Long-term nitrogen fertilization increases drought sensitivity of gross primary productivity capacity in a boreal Scots pine forest

Abstract. Nitrogen (N) is a key limiting element for plant photosynthesis in boreal forests. Thus, N fertilization is proposed as an effective management strategy to increase forest productivity and the associated carbon (C) sink in the N-limited boreal biome. However, there is a limited understanding of how N fertilization can affect the sensitivity of the C sink to drought stress, which is predicted to occur more frequently in the boreal region in a changing climate. This study was based on a 15-year controlled N fertilization experiment in a boreal Scots pine stand. Ecosystem light-saturated photosynthetic capacity (GPP2000) is a good indicator of forest photosynthesis response to environmental stress. Here, we used eddy covariance measurements of C fluxes data and environmental data from paired sites to investigate whether long-term N fertilization altered the drought sensitivity of the GPP2000. We found that long-term N fertilization significantly increased ecosystem GPP2000 even on dry days during summer (June, July, and August). However, a significantly divergent drought sensitivity of GPP2000 between the N Fertilized and Reference sites was detected. Specifically, N fertilization increased the sensitivity of GPP2000 to both atmospheric and soil drought to the extent that it may offset the positive effect of N fertilization on GPP2000. Moreover, using the random forest model, we found that the absolute GPP2000 difference between fertilization and control sites was mainly influenced by air and soil drought proxies followed by canopy conductance rather than the air temperature. These results advance our understanding of the mechanisms of forest response to drought with long-term N fertilization.