Nitrogen (N) availability is key to the functioning of ecosystems and the cycling of nutrients and energy through the biosphere. However, there is growing evidence that N availability is decreasing in many terrestrial ecosystems. The consequences of declining N availability will be widespread. For example, a decreased concentration of N in leaves reduces the availability of N to insects, contributing to population declines that may then cascade through higher trophic levels. Mason et al. reviewed the extent of this phenomenon, and the anthropogenic factors that might be driving it (including climate change and increasing atmospheric carbon dioxide), and discuss how its damaging effects might be mitigated.

The availability of nitrogen (N) to plants and microbes has a major influence on the structure and function of ecosystems. Because N is an essential component of plant proteins, low N availability constrains the growth of plants and herbivores. To increase N availability, humans apply large amounts of fertilizer to agricultural systems. Losses from these systems, combined with atmospheric deposition of fossil fuel combustion products, introduce copious quantities of reactive N into ecosystems. The negative consequences of these anthropogenic N inputs—such as ecosystem eutrophication and reductions in terrestrial and aquatic biodiversity—are well documented. Yet although N availability is increasing in many locations, reactive N inputs are not evenly distributed globally. Furthermore, experiments and theory also suggest that global change factors such as elevated atmospheric CO2, rising temperatures, and altered precipitation and disturbance regimes can reduce the availability of N to plants and microbes in many terrestrial ecosystems. This can occur through increases in biotic demand for N or reductions in its supply to organisms. Reductions in N availability can be observed via several metrics, including lowered nitrogen concentrations ([N]) and isotope ratios (δ15N) in plant tissue, reduced rates of N mineralization, and reduced terrestrial N export to aquatic systems. However, a comprehensive synthesis of N availability metrics, outside of experimental settings and capable of revealing large-scale trends, has not yet been carried out.