『Abstract
Flux of dissolved inorganic nitrogen (DIN - primarily nitrate)
from terrestrial ecosystems has been considered an important contributor
to acidification of linked aquatic systems. The basis of this
concern is the nitrogen (N) saturation hypothesis, positing that
additions of N to terrestrial ecosystems in excess of biological
requirements will result in DIN leaching. There is a consensus
(implicit hypothesis) in the literature that atmospheric deposition
of DIN in excess of a threshold of approximately 10 kg ha-1
year-1 leads to significant flux. Diverse data from
USA indicate that DIN flux is highly variable both in space and
time; the spatial uncertainty as measured by the pooled coefficient
of variation is about 0.95, and the temporal (inter-year) uncertainty
is about 0.75. The relationship between atmospheric s\deposition
of DIN and annual flux is near-linear within the range of current
deposition for US sites (≦8 kg ha-1 year-1
wet deposition). If wet and dry depositions are approximately
equal, over 85% of total DIN deposition is retained. This is nearly
equal to the retention reported by the US Geological Survey National
Water-Quality Assessment Program, which considered all nonpoint
sources of N as inputs and both DIN and organic N as fluxes. Although
input-output data have high uncertainty, the 85% retention of
atmospheric DIN by terrestrial watersheds casts doubt on its importance
as a contributor to aquatic acidification. There is no obvious
threshold of deposition leading to DIN leaching. The nitrogen
saturation hypothesis may not fully explain N behavior in terrestrial
ecosystems.
Keywords: Nitrogen saturation; Aquatic acidification; Nitrogen
retention; SIN variation』
1. Introduction
2. Methods
2.1. Datasets
2.2. Atmospheric deposition
2.3. Data screening
2.4. Statistical analysis
3. Results and discussion
3.1. Variation
3.2. Functional relationships
4. Conclusions
Acknowledgments
References