Does nitrogen deposition increase forest production? The role of phosphorus

Abstract

Effects of elevated N deposition on forest aboveground biomass were evaluated using long-term data from N addition experiments and from forest observation plots in Switzerland. N addition experiments with saplings were established both on calcareous and on acidic soils, in 3 plots with Fagus sylvatica and in 4 plots with Picea abies. The treatments were conducted during 15 years and consisted of additions of dry NH₄NO₃ at rates of 0, 10, 20, 40, 80, and 160 kg N ha⁻¹ yr⁻¹. The same tree species were observed in permanent forest observation plots covering the time span between 1984 and 2007, at modeled N deposition rates of 12–46 kg N ha⁻¹ yr⁻¹. Experimental N addition resulted in either no change or in a decreased shoot growth and in a reduced phosphorus concentration in the foliage in all experimental plots. In the forest, a decrease of foliar P concentration was observed between 1984 and 2007, resulting in insufficient concentrations in 71% and 67% of the Fagus and Picea plots, respectively, and in an increasing N:P ratio in Fagus. Stem increment decreased during the observation period even if corrected for age. Forest observations suggest an increasing P limitation in Swiss forests especially in Fagus which is accompanied by a growth decrease whereas the N addition experiments support the hypothesis that elevated N deposition is an important cause for this development.

Introduction

Anthropogenic nitrogen emission from combustion and agricultural activities has led to eutrophication of forest ecosystems in large parts of the industrialized world (Nihlgård, 1985, Aber et al., 1989, Galloway, 1995). As a consequence, previously N limited ecosystems are getting saturated with nitrogen (Aber et al., 1998) and leaching nitrate (Dise and Wright, 1985, Gundersen et al., 1998). The release of N limitation stimulates growth at least initially; an increase in foliar N is expected to increase photosynthesis (Reich et al., 1995). It has therefore been postulated that nitrogen will increase carbon sink strength of forests by a substantial amount (Townsend et al., 1996, IPCC, 2001, Reich et al., 2006). A larger forest growth has indeed been observed in the second half of the last century that was attributed to higher N availability (Spiecker et al., 1996). There are, however, observations that this growth increase after N addition is not sustainable as other limitations will come into effect. The newest IPCC report acknowledges these limitations (Nabuurs et al., 2007). Aber et al. (1989) suggest that phosphorus is getting limiting to plant growth when N limitation is removed. This concept of nutrient limitation would, however, explain the absence of further growth stimulation but not a decrease below pristine situation as observed e.g. by Nellemann and Thomsen (2001) and by Boxman et al. (1998) in Norway spruce, by Magill et al. (2004) in pine and by McNulty et al. (2005) in red spruce.

A network of forest observation plots in Switzerland, with a gradient of modeled N deposition ranging from 12 to 46 kg N ha⁻¹ yr⁻¹, allows to analyze N effects on growth and nutrition by epidemiological methods. The mechanistic background for such statistical analyses is given by complementary N addition experiments with saplings of Fagus sylvatica and Picea abies in different regions. Both datasets will be presented here and the conclusions compared.