Department of Geological Sciences, The University of Alabama, 201 7th Ave, Tuscaloosa, AL 35485, USA.
Department of Geological Sciences, The University of Alabama, 201 7th Ave, Tuscaloosa, AL 35485, USA.
Sci Total Environ. 2018 Jan 15;612:1442-1453. doi: 10.1016/j.scitotenv.2017.08.322. Epub 2017 Sep 25.
Human land use has led to significant changes in the character of dissolved organic matter (DOM) in lotic ecosystems. These changes are expected to have important environmental and ecological consequences. However, high spatiotemporal variability has been reported in previous studies, and the underlying mechanisms remain inadequately understood. This study assessed variation in the properties of stream water DOM within watersheds across a gradient of agricultural land use with grazing pasture lands as the dominant agricultural type in the southeastern United States. We collected water samples under baseflow conditions five times over eight months from a regional group of first- to fourth-order streams. Samples were analyzed for dissolved organic carbon (DOC) concentration, DOM quality based on absorbance and fluorescence properties, as well as DOM biodegradability. We found that air temperature and antecedent hydrological conditions (indicated by antecedent precipitation index and stream water sodium concentrations) positively influenced stream water DOC concentration, DOM fluorescence index, and the proportion of soil-derived, microbial humic fluorescence. This observation suggests that elevated production and release of microbial DOM in soils facilitated by high temperature, in conjunction with strong soil-stream hydrological connectivity, were important drivers for changes in the concentration and composition of stream water DOM. By comparison, watersheds with a high percentage of agricultural land use showed higher DOC concentration, larger proportion of soil-derived, humic-like DOM compounds, and higher DOC biodegradability. These observations reflect preferential mobilization of humic DOM compounds from shallow organic matter-rich soils in agricultural watersheds, likely due to enhanced soil erosion, organic matter oxidation and relatively shallow soil-to-stream flow paths.
人类土地利用导致了流水生态系统中溶解有机物质（DOM）性质的显著变化。这些变化预计将产生重要的环境和生态后果。然而，以前的研究报告显示，存在高时空变异性，其潜在机制仍未得到充分理解。本研究评估了美国东南部以放牧牧场为主导农业类型的流域内农业土地利用梯度下，流域内溪流水中 DOM 特性的变化。我们在八个月的时间里，在基流条件下五次采集了一个地区一级到四级溪流的水样。对水样进行了分析，以测定溶解有机碳（DOC）浓度、基于吸光度和荧光特性的 DOM 质量以及 DOM 可生物降解性。我们发现，空气温度和前水文条件（以前期降水指数和溪流水中钠浓度表示）对溪流水中的 DOC 浓度、DOM 荧光指数以及土壤衍生的微生物腐殖质荧光的比例有积极影响。这一观察结果表明，高温促进了土壤中微生物 DOM 的产生和释放，再加上土壤与溪流之间较强的水文连通性，是溪流水中 DOM 浓度和组成变化的重要驱动因素。相比之下，农业用地比例较高的流域具有更高的 DOC 浓度、更大比例的土壤衍生的腐殖质样 DOM 化合物以及更高的 DOC 可生物降解性。这些观察结果反映了农业流域中浅层富含有机质土壤中腐殖质 DOM 化合物的优先迁移，这可能是由于增强的土壤侵蚀、有机质氧化和相对较浅的土壤到溪流的流动路径。
