Nitrogen saturation and net ecosystem production - Reply

Nitrogen (N) deposition alters ecosystem function in several ways, with important effects on N leaching and water quality, as well as on interspecific competition and biodiversity. These changes have been attributed to ecosystem N saturation, defined as the alleviation of N limitations on rates of biological function1. After an initial fertilization effect, N saturation has also been suggested to reduce plant function and growth2, eventually leading to forest dieback. Although our observation of a substantial positive effect of N deposition on forest carbon (C) sequestration3 does not imply the absence of nitrate losses or other negative effects, as rightly stressed by De Schrijver et al.4, the sustained response observed demonstrates that the fear of a generalized forest decline in response to N fertilization could be overstated, at least within the rather broad N deposition range explored in our analysis. The nature of the observed response of forest C sequestration toNdeposition, however, has been questioned outright by de Vries et al.5, who suggested that it could be an artefact resulting from the covariation between N deposition and other environmental variables. The arguments proposed against an overwhelming N effect, however, do not seemto stand up to close scrutiny. Far from implausible, a 200:1 sensitivity is nevertheless higher than suggested by long-term forest fertilization experiments. Potential problems withNmanipulation studies have already been discussed. In particular, they overlook the role of canopy N uptake, which enables plants to absorb a relevant fraction of incoming N without any competition from soil microbes. Canopy N uptake amounts to up to 70% of N deposition, providing as much as one-third of tree N requirements. The critical comparison of results from ecosystem manipulation and observational studies could be providing a rare, unforeseen insight into the key factors controlling C–N relations in forest ecosystems.