Browning or Clearing: Recovery from acidification does not consistently alter dissolved organic carbon exports from small watersheds

Abstract

Long-term changes in dissolved organic carbon (DOC) concentrations in streams are widely documented, and increasing DOC (“browning”) is often attributed to watershed recovery from acid deposition. Recovery from acid rain is expected to reduce sulfate deposition, raise pH, and lower ionic strength in soils and streams, collectively increasing DOC solubility and mobility. However, whether DOC responses to acidification recovery are consistent across systems, and whether rising pH reliably predicts increasing DOC export at continental scales, remains unclear given heterogeneity in buffering capacity, soil chemistry, and hydrologic setting. We analyzed multi-decadal records of DOC, pH, ionic strength, atmospheric sulfate deposition, and selected climate variables across minimally disturbed watersheds in the MacroSheds database. We report results for sites with sufficient data for long-term trend analyses (10+ years). Atmospheric sulfate deposition declined at 100% of sites, indicating a consistent, continental-scale recovery forcing. Stream pH increased significantly at 48% of qualifying sites, demonstrating widespread chemical recovery. DOC responses were heterogeneous: only ~6% of qualifying sites exhibited significant DOC increases and ~14% exhibited significant DOC decreases, indicating that significant DOC trends are confined to a minority of chemically responsive systems. Sites with significant DOC trends include cases of both increasing and decreasing DOC alongside rising pH, suggesting that chemical recovery alone does not guarantee browning. These results show that while the potential for acidification recovery is widespread, DOC responses frequently decouple from recovery trajectories. Ionic strength emerged as a better predictor of DOC trend direction than pH among sites with significant trends, suggesting that co-occurring shifts in ionic strength and pH jointly control DOC mobility. This decoupling indicates that recovery forcing alone cannot predict browning and that watershed biogeochemical and hydrologic context mediates whether recovery translates into long-term changes in DOC export. Projections of terrestrial-to-aquatic carbon export under continued recovery and ongoing climate change will likely require accounting for watershed-specific controls rather than relying on pH or deposition trends alone.