Seasonal regulation of nitrogen cycling in a California grassland

Background/Question/Methods

Nitrogen (N) limits plant growth in most terrestrial ecosystems, and as such, it helps to mediate inter- and intra-specific plant competition, net primary production (NPP), plant composition, and species richness. For this reason, understanding temporal variability of N pools and processes is important, especially in ecosystems that experience dramatic seasonal shifts in temperature and precipitation, such as the California grassland. Most studies of California grasslands have focused on N cycling during the growing season (October – May); however, recent studies suggest that the hot, dry summer months may play a crucial role in year-round N dynamics. In order to contribute to this growing body of literature, we examined seasonal shifts in soil N pools and processes in a constructed California grassland comprised of native annual and perennial species (Bromus carinatus, Elymus glaucus, Leymus triticoides, Lotus purshianus, Lupinus bicolor, Nasella pulchra, Poa secunda, Vulpia microstachys). The study was conducted near Davis, CA, as part of a larger project aimed at understanding the impacts of precipitation change, N deposition, and simulated livestock grazing on plant community interactions. To address our objective, soil samples were collected once every 12 weeks (n=8), starting in October 2011, thereby capturing major seasonal changes.

Results/Conclusions

We found that nitrification potentials were significantly higher during the summer than the growing season. Similarly, denitrification potentials were equal to or higher than growing season rates. Pools of ammonium (NH₄⁺) were highest during the summer, while pools of nitrate (NO₃^-) were either equal to or lower than the growing season. Total soil microbial biomass was highest during the summer. All variables were significantly negatively correlated with gravimetric soil moisture content, with the exception of NO₃^- pools. These results differ from the established paradigm that low rates of N cycling occur during the summer months due to low water availability. Our findings, which are similar to other recent studies on N cycling in Mediterranean and semi-arid ecosystems, have large implications for our understanding of controls on N cycling and seasonal regulation of plant-available N.