Long‐Term Change in the Concentration‐Discharge Relationship Suggests Controls on Watershed Exports of Dissolved Organic Carbon From Forested Headwater Streams

Abstract

Dissolved organic carbon (DOC) export from watersheds by streams is an important and changing component of the global carbon cycle. We examined controls on DOC export by quantifying changes in the DOC concentration‐discharge relationship between 1992 and 2022 for nine forested headwater catchments at the Hubbard Brook Experimental Forest (northeastern United States), which historically received high atmospheric acid deposition. We observed an increase in the intercept of the log‐log concentration‐discharge relationship and a weaker increase in the slope of that relationship, along with seasonal and watershed‐level differences. The intercept, which indicates the average stream DOC concentration at a given discharge, was inversely related to the ionic strength of the soil solution as predicted by electrolyte solubility theory. This relationship varied between the two watersheds where soil solution chemistry data were available in response to soil pH. The intercept was not strongly related to annual precipitation or air temperature. DOC export ranged from 13 to 156 kg C ha ⁻¹  y ⁻¹ among study watersheds and years, and was correlated with annual precipitation and discharge. Historical data suggest that DOC export has increased over the past 50 years, likely due to both increases in precipitation and runoff and increases in the intercept and slope of the concentration‐discharge relationship. Our results suggest the potential for long‐term legacy effects of acidification on DOC solubility and stream DOC concentrations in acid‐impacted watersheds, despite reductions in acid deposition, as mineral weathering slowly replenishes the ionic strength and buffering capacity of soil solutions.