The Role of Mineral Weathering Dynamics in the Calcium Cycle of Northern Hardwood Forest Soils in a Changing World: Reconciling Mineral Depletion and Mass Balance Studies.

The northern hardwood forest is a widespread biome across northeastern North America that provides a broad range of valuable ecosystem services. Over the last couple of centuries, tree species composition and disturbance regimes in this ecosystem have been altered by timber harvests, air pollution, and potentially by climate change. Some changes in forest productivity and tree disease patterns have been tied to reduced availability of nutrients such as calcium and magnesium that are primarily supplied via mineral weathering processes in the soil.  Quantifying the rates of these reactions and determining where they occur relative to the rooting zone and hydrologic pathways is critical to understanding how forests will continue to change in the future. We review long term monitoring of watershed-scale calcium mass balance at three sites spanning a range of calcium supply across the region – at the Neversink River, New York, with Ca-poor soils formed in sandstone dominated glacial drift, at Sleepers River, Vermont, with Ca-rich soils formed in calc-silicate drift, and at Hubbard Brook, New Hampshire, with intermediate soils formed in granitic/schistose drift. We compare several methods to evaluate mineral weathering rates and processes to address whether resupply of the cation exchange complex via mineral weathering is sufficient to sustain forest nutrient pools. After approximately 10,000 years of soil development, many of the most reactive phases have been depleted in the solum, pointing to the importance of mineral weathering in the subsoil coupled with hydrologic transfer to the solum as mechanisms that control resupply of the exchange pool within the rooting zone. Understanding spatial variation in hydrologic pathways, and the role of climate change in altering groundwater flow regimes will be critical to predicting future changes in base cation supply within the rooting zone.