Effects of different N sources on riverine DIN export and retention in a subtropical high-standing island, Taiwan
Increases in nitrogen (N) availability and mobility resulting from anthropogenic activities have substantially altered the N cycle, both locally and globally. Taiwan characterized by the subtropical montane landscape with abundant rainfall, downwind of the most rapidly industrializing eastern coast...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-03-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/1787/2016/bg-13-1787-2016.pdf |
| Summary: | Increases in nitrogen (N) availability and mobility resulting from
anthropogenic activities have substantially altered the N cycle, both locally
and globally. Taiwan characterized by the subtropical montane landscape with
abundant rainfall, downwind of the most rapidly industrializing eastern coast
of China, can be a demonstration site for extremely high N input and riverine
DIN (dissolved inorganic N) export. We used 49 watersheds with similar
climatic and landscape settings but classified into low, moderate, and highly
disturbed categories based on population density to illustrate their
differences in nitrogen inputs (through atmospheric N deposition, synthetic
fertilizers, and human emission) and DIN export ratios. Our results showed
that the island-wide average riverine DIN export is ∼ 3800 kg N km<sup>−2</sup> yr<sup>−1</sup>, approximately 18 times the global
average. The average riverine DIN export ratios are 0.30–0.51, which are
much higher than the averages of 0.20–0.25 of large rivers around the world,
indicating excessive N input relative to ecosystem demand or retention
capacity. The low disturbed watersheds have a high N retention capacity and
DIN export ratios of 0.06–0.18 in spite of the high N input
(∼ 4900 kg N km<sup>−2</sup> yr<sup>−1</sup>). The high retention capacity is
likely due to effective uptake by secondary forests in the watersheds. The
moderately disturbed watersheds show a linear increase in DIN export with
increases in total N inputs and mean DIN export ratios of 0.20 to 0.31. The
main difference in land use between low and moderately disturbed watersheds
is the greater proportion of agricultural land cover in the moderately
disturbed watersheds. Thus, their greater DIN export could be attributed to N
fertilizers used in the agricultural lands. The greater export ratios also
imply that agricultural lands have a lower proportional N retention capacity
and that reforestation could be an effective land management practice to
reduce riverine DIN export. The export ratios of the highly disturbed
watersheds are very high, 0.42–0.53, suggesting that much of the N input is
transported downstream directly, and urges the need to increase the
proportion of households connected to a sewage system and improve the
effectiveness of wastewater treatment systems. The increases in the riverine
DIN export ratio along the gradient of human disturbance also suggest a
gradient in N saturation in subtropical Taiwan. Our results help to improve
our understanding of factors controlling riverine DIN export and provide
empirical evidence that calls for sound N emission/pollution control
measures. |
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| ISSN: | 1726-4170 1726-4189 |