Nutrient Limitation in Sequoia National Park
Personnel:	Steven Perakis, Peter Vitousek (Stanford University), Chev Kellogg (OSU), Jon Keeley (USGS-WERC)
Funding:	National Park Foundation Ecological Research Program

The pervasiveness of nitrogen (N) limitation in terrestrial plant communities has lead to a search for general mechanisms that can maintain N limitation in a variety of environments over vastly different time scales. Constraints on N inputs from biological fixation are broadly acknowledged (though not fully understood) as a primary cause of N-limitation on land. At the same time, pathways of N loss that are not under direct biotic control may further prevent N accumulation and intensify N-limitation in a wide variety of ecosystems. This study seeks to understand constraints on N supply and accumulation in semi-arid terrestrial ecosystems. We are conducting nutrient fertilization experiments in grasslands of Sequoia National Park to document patterns of nutrient limitation in these ecosystems, as well as to understand potential shifts in community composition (native, invasive, N-fixer) that arise from differences in nutrient supply. We are also manipulating precipitation inputs to grassland microcosms in the greenhouse to examine how variations in moisture regimes (episodic, seasonal, inter-annual) drive asynchrony in N cycling and N loss from soils. This information is being coupled to simulation modeling to examine how constraints on ecosystem N input-output balances contribute to the widespread occurrence of N-limitation on land.

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