PARENT SESSION
Oral Session # 19: Biogeochemistry II: Grasslands.
Presiding: JT Lennon
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

The dominant role of plant species in controlling the response of N mineralization to altered plant diversity, atmospheric CO₂, and N deposition.

West, Jason^*,1, Wedin, David², Hobbie, Sarah¹, Reich, Peter¹, ¹ University of Minnesota, St. Paul, MN² University of Nebraska, Lincoln, NE

ABSTRACT- Declining biodiversity and increasing nitrogen inputs and atmospheric CO₂ are important global changes that strongly affect terrestrial ecosystems. We present results from a long-term experiment where we manipulated plant species biodiversity, atmospheric CO₂, and N deposition (BioCON) in N-limited grassland plots in central Minnesota and examined the consequences for net N mineralization rates (Nmin). As N limitation is a common feature of terrestrial ecosystems, the response of N cycling to global change factors may shape overall ecosystem response to global change. Consistent with our predictions, diversity strongly decreased Nmin. This effect was evident in the first year of plant growth. The difference among diversity treatments dampened over time, suggesting that greater species diversity may accelerate the return of plant-soil interactions characteristic of prairie ecosystems. The results from year 5 revealed an interaction between elevated CO₂ and N. Elevated CO₂ decreased Nmin at ambient N, whereas it stimulated Nmin at elevated N. The observed changes in Nmin in mixed-species plots are likely the result of shifts in dominance in response to the treatments. Consistent with previous research, we observed large species effects on Nmin. Most of the effects in ambient plots corresponded to known plant trait differences between and within functional groups (i.e. C3 & C4 grasses, C3 forbs, legumes). However, both the magnitude and direction of changes in Nmin in response to CO₂ and N differed widely among species. For example, elevated CO₂ dramatically stimulated Nmin in Koeleria cristata plots, whereas there was no effect of CO₂ for Agropyron repens, both C3 grasses. The overall time course likely represents initial immobilization of N, followed by N cycle changes from plant effects. Understanding the mechanisms of these responses improves our ability to generalize about the effects of global change. Gross N mineralization measured in year five suggests that plant effects on Nmin are caused by changes in N immobilization. Our results demonstrate important effects of global changes on N cycling, and the primary role of plant composition in determining the direction and magnitude of these responses.

Key words: biodiversity, ecosystem function, nitrogen , atmospheric CO₂