Nitrogen partitioning between microbes and plants in the shortgrass steppe

Nitrogen (N) and water additions in the shortgrass steppe change the dominance of plant functional types (PFT) that are characterized by different photosynthetic pathways and phenologies. We aimed to examine monthly patterns of plant N and microbial N storage during the growing season, and to assess whether N fertilization last applied 30 years ago alters the timing and magnitude of N storage. We measured plant biomass and N, and microbial biomass N monthly during the growing season. We found differences in temporal patterns of plant and microbial N storage in the control plots, with microbial storage higher than plant storage in July, and the opposite trend in September. Unlike the control plots, the plots fertilized 30 years ago exhibited overlapping peaks of N storage in plants and microbes in August. Seasonal trends indicated that rainfall was an important control over plant and microbial activity at the beginning of the growing season, and that temperature limited these activities at the end of the growing season. PFT affected the amount of microbial N, which was in general higher under C3 grasses than other PFTs, independent of fertilization. Historical resource additions increased plant biomass and N, but had little effect on microbial N. These results highlight the complexity of the microbial response. Changes in climate that influence precipitation timing will affect the temporal pattern for microbial biomass N, while management practices resulting in altered plant community composition will influence the magnitude of microbial biomass N.