ABSTRACT

Surface geology is a key factor regulating the geographical patterns of P export; however, geographically extensive data on P export mechanisms from natural coastal mountain regions are lacking. We compared stream-water characteristics in forested headwater catchments with different types of surface geology (marine sedimentary rocks [MSRs] and igneous rocks [IRs]) and evaluated the factors regulating soluble reactive phosphorus (SRP) concentrations in stream water. Geographically extensive stratified sampling was conducted: stream-water and sediment samples were obtained from 66 MSR catchments and 55 adjacent IR catchments along the coastal forested mountain regions in Akita, Japan. SRP and major cations and anions in stream waters were analyzed. To evaluate the effect of ionic composition on SRP, sediments were extracted with NaCl and CaCl₂ solutions and the SRP concentrations in the extracts were measured. Stream SRP was significantly higher in MSR catchments (median 23 µg L⁻¹) than in IR catchments (median 8 µg L⁻¹) (P < 0.001), and tended to be higher in MSR catchments with younger rocks. Dissolved inorganic nitrogen (DIN) did not differ between catchments with different geologies; therefore, DIN/SRP was significantly lower in MSR catchments (median 18), close to the Redfield ratio. On a Piper diagram, high-SRP stream waters were of the NaCl water type, and stream SRP was significantly correlated with Na⁺ (r = 0.642, P < 0.0001). In IR catchments, stream waters had relatively higher Ca²⁺ and Mg²⁺ concentrations. SRP in the sediment extracts was significantly higher in MSR catchments and samples obtained by NaCl extraction than in IR catchments and CaCl₂-extracted samples. Exchangeable Na⁺ in sediments was significantly higher in MSR catchments, decreasing Ca²⁺ in the liquid phase by an ion-exchange reaction, which is likely also key in maintaining high SRP levels. We conclude that MSR catchments in coastal Akita are a significant P source for streams even in headwater forests, and that site-specific NaCl water types and higher exchangeable Na⁺ concentrations in sediments result in higher SRP concentrations in stream waters. These results have critical implications for P cycles in freshwater ecosystems influenced by Na⁺.

摘要

地表地质是调控磷出口地理格局的关键因素；然而，缺乏关于天然沿海山区磷出口机制的地理广泛数据。我们比较了具有不同表面地质类型（海洋沉积岩 [MSR] 和火成岩 [IR]）的森林源头集水区的河流水特征，并评估了调节河流水中可溶性活性磷 (SRP) 浓度的因素。进行了地理范围广泛的分层采样：从日本秋田沿海森林山区的 66 个 MSR 集水区和 55 个相邻的 IR 集水区获取溪流水和沉积物样本。分析了河流水中的 SRP 和主要阳离子和阴离子。为了评估离子组成对 SRP 的影响，用 NaCl 和 CaCl 提取沉积物_{测量了 2 种}溶液和提取物中的 SRP 浓度。MSR 流域（中值 23 µg L ^-1）的溪流 SRP 显着高于IR 流域（中值 8 µg L ^-1）（P < 0.001），并且在具有较年轻岩石的 MSR 流域中往往更高。不同地质流域的溶解无机氮 (DIN) 没有差异；因此，MSR 集水区的 DIN/SRP 显着较低（中值为 18），接近雷德菲尔德比率。在派珀图上，高 SRP 河流水是 NaCl 水类型，河流 SRP 与 Na ⁺显着相关（r = 0.642，P < 0.0001）。在 IR 集水区，河流水中的 Ca ²⁺相对较高和 Mg ²⁺浓度。MSR 流域和通过 NaCl 提取获得的样品中沉积物提取物中的 SRP 显着高于 IR 流域和 CaCl ₂提取的样品。MSR 集水区沉积物中可交换的 Na ⁺显着更高，通过离子交换反应降低液相中的Ca ²⁺，这可能也是维持高 SRP 水平的关键。我们得出结论，秋田沿海的 MSR 集水区是溪流的重要 P 源，即使在源头森林中也是如此，并且特定地点的 NaCl 水类型和较高的可交换 Na ⁺沉积物中的浓度会导致河流水中的 SRP 浓度更高。这些结果对受 Na ⁺影响的淡水生态系统中的 P 循环具有重要意义。