Nitrogen deposition shows no consistent negative nor positive effect on the response of forest productivity to drought across European FLUXNET forest sites

S C Van Der Graaf,T A J Janssen,J W Erisman,M Schaap

doi:10.1088/2515-7620/ac2b7d

Abstract

Atmospheric reactive nitrogen (N) deposition is an important driver of carbon (C) sequestration in forest ecosystems. Previous studies have focused on N-C interactions in various ecosystems; however, relatively little is known about the impact of N deposition on ecosystem C cycling during climate extremes such as droughts. With the occurrence and severity of droughts likely to be exacerbated by climate change, N deposition—drought interactions remain one of the key uncertainties in process-based models to date. This study aims to contribute to the understanding of N deposition-drought dynamics on gross primary production (GPP) in European forest ecosystems. To do so, different soil water availability indicators (Standardized Precipitation Evapotranspiration Index (SPEI), soil volumetric water) and GPP measurements from European FLUXNET forest sites were used to quantify the response of forest GPP to drought. The computed drought responses of the forest GPP to drought were linked to modelled N deposition estimates for varying edaphic, physiological, and climatic conditions. Our result showed a differential response of forest ecosystems to the drought indicators. Although all FLUXNET forest sites showed a coherent dependence of GPP on N deposition, no consistent or significant N deposition effect on the response of forest GPP to drought could be isolated. The mean response of forest GPP to drought could be predicted for forests with Pinus trees as dominant species (R 2 = 0.85, RMSE = 8.1). After extracting the influence of the most prominent parameters (mean annual temperature and precipitation, forest age), however, the variability remained too large to significantly substantiate hypothesized N deposition effects. These results suggest that, while N deposition clearly affects forest productivity, N deposition is not a major nor consistent driver of forest productivity responses to drought in European forest ecosystems.

Highlights

Terrestrial ecosystems have the potential to take up significant amounts of carbon dioxide from the atmosphere through photosynthesis and growth and thereby mitigate climate change
This study discusses various pathways in which N deposition and drought can interact as co-stressors on forest productivity in European forest ecosystems
Our initial hypothesis was that the response of forest gross primary production (GPP) to drought is relatively small in forest ecosystems that experience intermediate levels of N deposition (5–10 kg ha−1 yr−1)

Summary

Introduction

Terrestrial ecosystems have the potential to take up significant amounts of carbon dioxide from the atmosphere through photosynthesis and growth and thereby mitigate climate change. The compound effect of many interacting drivers determines whether a forest is a net sink of C (i.e., taking up C from the atmosphere over multi-year timescales) or a net source of C (i.e., releasing C to the atmosphere over multi-year timescales). These drivers include edaphic and climatic factors such as soil nutrient and moisture conditions and air temperature and humidity. Fires and outbreaks of insect herbivores and fungal pathogens can reduce forest productivity for many years and can cause widespread forest mortality (e.g., Anderegg et al 2020). Some factors can potentially increase ecosystem carbon storage, such as CO2 fertilization, ozone exposure, nitrogen (N) deposition and forest management

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