COS 120-1 - A “model-based analysis of causes of nitrogen limitation in terrestrial ecosystems

Abstract: Nitrogen (N) limitation to net primary production is widespread and influences the responsiveness of ecosystems to many components of global environmental change. Both logic and models demonstrate that the co-occurrence of losses of N in forms that organisms cannot control and barriers to biological N fixation (BNF) that keep it from responding to N deficiency are necessary to N limitation. We developed a simulation model for N limitation, applied it across a climate gradient in Hawaii, and validated it with patterns in ¹⁵N across that gradient. Earlier models have focused on the continuous process of leaching losses of dissolved organic N in biologically unavailable forms, but here we show that discontinuous losses of ammonium and nitrate when they are temporariy present in excess, normally forms of N whose losses organisms can control, can be uncontrollable by organisms and can contribute to N limitation under realistic conditions. These discontinuous losses can be caused by temporal variation in precipitation or by ecosystem-level disturbance like harvest, fire, and windthrow. Temporal variation in precipitation is likely to increase and to become increasingly important in causing N losses as anthropogenic climate change proceeds. We also demonstrate that under the conditions simulated here, differentially intense grazing on N- and P-rich symbiotic N fixers is the most important barrier to the responsiveness of BNF to N deficiency.