APPLYING CA AND MG ISOTOPES AND MASS-BALANCE APPROACHES TO QUANTIFY ROCK WEATHERING, SOIL WEATHERING AND TREE-SOIL RECYCLING IN MANAGED FOREST NUTRIENT BUDGET

Quantifying nutrient cycling in managed forestry systems is essential for assessing the long-term nutrition for forest productivity, but is difficult due to the co-occurring pathways of trees-soils recycling and primary mineral weathering in rocks and soils. Two elements of importance to investigate are Ca and Mg due to tree meso-nutritional demand, decades of enhanced losses from acid rain, centuries of export via woody biomass during timber harvesting, and variability in lithology and soil parent material. Here, we have measured aboveground and belowground pools and fluxes of Ca and Mg as well as their isotopic signatures to quantify their biogeochemical cycling in two mixed-aged forest stands, nutrient-rich COR in Vermont and nutrient poor BEF in New Hampshire. Samples of rock, soil, soil leachate, rock weathering solution, litterfall, throughfall, branch wood, and green leaves from each forest were measured using a Neptune Multi-Collector Inductively Coupled Plasma Mass Spectrometer. δ^44/42Ca and δ^44/40Ca at nutrient-poor BEF indicate soil leachate exiting the rooting zone is predominantly fresh rock weathered Ca, suggesting strong internal recycling between plant-soil and limited soil weathering inputs. δ^26/24Mg values for soil leachate closely aligns with litterfall values, suggesting abundant Mg in the tree-soils system are not tightly cycling and rocks and soils are not important Mg sources. δ^44/42Ca, δ^44/40Ca, and mass fluxes at nutrient-rich COR indicate soil leachate exiting the rooting zone is predominantly soil and rock Ca, suggesting mineral weathering dominates the Ca cycle and tree-soil recycled Ca is only a small mass in the system. The nutrient-rich COR has δ^26/24Mg values for soil leachate that align closest with litterfall values, suggesting excess Mg is present and trees are not tightly cycling Mg or substantial fractionation between solid phase dissolution and dissolve phased Mg. Our findings demonstrate that nutrient abundance in the soils and rocks as well as internal recycling play a substantial role in Ca and Mg cycling in managed forests.