Catchment afforestation increases lake CO2 by reducing pH and alkalinity

Many lakes in the United States currently exhibit elevated partial pressures of carbon dioxide (<i>p</i>CO₂) and release it to the atmosphere. While CO₂ supersaturation often arises from the influx and mineralization of terrestrial dissolved organic matter, little is known about the mechanisms regulating continental- and decadal-scale variability in lentic dissolved CO₂ levels. By integrating data from four national lake surveys (3,498 lakes, <i>n</i> = 5,733) with high-resolution estimates of land use, we show that the extent of catchment forest cover explains a large part of the variability in topsoil pH (<i>R</i>²= 0.39) and, in turn, freshwater pH (<i>R</i>^<em>2</em> = 0.29), the main control of lentic <i>p</i>CO₂ (<i>R</i>²_{lake pH-}_<em>p</em>_CO2= 0.55). The extent of forest cover also reduces lake alkalinity, consistent with stabilization of soils and reduced weathering inputs. Machine-learning models show that topsoil pH declines by 0.2 units for every 10% increase in forest cover, resulting in a pH decline in downstream lakes, a shift in the carbonate equilibrium from HCO₃^– to CO₂, and a predicted increase in CO₂ emissions. Conversely, catchments with lower forest cover and higher nutrient levels exhibited lower <i>p</i>CO₂. Our results demonstrate that increased forest cover, a key climate mitigation strategy, may inadvertently elevate <i>p</i>CO₂ levels in U.S. lakes, potentially offsetting about 22.3 Tg C of carbon sequestration over the past three decades, equivalent to ~ 3.2% of the organic carbon buried in conterminous United States lakes and reservoirs in the same period.