Data from: Solute and sediment export from Amazon forest and soybean headwater streams

Intensive crop agriculture commonly increases streamwater solute concentrations and export from small watersheds. In recent decades, the lowland tropics have become the world's largest and most important region of cropland expansion. Although the effects of intensive cropland agriculture on streamwater chemistry and watershed export have been widely studied in temperate regions, their effects in tropical regions are poorly understood. We sampled seven headwater streams draining watersheds in forest (n=3) or soybeans (n=4) to examine the effects of soybean cropping on stream solute concentrations and watershed export in a region of rapid soybean expansion in the Brazilian state of Mato Grosso. We measured stream flows and concentrations of NO3-, PO43-, SO42-, Cl-, NH4+, Ca2+, Mg2+, Na+, K+, Al3+, Fe and dissolved organic carbon (DOC) biweekly to monthly to determine solute export. We also measured stormflows and stormflow solute concentrations in a subset of watersheds (2 forest, 2 soybean) during 2 to 3 storms, and solutes and δ18O in groundwater, rainwater and throughfall to characterize watershed flowpaths. Concentrations of all solutes except K+ varied seasonally in streamwater, but only Fe concentrations differed between land uses. The highest streamwater and rainwater solute concentrations occurred during the peak season of wildfires in Mato Grosso, suggesting that regional changes in atmospheric composition and deposition influence seasonal stream solute concentrations. Despite no concentration differences between forest and soybean land uses, annual export of NH4+, PO43-, Ca2+, Fe, Na+, SO42-, DOC and TSS were significantly higher from soybean than forest watersheds (5.6-fold mean increase). This increase largely reflected a 4.3-fold increase in water export from soybean watersheds. Despite this increase, total solute export per unit watershed area (i.e., yield) remained low for all watersheds (<1 kg NO3- N/ha/yr, <2.1 kg NH4+-N/ha/yr, <0.2 kg PO43--P/ha/yr, <1.5 kg Ca2+/ha/yr). Responses of both streamflows and solute concentrations to crop agriculture appear to be controlled by high soil hydraulic conductivity, groundwater-dominated hydrologic flowpaths on deep soils, and the absence of nitrogen fertilization. To date, these factors have buffered streams from the large increases in solute concentrations that often accompany intensive croplands in other locations.

集约化作物种植（intensive crop agriculture）通常会提升小型流域的河流水体溶质浓度与溶质输出通量。近几十年来，低纬度热带地区已成为全球规模最大、地位最为重要的耕地扩张区域。尽管集约化耕地农业对河流水化学及流域输出通量的影响在温带地区已得到广泛研究，但在热带地区的相关影响却鲜为人知。我们在巴西马托格罗索州一处大豆种植快速扩张的区域，选取了7条汇水流域分别为林地（n=3）或大豆种植区（n=4）的源头溪流（headwater streams），以此探究大豆种植对河流水体溶质浓度及流域溶质输出通量的影响。我们以每两周至每月一次的频率测定了溪流流量以及硝酸根（NO₃⁻）、磷酸根（PO₄³⁻）、硫酸根（SO₄²⁻）、氯离子（Cl⁻）、铵根（NH₄⁺）、钙离子（Ca²⁺）、镁离子（Mg²⁺）、钠离子（Na⁺）、钾离子（K⁺）、铝离子（Al³⁺）、铁（Fe）以及溶解性有机碳（dissolved organic carbon, DOC）的浓度，以此计算溶质输出通量。我们还在2~3场暴雨事件中，对部分流域（2个林地下游流域、2个大豆种植流域）的暴雨径流（stormflows）及其溶质浓度进行了测定；同时对地下水、雨水以及林冠穿透水（throughfall）中的溶质与δ¹⁸O进行了检测，以此解析流域水文路径特征。除钾离子（K⁺）外，所有溶质的河流水体浓度均呈现季节变化，但不同土地利用类型间仅铁（Fe）的浓度存在显著差异。马托格罗索州野火高发季的河流水体与雨水溶质浓度达到峰值，这表明区域大气组成与沉降的变化会影响河流水体溶质浓度的季节动态。尽管林地与大豆种植区的河流水体溶质浓度并无显著差异，但大豆种植流域的铵根（NH₄⁺）、磷酸根（PO₄³⁻）、钙离子（Ca²⁺）、铁（Fe）、钠离子（Na⁺）、硫酸根（SO₄²⁻）、溶解性有机碳以及总悬浮颗粒物（Total Suspended Solids, TSS）的年输出通量均显著高于林地下游流域（平均增幅达5.6倍）。该输出通量的提升主要源于大豆种植流域的径流出水量较林地下游流域增加了4.3倍。尽管如此，所有流域的单位面积溶质总输出通量（即产通量）仍处于较低水平（分别为：硝酸态氮<1 kg N/ha·yr、铵态氮<2.1 kg N/ha·yr、磷酸态磷<0.2 kg P/ha·yr、钙离子<1.5 kg Ca²⁺/ha·yr）。作物种植对溪流流量与溶质浓度的影响，似乎受以下因素调控：较高的土壤导水率、深层土壤上以地下水为主导的水文路径，以及未施加氮肥的种植模式。迄今为止，这些因素使得溪流免受了其他地区集约化耕地通常伴随的溶质浓度大幅升高问题。