Institute of Tropical Agriculture and Forestry, Hainan University, Haikou 570228, China.
Institute of Tropical Agriculture and Forestry, Hainan University, Haikou 570228, China.
Sci Total Environ. 2019 Apr 1;659:1484-1495. doi: 10.1016/j.scitotenv.2018.12.237. Epub 2018 Dec 19.
Landscape urbanization and intensive agriculture dramatically alter stream ecosystems, but it is little known how urban and agriculture land use change the quantity, quality and ultimate fate of dissolved organic matter (DOM) in stream ecosystems via nonpoint source pathways. Size-exclusion chromatography with fluorescence excitation-emission matrices, as well as absorbance and lignin biomarkers were applied to investigate the characteristics and microbial fate of DOM in 15 first-order agricultural, urban and forest headwater streams in Taihu Lake Watershed, a subtropical region of SE China. Results showed that both urban and agricultural land cover increased the amount of dissolved organic matter (DOC) with a lower C/N ratio, and that the majority of DOC was bound to terrestrial humic-like substances. Compared to forest-impacted headwater streams, the aromaticity and molecularity of OM were greatly decreased as the amounts of anthropogenic/aquagenic fulvic acid-like and protein-like DOM enriched in urban streams, and that of microbially soil-derived humic-like DOM and low molecular-weight substances (e.g., neutrals and acids) increased in agricultural streams, respectively. Human-influenced land use also influenced the seasonal variability of stream DOM biogeochemistry. Natural watersheds produced high DOC specific loads and concentrations only in the rainy season (high-flow period). Agricultural streams contributed a higher amount of terrestrial humic-like and protein-like fractions and urbanized watersheds had high values of autochthonous protein-like fractions only. Redundancy analysis (RDA) revealed that DOM quality (explainable variables: molecular source, 43.29%; molecular-weight, 29.74%) were the most prominent factor impacting microbial carbon processing, followed by inorganic nutrients (17.29%). A higher proportion of DOM from urban streams was eventually mineralized to carbon dioxide mainly due to the inefficient utilization of humic and nonhumic (e.g., protein, polysaccharides and lignin) substances as well as higher levels of inorganic nitrogen and phosphorus, whereas a significant fraction of DOM from agricultural and forest streams tended to enter microbial production and the recalcitrant-DOM pool due to the presence of less labile substrates. Our findings indicate that differences in stream-DOM and environmental properties due to urbanization and farming practices may influence stream microbial carbon processing and cause bottom-up changes in the fate of organic carbon moving through freshwater ecosystems.
景观城市化和集约化农业极大地改变了溪流生态系统，但人们对城市和农业土地利用如何通过非点源途径改变溪流生态系统中溶解有机物质 (DOM) 的数量、质量和最终归宿知之甚少。采用尺寸排阻色谱法结合荧光激发-发射矩阵以及吸光度和木质素生物标志物，研究了中国东南部亚热带太湖流域 15 条一级农业、城市和森林溪流中 DOM 的特征和微生物命运。结果表明，城市和农业土地覆被均增加了溶解有机物质 (DOC) 的含量，且具有较低的 C/N 比，并且大部分 DOC 与陆地腐殖质样物质结合。与受森林影响的溪流相比，由于城市溪流中富含人为/水生富里酸样和蛋白样 DOM，而农业溪流中微生物土壤衍生的腐殖质样 DOM 和低分子量物质（如中性和酸性物质）的芳香度和分子度大大降低，受人为影响的土地利用也影响了溪流 DOM 生物地球化学的季节性变化。自然流域仅在雨季（高流量期）产生高 DOC 特定负荷和浓度。农业溪流贡献了更多的陆地腐殖质样和蛋白样物质，而城市化流域只有高含量的自生蛋白样物质。冗余分析（RDA）表明，DOM 质量（可解释变量：分子来源，43.29%；分子量，29.74%）是影响微生物碳处理的最主要因素，其次是无机养分（17.29%）。由于腐殖质和非腐殖质（如蛋白质、多糖和木质素）物质的利用效率较低以及无机氮和磷水平较高，城市溪流中的 DOM 最终更多地矿化为二氧化碳，而农业和森林溪流中的 DOM 则由于存在较少的易降解底物而倾向于进入微生物生产和难降解 DOM 库。我们的研究结果表明，城市化和农业耕作导致的溪流 DOM 和环境特性的差异可能会影响溪流微生物碳处理，并导致有机碳在淡水生态系统中的归宿发生自下而上的变化。
