Nitrogen mineralization, groundwater dynamics, and forest growth on a Minnesota outwash landscape

We measured aboveground biomass and aboveground net primary productivity (ANPP), groundwater depth and fluctuation, and in situ nitrogen (N) mineralization in 13 upland and 4 wetland forest stands at Cedar Creek Natural History Area (CCNHA). The area, in east central Minnesota (45°25′ N, 93°10′ W), is on well-sorted glacial outwash of very uniform fine sand. Uplands are interspersed with peadands and the area has shallow groundwater. Stands were aggregated into six ecosystem types based on overstory composition: oak, pine-oak, mesic hardwoods, northern white-cedar, lowland hardwoods, and savanna. Aboveground overstory biomass ranged from 35 to 250 Mg ha^-1; lowest in the savanna and highest in the pine-oak. The ANPP ranged from about 2 to 7.5 Mg ha^-1; also lowest in the savanna but highest in the white-cedar. Over all types, the annual aboveground uptake of N was poorly related to available N measured by in situ mineralization (r² = 0.01), but the relationship was better (r² = 0.88) if N availability in the wetland stands was assumed to be a fixed proportion of N in the surface soil (1.5%). Over all types, in situ N mineralization was poorly related to ANPP (r² = 0.05) and biomass (r² = 0.38). Both ANPP and overstory biomass were more closely related to groundwater fluctuation (r² = 0.87 and 0.28, respectively) than to depth (r² = 0.01 and 0.21, respectively)). The strength of all relationships varied with the inclusion or exclusion of data from the wetland types or the savanna. Total soil N and rates of mineralization were inversely related (r² = 0.42) because of data from wetland stands. Results demonstrate that the positive relationships between aboveground productivity and measured in situ N mineralization observed in upland forests are not valid for the landscape that includes wetland forests either because in situ measurements do not indicate N availability in wetlands or because of the presence of other limiting factors. The north temperate landscape includes an abundance of wetland forests with potentially strong linkages to uplands. This study suggests that the commonly-used measure of N availability provides inconsistent information about controls on ecosystems processes in this diverse landscape.