Nitrogen states in plant ecosystems: A viewpoint

Abstract

Terrestrial ecosystems are considered to be in only two possible states: N unsaturated or N saturated. This view lacks rigour and has led to three differing concepts of N saturation: (1) a continuum of changes in N states and processes; (2) a threshold point, when N output increases; and (3) any equilibrium state, when N output equals N input. A simple model of ecosystem N content, input and output is used to point out that, strictly, there are four possible N states of ecosystems: (1) at equilibrium, but N saturated and N-limited, so that they will respond to additional N input with increased N accumulation; (2) not at equilibrium and accumulating N, either in response to additional N input, or more commonly in nature, when recovering from fire, other disturbance or unfavourable conditions; (3) at equilibrium but truly N saturated and not N-limited, so that any additional N input is matched by equal N output; and (4) not at equilibrium and losing N because of disturbance or soil changes induced by N addition itself or other factors. Most natural ecosystems are, for most of the time, in one of the two non-equilibrium states, especially in short-term N-addition experiments. It is not meaningful to regard them as being either N saturated or N-limited. © (C) 2000 Annals of Botany Company.