Abstract
Increasing anthropogenic nitrogen (N) emission via different pathways has shown prominent impact on aquatic ecosystems for decades, but the effects of interaction among climate-, landscape- and human-associated variables on riverine DIN (dissolved inorganic nitrogen, mainly NO3− and NH4+) export are unclear. In this study, the data of 43 watersheds with a wide range of climate-, landscape- and human-associated gradients across Taiwan were evaluated with partial redundancy analysis (pRDA) to examine their interactive controls on riverine DIN export. Results show that the annual riverine DIN export in Taiwan is approximately 3100 kg-N km−2 yr−1, spanning from 230 kg-N km−2 yr−1 in less disturbed watersheds (eastern and central Taiwan) to 10,000 kg-N km−2 yr−1 in watersheds with intensive human intervention (southwestern and northern Taiwan). NO3− is generally the single dominant form of DIN, while NH4+ renders significance in disturbed watersheds. Nearly all environmental variables display a positive correlation with DIN export, except for landscape setting variables (e.g., slope, area, channel length), which show a negative relationship. In terms of seasonal pattern, climate and human-landscape variables are related to NO3− export independently in the wet season, yet in the dry season climate-human variables jointly dominate NO3− export. Meanwhile, human-landscape (LH) variables (λ1 of LH > 0.60) control NH4+ exports in both seasons, and human-associated (H) variables (λ1 of H = 0.13) have a minor effect on NH4+ exports in dry season. Precisely, the contribution of controlling variables on DIN export vary with species and seasons, indicating water quality management could be time-dependent, which should be taken into consideration for designing mitigation strategies.
Highlights
Reactive nitrogen, a vital and essential nutrient for organisms and ecosystems, plays a key role in maintaining biodiversity and functions of ecosystems [1,2]
According to the hydrologic seasonality, we summarized the daily export from May to October as wet season export and the summation of other daily exports as dry season export
35 predictor variables among climatic, landscape setting and human disturbance dimensions were applied using principal components analysis (PCA) and partial redundancy analysis (RDA) (pRDA) analysis based upon data derived from 43 watersheds island-wide in Taiwan
Summary
A vital and essential nutrient for organisms and ecosystems, plays a key role in maintaining biodiversity and functions of ecosystems [1,2]. Over the past half century, the rapid increasing anthropogenic N emissions inevitably accelerated N deposition into the biosphere [3], and exceeded the N-requirement for terrestrial ecosystems [4]. Studies show that N emissions and depositions have been declining in Europe and the U.S since 2000, East and South Asia, in contrast, have become the hot spots of pollutant emissions due to population growth and intense agricultural activities [5,6]. Regions located near the emission source of East Asia receive excessive N deposition under favorable climatic conditions, such as the East Asian monsoon that blows directly to Taiwan with abundant rainfall [7]. The DIN exports, for seasonal changes, in subtropical mountainous watersheds are still unclear
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