Wildfires lead to decreased carbon and increased nitrogen concentrations in upland arctic streams

The Central Siberian Plateau (CSP) is undergoing rapid climate change that has resulted in increased frequency of forest fires and subsequent alteration of watershed carbon and nutrient dynamics. Across a watershed chronosequence (3 to >100 years since wildfire) we quantified the effects of fire on quantity and composition of dissolved organic matter composition (DOM), stream water nutrients concentrations, as well as in-stream nutrient uptake. Wildfires increased concentrations of nitrate for a decade, while decreasing concentrations of dissolved organic carbon and nitrogen (DOC and DON) and aliphatic DOM contribution for five decades. These post-wildfire changes in stream DOM result in lower uptake efficiency of in-stream nitrate in recently burned watersheds. Nitrate uptake (as uptake velocity) is strongly dependent on DOM quality (e.g. polyphenolics), ambient dissolved inorganic nitrogen (DIN), and DOC to DIN ratios. Our observations and experiments suggest that a decade-long pulse of inorganic nitrogen and a reduction of DOC export occur following wildfires in streams draining the CSP. Increased fire frequency in the region is thus likely to both decrease DOM and increase nitrate delivery to the main stem Yenisei River, and ultimately the Arctic Ocean, in the coming decades.

中西伯利亚高原（Central Siberian Plateau, CSP）正经历快速气候变化，导致森林野火发生频次上升，进而改变了流域碳与营养盐动态。本研究针对野火发生后3年至100余年的流域时间序列，量化了野火对溶解态有机物（dissolved organic matter, DOM）的数量与组成、河流水体营养盐浓度，以及河道内营养盐摄取过程的影响。野火可使硝酸盐浓度升高长达十年，同时在五十年内降低溶解态有机碳与氮（dissolved organic carbon and nitrogen, DOC与DON）以及脂肪族DOM的占比。野火后溪流DOM的上述变化，会导致近期过火流域内的河道硝酸盐摄取效率降低。硝酸盐摄取速率（以摄取速度表征）强烈依赖于DOM质量（如多酚类物质）、环境溶解态无机氮（dissolved inorganic nitrogen, DIN）以及DOC与DIN的比值。本研究的观测与实验结果表明，在CSP流域的排水溪流中，野火过后会出现为期十年的无机氮脉冲现象，并伴随DOC输出量的下降。因此，该区域野火频次的增加，在未来数十年内可能同时降低DOM含量，并提升向叶尼塞河干流乃至北冰洋输送的硝酸盐通量。