Century-scale Variations in Plant and Soil Nitrogen Pools, Isotopic Composition, and Microbial Processes in Northern Hardwood Forests

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The enormous size and spatial heterogeneity of soil nitrogen regularly impede study of the role of mineral soil as an N source or sink over the course of forest succession.  Across three forest stands of varying time since harvest in and near Hubbard Brook, New Hampshire (40-year W4, ~100-year west of W6, and old-growth Mt. Moosilauke), we have recently completed measurements of tree and soil stocks of ecosystem N and 15N and microbial activity to 50 cm depth. Measurements show substantial differences in soil N stock and 15N patterns, across sites and with soil depth.  Tree growth decreased with stand age, while plant tissue 15N varied more by tree species than by site. Decomposition enzyme activity per gram soil C varied by site, but stayed surprisingly steady with increasing soil depth. Gross N cycling rates decreased with soil depth when considered per gram of soil, but the vast size of the mineral soil enables this pool to cycle several times more N per hectare than in the forest floor.  These observations demonstrate the great capacity of the mineral soil to store and cycle N in northern hardwood forests.