second approach examined ecosystem allocation of added N using a stable isotope tracer. Open grassland with high N:P had the most label recovery; meaning that it is more N-limited than under canopy. A final tract looked at changes in microbial activity after N and P addition where carbon-use efficiency (CUE) was measured on soils with short or long-term fertilization history. Treatments had no effect on CUE under canopies. Grassland had lower CUE when single nutrients were added in the short-term, but unchanged when combined. Thus, stoichiometric imbalance reduces CUE in the short term, but more study is needed. Overall, oak-savanna microbial activity is not nutrient-limited, but limited by C availability. Grassland soil of oak-savannas is more sensitive to changes in nutrient availability than that under canopies. Grassland soil C is more susceptible to loss when N:P stoichiometry is higher due to increased respiration. Greater mechanistic understanding is needed of how N and P alter microbial activity.