Microbial activity may be affected by nutrient additions and plant functional types (PFT) through influence on the levels of inorganic nitrogen (N) and labile carbon in the rhizosphere. To understand the effect of PFT (C3 and C4 species) and historical nutrient additions on temporal patterns of N partitioning between microbes and plants, I estimated seasonal trends in plant biomass and N content, microbial N, and soil N availability. I evaluated monthly emissions of CO₂ and N₂O, discriminating between fungal and bacterial production through incubations of soils under the influence of different PFTs and historical N additions. Last, I tested the effect of biosolid application on CO₂ and N₂O emissions from fungi and bacteria in SGS soils.

Seasonal trends in plant and microbial N concentration indicated that the two were synchronous during most of the plant growing season and both strongly influenced by precipitation. Plant functional type did not explain differences in microbial N and available soil N, but historical N amendments increased plant N content, decreased microbial N, and had no detectable effect on soil available N.

Fungi showed higher emissions of CO₂ and N₂O compared to bacteria, whereas there was no difference in emissions between the two groups in the historically N amended plots. There were no effects of PFT on bacterial and fungal emissions of CO₂ and N₂O but high historical N fertilization resulted in increased CO₂ and N ₂O emissions from bacteria.

Fungal emissions of CO₂ were higher than bacterial emissions in SGS sites compared to biosolid amended sites, but I detected no differences between microbial groups in N₂O emissions. CO₂ and N ₂O emissions were higher in biosolid treated sites than non-treated SGS sites even 20 years after amendments ceased. Biosolid treated sites dominated by forbs showed higher CO₂ emissions compared to sites dominated by C3 grasses, while C3-dominated sites with high available inorganic N had higher N₂O emissions than C4-dominated sites.

In summary, historical N additions had long lasting effects on SGS by increasing plant biomass and N. Given that N additions to ecosystems are increasing worldwide, it may be important to evaluate the impacts of these changes in processes on ecosystems services that grasslands provide. My results suggest that high levels of nutrient additions have unintended consequences such as increased CO₂ and N₂O emissions, and in particular carbon additions through biosolids increase fungal activity, which is also conducive to N₂O production.