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IS NITROGEN DEPOSITION ALTERING THE NITROGEN STATUS OF NORTHEASTERN FORESTS?
Citation:
Aber, J. D., C. L. Goodale, S. V. Ollinger, M. L. Smith, A. H. Magill, M. E. Martin, R. A. Hallett, AND J L. Stoddard. IS NITROGEN DEPOSITION ALTERING THE NITROGEN STATUS OF NORTHEASTERN FORESTS? BIOSCIENCE 53:1-15.
Impact/Purpose:
Article is intended for use by academic science community, policy analysts in NGOs and all levels government organizations, and the interested public.
Description:
This paper reviews literature and compiles existing data to address the question "Is N deposition altering the N status in Northeastern forests?" Using correlational techniques and large sample size, three different categories of indicators appear to give different results. The surface water data suggests a very strong trend between nitrate concentration and flux across the N deposition gradient. The soils data show a strong relationship between soil C:N ratio and nitrification, but weaker relationships between N deposition and either soil C:N or nitrification. In the foliar data set, relationships with N deposition are no stronger than those with temperature and elevation.
The confounding factors described in the paper (climate variation, disturbance, species composition, hydrological pathways) affect foliar, soil, and stream chemistry at different spatial and temporal scales, as we have discussed. We suggest that the relative sensitivity of the three indicators to these confounding variables affects the strength of the relationship exhibited with N deposition. Specifically, foliar samples integrate over a very small area (one tree crown) and are subject to strong and unexplained inter-annual variation. We did not attempt to control for year of sampling in our foliar analyses and could not be certain that this would be helpful as we do not know if the factors controlling inter-annual variability are local or regional. Although soil characteristics do not change rapidly over time, they are also subject to large variation over very short spatial scales due to the fine scale nature of both disturbance forces and species distribution. The close correlation between C:N and nitrification suggests that N status is reflected in nitrification, while the weaker relationship with N deposition may reflect the influence of disturbance and species effects. Surface water samples integrate over much larger areas (watersheds), and although decadal scale changes in stream nitrate appear to occur, all samples used in this analysis were taken during the 1990s when stream nitrate concentrations were relatively stable and generally low compared to earlier periods.
We conclude that, of the three indicators discussed here, surface waters integrate most effectively over space and time, and yield the most comprehensive view of the state of N saturation across the northeast as they integrate over larger areas. Significantly larger sample sets for soils and foliage would be required to cover the same effective area as stream sampling, and, if spread more systematically across the region, could also provide valuable insights. Given the inter-annual variation in foliar chemistry, synoptic, large-scale, repeatable sampling, such as is becoming possible for foliage through remote sensing, may be the only method by which foliar chemistry could be a valuable broad-scale spatial predictor of forest N status.