Mangal V, Lam W Y, Emilson E J S, Mackereth R W, Mitchell C P J
Chemistry Department, Brock University. 1812 Sir Isaac Brock Way, St. Catherine's, ON, L2S 3A1, Canada.
Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada.
Environ Sci Process Impacts. 2024 May 22;26(5):942-956. doi: 10.1039/d3em00305a.
Small headwater streams can mobilize large amounts of terrestrially derived dissolved organic matter (DOM). While the molecular composition of DOM has important controls on biogeochemical cycles and carbon cycling, how stationary landscape metrics affect DOM composition is poorly understood, particularly in relation to non-stationary effects from hydrological changes across seasons. Here, we apply a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and absorbance spectroscopy to characterize stream DOM from 13 diverse watersheds across the central Canadian boreal forests and statistically relate DOM compositional characteristics to landscape topography and hydrological metrics. We found that watershed runoff across different surface physiographies produced DOM with distinctly different chemical compositions related to runoff pH. Specifically, streams in sandy soil watersheds contained more abundant aromatic, nitrogenated and sulfurized fractions of DOM, likely due to a combination of lower soil capacity to absorb DOM than other soil types and high conifer forest coverage that generated acidic litterfall in more sandy watersheds. In contrast, streams with more neutral pH in watersheds with shallow soils had DOM resembling low oxidized phenolic molecules mainly due to increased brush/alder and deciduous vegetation coverage in relatively steeper watersheds. However, as precipitation and flows increased in the fall, the overall water chemistry of streams became more similar as runoff pH increased, the overall chemical diversity of DOM in streams decreased, and stream DOM resembled fresher, lower molecular weight lignin material likely originating from freshly produced leaf litter. Together, our findings show that during hydrologically disconnected periods, pH and landscape characteristics have important controls on the mobilization of aromatic DOM but that many landscape-specific characteristics in the Canadian boreal forest are less influential on DOM processing during wetter conditions where chemically similar, plant-derived DOM signatures are preferentially mobilized. These findings collectively help predict the composition of DOM across diverse watersheds in the Canadian boreal to inform microbial and contaminant biogeochemical processes in downstream ecosystems.
小源头溪流能够搬运大量源自陆地的溶解有机物质（DOM）。虽然DOM的分子组成对生物地球化学循环和碳循环具有重要的控制作用，但静止景观指标如何影响DOM组成却鲜为人知，特别是与季节间水文变化的非静止效应相关的情况。在这里，我们结合傅里叶变换离子回旋共振质谱（FT-ICR-MS）和吸光光谱法，对加拿大中部北方森林中13个不同流域的溪流DOM进行表征，并将DOM组成特征与景观地形和水文指标进行统计关联。我们发现，不同地表地貌的流域径流产生的DOM具有与径流pH值明显不同的化学组成。具体而言，沙质土壤流域的溪流中DOM的芳香族、含氮和含硫部分更为丰富，这可能是由于沙质土壤比其他土壤类型吸收DOM的能力较低，以及高针叶林覆盖率导致更多沙质流域产生酸性凋落物的综合结果。相比之下，浅土壤流域中pH值更中性的溪流中的DOM类似于低氧化酚类分子，这主要是由于相对陡峭流域中灌木/桤木和落叶植被覆盖率增加。然而，随着秋季降水量和流量的增加，随着径流pH值升高，溪流的整体水化学变得更加相似，溪流中DOM的整体化学多样性降低，溪流DOM类似于可能源自新产生的落叶的更新鲜、分子量更低的木质素物质。总之，我们的研究结果表明，在水文断开时期，pH值和景观特征对芳香族DOM的搬运具有重要控制作用，但在湿润条件下，当化学性质相似的植物源DOM特征被优先搬运时，加拿大北方森林中许多特定于景观的特征对DOM处理的影响较小。这些研究结果共同有助于预测加拿大北方不同流域的DOM组成，为下游生态系统中的微生物和污染物生物地球化学过程提供信息。
