BELOWGROUND LITTER CONTRIBUTIONS TO NITROGEN CYCLING AT A NORTHERN GRASSLAND–FOREST BOUNDARY

Abstract

Forest expansion at the northern edge of the Great Plains is associated with increased availability of soil nitrogen (N). Studies of N dynamics typically focus on above- ground litter production, but in semiarid ecosystems, fine-root production greatly exceeds shoot production. We explored the contribution of root and shoot litter to N cycling in adjacent grassland and aspen (Populus tremuloides) forest at the northern edge of the Great Plains. We used a new approach to measure N inputs from root litter production: we combined root productivity data from minirhizotron images with N content data from several root diameter and color classes. We also measured the production and N content of above- ground litter. The novel contribution of our study comes from the simultaneous measure- ments of above- and belowground productivity and N input from litter production in adjacent forest and grassland habitats. Aboveground litter production was threefold greater in forest than in grassland (330 vs. 136 g-m-2.yr-1), but fine roots accounted for 80-90% of total litter production. As a result, total production was not significantly different between hab- itats, and the N contribution from total litter production was surprisingly similar between grassland (16.8 g.m-2.yr-1) and forest (17.1 g.m-2.yr-1). Thus in spite of great differences between habitats in aboveground litter production, N inputs from total litter production cannot explain the higher availability of N in forest soils. However, we found differences between habitats in root litter quality (forest, 1.14% N; grassland, 0.81% N, as well as in the seasonal and vertical distribution of root production. Grassland root production was significantly greater than forest root production in early summer in the top 20 cm of soil. Conversely, forest produced more root length at the end of the growing season in deeper soil layers (>50 cm). These differences may increase available N in forest soils, but this increase is not attributable to differences in total litter production between habitats.