  The global rise in anthropogenic reactive nitrogen (N) and the negative
impacts of N deposition on terrestrial plant diversity are well-documented. The
R* theory of resource competition predicts reversible decreases in plant
diversity in response to N loading. However, empirical evidence for the
reversibility of N-induced biodiversity loss is mixed. In a long-term
N-enrichment experiment in Minnesota, a low-diversity state that emerged during
N addition has persisted for decades after additions ceased. Hypothesized
mechanisms preventing recovery of biodiversity include nutrient recycling,
insufficient external seed supply, and litter inhibition of plant growth. Here
we present an ODE model that unifies these mechanisms, produces bistability at
intermediate N inputs, and qualitatively matches the observed hysteresis at
Cedar Creek. Key features of the model, including native species' growth
advantage in low-N conditions and limitation by litter accumulation, generalize
from Cedar Creek to North American grasslands. Our results suggest that
effective biodiversity restoration in these systems may require management
beyond reducing N inputs, such as burning, grazing, haying, and seed additions.
By coupling resource competition with an additional inter-specific inhibitory
process, the model also illustrates a general mechanism for bistability and
hysteresis that may occur in multiple ecosystem types.
