The effects of gap disturbance on nitrogen cycling and retention in late-successional northern hardwood–hemlock forests

Late-successional forests in the upper Great Lakes region are susceptible to nitrogen (N) saturation and subsequent nitrate (NO3−) leaching loss. Endemic wind disturbances (i.e., treefall gaps) alter tree uptake and soil N dynamics; and, gaps are particular susceptible to NO3− leaching loss. Inorganic N was measured throughout two snow-free periods in throughfall, forest floor leachates, and mineral soil leachates in gaps (300–2,000 m2, 6–9 years old), gap-edges, and closed forest plots in late-successional northern hardwood, hemlock, and northern hardwood–hemlock stands. Differences in forest water inorganic N among gaps, edges, and closed forest plots were consistent across these cover types: NO3− inputs in throughfall were significantly greater in undisturbed forest plots compared with gaps and edges; forest floor leachate NO3− was significantly greater in gaps compared to edges and closed forest plots; and soil leachate NO3− was significantly greater in gaps compared to the closed forest. Significant differences in forest water ammonium and pH were not detected. Compared to suspected N-saturated forests with high soil NO3− leaching, undisturbed forest plots in these late-successional forests are not losing NO3− (net annual gain of 2.8 kg ha−1) and are likely not N-saturated. Net annual NO3− losses were observed in gaps (1.3 kg ha−1) and gap-edges (0.2 kg ha−1), but we suspect these N leaching losses are a result of decreased plant uptake and increased soil N mineralization associated with disturbance, and not N-saturation.