National High Magnetic Field Laboratory Geochemistry Group and Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, USA.
Department of Environmental Systems Science, ETH Zürich, Zurich, Switzerland.
Glob Chang Biol. 2020 Mar;26(3):1374-1389. doi: 10.1111/gcb.14889. Epub 2020 Jan 11.
The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape-level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial-to-aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%-77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra-high-resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land-use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular-plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere.
陆地向河流输送的碳（C）通量和组成是全球 C 循环的关键组成部分，也是整合景观水平过程的有力因素。在刚果盆地，广阔的溪流和河流网络将源自地球上最大的原始热带森林的陆源 C 进行传输和循环。盆地内森林砍伐和向农业的转化速度不断加快，正在改变当前陆地向水生生物地球化学 C 循环的格局。为了研究森林砍伐对刚果盆地溶解有机和无机 C（DOC 和 DIC，分别）生物地球化学的作用，我们每月对 6 条不同森林比例（12%-77%）流域的低地溪流进行采样，为期 1 年。DOC 的年平均浓度对森林损失表现出渐近响应，而 DIC 浓度则随着森林损失的增加而持续增加。DIC 的同位素特征随着森林砍伐变得更加丰富，表明控制 DIC 产生的源和过程发生了变化。超高效分辨率质谱揭示的溶解有机物（DOM）组成表明，森林砍伐流域输出的相对更多的来自土壤微生物生物量的脂肪族和杂原子 DOM。DOM 组成结果表明，与森林相比，来自森林砍伐地点的 DOM 更容易生物降解，这与相应的溪流 CO2 浓度升高一致。简而言之，森林砍伐导致相关溪流的 C 生物地球化学发生重大而全面的变化。显然，土地利用的转换有可能通过减少稳定的、来自维管束植物的 DOC 的下游通量，同时增加生物可利用的土壤 C 向大气的转移，从而极大地影响刚果盆地的 C 循环。
