Nitrogen limitation: which process is limited and how rapidly can processes adjust?

Nitrogen limitation is a concept that has been widely explored and applied to ecosystem carbon cycle research. However, it has not been carefully examined which carbon process is more limited by nitrogen availability than others and how rapidly different nitrogen processes adjust to influence its availability under global change. By comprehensively synthesizing the literatures, here we show that nitrogen limitation is mostly restricted to plant productivity because nitrogen enrichment only increases plant productivity but decreases microbial biomass and activity. Neither net ecosystem productivity nor soil carbon storage consistently increases or decreases across studies under nitrogen enrichment. While nitrogen leaching and gas emission increase almost simultaneously with nitrogen addition, soil inorganic nitrogen concentrations return to ambient levels within a few years after cessation of decadal nitrogen addition. On the other hand, both elevated CO₂ and ecosystem succession almost always increase plant nitrogen content simultaneously with plant carbon accumulation, mainly due to increasing nitrogen fixation and decreasing nitrogen losses. Thus, soil nitrogen cycle processes continuously adjust to change its availability in response to either overload under nitrogen addition or deficiency under CO₂ enrichment and ecosystem succession. Indeed, processes of both carbon and nitrogen cycles continuously adjust under global change, leading to shifts in carbon and nitrogen coupling. Ecosystem models failing to simulate these adjustments can't well simulate carbon-nitrogen coupling nor predict ecosystem carbon sequestration.