Emergence and maintenance of nutrient limitation over multiple timescales in terrestrial ecosystems.

Nutrient availability often limits primary production, yet the processes governing the dynamics of nutrient limitation are poorly understood. In particular, plant-available (e.g., nitrate) versus plant-unavailable (e.g., dissolved organic nitrogen) nutrient losses may have qualitatively different impacts on nutrient limitation. We examine processes controlling equilibrium and transient nutrient dynamics at three separate timescales in a model of a nutrient cycling through plants and soil. When the only losses are from the plant-available nutrient pool, nutrient limitation at a long-term equilibrium is impossible under a wide class of conditions. However, plant biomass will appear to level off on a timescale controlled by plant nutrient turnover (years in grasslands, decades to centuries in forests), even though it can grow slowly forever. Primary production can be nutrient limited in the long-term when there are losses of plant-unavailable nutrients or when the mineralization flux saturates with increasing detrital mass. The long timescale required for soil nutrient buildup is set by the plant-unavailable loss rate (centuries to millennia). The short timescale (hours to days) at which available nutrients in the soil equilibrate in the model is controlled by biotic uptake. These insights into processes controlling different timescales in terrestrial ecosystems can help guide empirical and experimental studies.