Huang Jr-Chuan, Lee Tsung-Yu, Lin Teng-Chiu, Hein Thomas, Lee Li-Chin, Shih Yu-Ting, Kao Shuh-Ji, Shiah Fuh-Kwo, Lin Neng-Huei
Department of Geography, National Taiwan University, Taipei, Taiwan.
Department of Geography, National Taiwan Normal University, Taipei, Taiwan.
Biogeosciences. 2016;13(6):1787-1800. doi: 10.5194/bg-13-1787-2016. Epub 2016 Mar 23.
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 yr, 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 yr). 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.
人为活动导致的氮（N）有效性和流动性增加，已在局部和全球范围内极大地改变了氮循环。台湾以亚热带山地景观和充沛降雨为特征，位于中国工业化发展最快的东部沿海地区的下风处，可作为氮输入极高和河流溶解无机氮（DIN）输出的一个示范地点。我们使用了49个气候和景观条件相似但根据人口密度分为低度、中度和高度干扰类别的流域，以说明它们在氮输入（通过大气氮沉降、合成肥料和人类排放）和DIN输出比率方面的差异。我们的结果表明，全岛河流DIN的平均输出量约为3800 kg N km⁻² yr⁻¹，约为全球平均水平的18倍。河流DIN的平均输出比率为0.30 - 0.51，远高于世界大型河流0.20 - 0.25的平均水平，这表明相对于生态系统需求或保留能力而言，氮输入过量。尽管氮输入量很高（约4900 kg N km⁻² yr⁻¹），低度干扰的流域仍具有较高的氮保留能力，DIN输出比率为0.06 - 0.18。高保留能力可能是由于流域内次生林的有效吸收。中度干扰的流域中，DIN输出随总氮输入的增加呈线性增加，平均DIN输出比率为0.20至0.31。低度和中度干扰流域土地利用的主要差异在于，中度干扰流域的农业用地覆盖比例更大。因此，它们较高的DIN输出可能归因于农业用地中使用的氮肥。较高的输出比率还意味着农业用地的氮保留比例较低，并且重新造林可能是减少河流DIN输出的一种有效土地管理措施。高度干扰流域的输出比率非常高，为0.42 - 0.53，这表明大部分氮输入直接被输送到下游，这就迫切需要提高接入污水处理系统的家庭比例，并提高废水处理系统的有效性。沿着人类干扰梯度，河流DIN输出比率的增加也表明了台湾亚热带地区氮饱和度的梯度变化。我们的结果有助于增进我们对控制河流DIN输出因素的理解，并提供了实证依据，呼吁采取合理的氮排放/污染控制措施。
