Precipitation control over inorganic nitrogen import-export budgets across watersheds: a synthesis of long-term ecological research

We investigated long-term and seasonal patterns of N imports and exports, as well as patterns following climate perturbations, across biomes using data from 15 watershed from nine Long-Term Ecological Research (LTER) sites in North America. Mean dissolved inorganic nitrogen (DIN) import-export budgets (N import via precipitation-N export via stream flow) for common years across all watersheds was highly variable, ranging from a net loss of -0.17 +/- 0.09 kg N ha(-1)mo(-1) to net retention of 0.68 +/- 0.08 kg N ha(-1)mo(-1). The net retention of DIN decreased (smaller import-export budget) with increasing precipitation as well as with increasing variation in precipitation during the winter, spring, and fall. Averaged across all seasons net DIN retention decreased as teh coefficient of variation (CV) in precipitation increased across all sites (r(2) = 0.48. P = 0.005). This trend was made stronger when the disturbed watersheds were withheld from the analysis (r(2) = 0.80, p < 0.001, n = 11). Thus, DIN exports were either similar to or exceeded imports in the tropiocal, boreal, and wet coniferous watershed, whereas imports exceeded exports in temperate deciduous watersheds. In general, forest harvesting, hurricanes, or floods corresponded with periods of increased DIN exports relative to imports. Periods when water throughput within a watershed was likely to be lower (i.e. low snow pack or EI Nino years) corresponded with decreased DIN exports relative to imports. These data provided a basis for ranking diverse sites in terms of their ability to retain DIN in the context of changing precipitation regimes likely to occur in the future. Copyright (C) 2008 John Wiley & Sons. Ltd.