Catchment afforestation increases lake CO2 by reducing pH and alkalinity

Many lakes in the United States currently exhibit elevated partial pressures of carbon dioxide (pCO2) 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, n = 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 (R2 = 0.39) and, in turn, freshwater pH (R2 = 0.29), the main control of lentic pCO2 (R2 lake pH-pCO2 = 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 pCO2. Our results demonstrate that increased forest cover, a key climate mitigation strategy, may inadvertently elevate pCO2 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.