Soil Moisture as a Mediator for Nitrogen Competition: Implications for Species Invasion and Climate Change

Abstract

Simple models of competition for one and two resources have been well investigated for both nitrogen and water. Ecosystem models of nitrogen and water, including their interactions are also well known, but competition and ecosystem processes have rarely been considered together. Using a simple ecosystem model of the nitrogen and water cycles including the dependence of the nitrogen cycle on soil moisture I examine the outcome of competition for nitrogen. This model shows that when there are species specific effects on soil moisture, the competitive outcome for nitrogen can be coexistence or alternative stable states. The last century has seen widespread invasion of Californian grasslands by annual grasses and forbs from the southern Mediterranean region. Both nitrogen and water have been shown to be limiting in this system. The native perennial grasses deplete the soil moisture and nitrogen to a lower level than the invasive annual grasses, suggesting that natives should be better competitors. Parameterising my model for a Californian grassland system I show that in fact the invasive annual grasses are the superior competitor for nitrogen at lower soil moisture when nitrogen competition is mediated by soil moisture. The competitive outcome at current rainfall levels in California is limited to competitive exclusion by the annuals or coexistence. Climate change caused by anthropogenic emissions of CO2 is already affecting communities and ecosystems. Global climate models predict changes in both mean amount and distribution of precipitation. I investigate the effect of changing precipitation distributions on both ecosystem and community processes in nitrogen limited systems. There is wide variation in ecosystem response to increased variation in rainfall. The outcome of N competition was more predictable, with the best competitor at lower soil moisture being consistently favoured with increasing rainfall variability. This might prove another problem in restoring native grasses in Californian grasslands.