Increases in dissolved organic carbon (DOC) have occurred in many freshwaters across Europe and North America over the last decades. Several mechanisms have been proposed to explain these trends, but consensus regarding the relative importance of recovery from acid deposition, climate change, and land management remains elusive. To advance our understanding of browning mechanisms, we explored DOC trends across 13 nested boreal catchments, leveraging concurrent hydrological, chemical, and terrestrial ecosystem data to quantify the contributions of different drivers on observed trends. We first identified the environmental factors related to DOC concentrations, then attributed the individual trends of DOC to potential drivers across space and time. The results showed that all catchments exhibited increased DOC trends from 2003 to 2021, but the DOC response rates differed five-fold. No single mechanism can fully explain the ongoing browning, instead the interaction of sulfate deposition, climate-related factors and site properties jointly controlled the variation in DOC trends. Specifically, the long-term increases in DOC were primarily driven by recovery from sulfate deposition, followed by terrestrial productivity, temperature, and discharge. However, catchment size and landcover type regulated the response rate of DOC trends to these drivers, creating the spatial heterogeneity in browning among the sub-catchments under similar deposition and climate forcing. Interestingly, browning has weakened in the last decade as sulfate deposition has fully recovered and other current drivers are insufficient to sustain the long-term trends. Our results highlight that multifaceted, spatially structured, and nonstationary drivers must be accounted for to predict future browning.

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 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. 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 determined by air and soil drought proxies and 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.