Multiple nitrogen saturation indicators yield contradicting conclusions on improving nitrogen status of temperate forests

Abstract

Nitrogen (N) depositions in Europe are decreasing, but this could not explain faster than expected improvement of N saturation indicators in temperate forests. Alongside there were local signs of initial recovery from acidification during the past three decades and enhanced leaching of dissolved organic carbon and nitrogen (DOC, DON). These two global change processes both affect total dissolved nitrogen (TDN) levels and often occur simultaneously, hence complicating mechanistic explanations for changing European forest N status. We aimed to test the hypothesis that forest N status in northwest Europe has started to improve. If this hypothesis is confirmed, we wanted to investigate to what extent such improvement is due to reduced N deposition. We evaluated the evolution of multiple N saturation indicators in five ICP Forests Level II plots in northern Belgium, using long-term soil solution and foliage datasets. The DON:TDN ratio (molar) in soil solution increased overall in the O horizon (mean 0.279–0.463, slope 0.023–0.037 yr−1) and in the mineral soil (mean 0.134–0.78, slope 0.007–0.051 yr−1) between 2005 and 2014. The DOC:NO3− ratio (molar) in soil solution increased in three plots in the O horizon (mean 6.84–22.15, slope 0.58–1.92 yr−1) and in four plots in the mineral soil (mean 2.07–25.32, slope −0.06–5.76 yr−1) between 2002 and 2014. The ratio of N and phosphorus (P) concentrations in foliage (mgg−1) and the ratio of base cations (Bc=Ca+K+Mg) and N concentrations in foliage (molar) remained unaltered between 1999 and 2013. Changes in the soil solution chemical composition thus confirmed an improvement in forest N status, despite sustained high NO3− concentrations, but biotic recovery appeared to be lagging behind. This demonstrates that insight in forest recovery from N saturation requires a multiple indicator approach, and further monitoring of tree nutritional status alongside soil processes is needed to monitor the evolution of European forest N status.