Soluble ammonium in plants as a bioindicator for atmospheric nitrogen deposition: Refinement and testing of a practical method

Abstract

Substrate nitrogen and soluble ammonium represent newly recognized parameters in the context of the bioindication of atmospheric nitrogen concentrations and deposition. The interest in these parameters originates from the analysis of plant carbon and nitrogen dynamics in ecosystem models in relation to the potential for plants to absorb or emit ammonia from the atmosphere, as regulated by the ammonia 'compensation point' (Sutton et al. 2004). The ammonia compensation point is a function of both agricultural and atmospheric N inputs. Enhanced nitrogen deposition raises the ammonia 'compensation point' and provides a limitation to further nitrogen deposition. In this context, the rates of atmospheric ammonia deposition, and hence total N deposition, depend partly on the extent to which the ecosystem has already responded to N deposition. As a consequence of these interactions, the level of substrate N or soluble ammonium (NH4-N) may be a convenient indicator of accumulated nitrogen deposition and ecosystem response (Sutton et al. 2004). Previous studies (Sutton et al. 2004; Leith et al. 2005) suggest that the relationship to atmospheric N is more precise for soluble ammonium (NH 4-N) than for total tissue N in plants. The steeper slope of the line in Fig. 17.1 for soluble NH4-N (=free ammonium) indicates that NH4-N is more sensitive to changes in ammonia air concentration than total tissue N (%N) and total soluble nitrogen (substrate nitrogen). © 2009 Springer Netherlands.