Dissolved organic matter (DOM) concentrations have risen by a factor of two or more across much of Europe and North America during recent decades. These increases have affected the carbon cycle, light regime, drinking water treatability, and the energy and nutrient budgets of lakes and streams. However, while trends in DOM quantity are well characterised, information on how/whether qualitative properties of DOM have changed are scarce. Here, we describe over 40 years of monitoring data from a forested headwater stream in the Gårdsjön experimental catchment, southwest Sweden, which provides a unique record of biogeochemical change, including optical and stoichiometric DOM quality metrics, spanning the entire period of recovery from acidification. For the period 1980–2020 we find a 71% reduction in decadal mean sulphate concentrations, and a similar reduction in inorganic aluminium concentrations, alongside a 64% increase in dissolved organic carbon (DOC) concentrations. Over the same period, colour (absorbance at 420 nm) increased almost twice as much as DOC, whereas dissolved organic nitrogen (DON) increased by only one third as much. These results demonstrate a shift in stream water composition, with DOM becoming dominated by highly coloured, complex, nitrogen-poor compounds. This material is likely more resistant to biological degradation, but more susceptible to photochemical degradation. Changes in DOM stoichiometry could lead to intensified nitrogen and/or phosphorus limitation in surface waters, while increased colour/DOC ratios could intensify light-limitation of primary production beyond that expected from DOC increases alone. We observed increases in organic matter associated metals (iron 117%, organically complexed aluminium 85%) that exceeded the increase in DOC, consistent with their increased mobilisation by more aromatic organic matter. All observed changes are consistent with recovery from acidification being the primary driver of change, implying that past acidification, and ongoing recovery, have profoundly affected terrestrial and aquatic biogeochemistry, ecology and the carbon cycle.

近几十年来，欧洲和北美大部分地区的溶解有机物 (DOM) 浓度上升了两倍或更多。这些增加影响了碳循环、光照状况、饮用水可处理性以及湖泊和溪流的能量和养分预算。然而，虽然 DOM 数量的趋势已得到很好的表征，但有关 DOM 的定性特性如何/是否发生变化的信息却很少。在这里，我们描述了瑞典西南部 Gårdsjön 实验流域森林覆盖的源头溪流 40 多年的监测数据，这些数据提供了生物地球化学变化的独特记录，包括光学和化学计量的 DOM 质量指标，涵盖了酸化恢复的整个时期。在 1980 年至 2020 年期间，我们发现十年平均硫酸盐浓度下降了 71%，无机铝浓度也有类似下降，同时溶解有机碳 (DOC) 浓度增加了 64%。同一时期内，颜色（420 nm 处的吸光度）增加几乎是 DOC 的两倍，而溶解有机氮 (DON) 仅增加三分之一。这些结果表明溪流水成分发生了变化，DOM 逐渐以颜色丰富、复杂、贫氮的化合物为主。这种材料可能更能抵抗生物降解，但更容易受到光化学降解的影响。 DOM 化学计量的变化可能导致地表水中氮和/或磷的限制加剧，而色度/DOC 比率的增加可能会加剧初级生产的光限制，超出仅 DOC 增加的预期。 我们观察到有机物相关金属（铁 117%，有机络合铝 85%）的增加超过了 DOC 的增加，这与更多芳香族有机物对它们的动员增加一致。所有观察到的变化都与酸化的恢复一致，酸化是变化的主要驱动力，这意味着过去的酸化和持续的恢复深刻影响了陆地和水生生物地球化学、生态和碳循环。