Freeze-thaw (FT) events profoundly perturb the biochemical processes of soil and water in mid- and high-latitude regions, especially the riparian zones that are often recognized as the hotspots of soil-water interactions and thus one of the most sensitive ecosystems to future climate change. However, it remains largely unknown how the heterogeneously composed and progressively discharged meltwater affect the biochemical cycling of the neighbor soil. In this study, stream water from a valley in the Chinese Loess Plateau was frozen at –10°C for 12 hours, and the meltwater (at +10°C) progressively discharged at three stages (T1 ∼ T3) was respectively added to rewet the soil collected from the same stream bed (Soil+T1 ∼ Soil+T3). Our results show that: (1) Approximately 65% of the total dissolved organic carbon and 53% of the total NO3--N were preferentially discharged at the first stage T1, with enrichment ratios of 1.60 ∼ 1.94. (2) The dissolved organic matter discharged at T1 was noticeably more biodegradable with significantly lower SUVA254 but higher HIX, and also predominated with humic-like, dissolved microbial metabolite-like, and fulvic acid-like components. (3) After added to the soil, the meltwater discharged at T1 (e.g., Soil+T1) significantly accelerated the mineralization of soil organic carbon with 2.4 ∼ 8.07-folded k factor after fitted into the first-order kinetics equation, triggering 125 ∼ 152% more total CO2 emissions. Adding T1 also promoted significantly more accumulation of soil microbial biomass carbon after 15 days of incubation, especially on the FT soil. Overall, the preferential discharge of the nutrient-enriched meltwater with more biodegradable DOM components at the initial melting stage significantly promoted the microbial growth and respiratory activities in the recipient soil, and triggered sizable CO2 emission pulses. This reveals a common but long-ignored phenomenon in cold riparian zones, where progressive freeze-thaw can partition and thus shift the DOM compositions in stream water over melting time, and in turn profoundly perturb the biochemical cycles of the neighbor soil body.

中文翻译：

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地表水的逐渐融化和不同 DOM 组分的不均匀排放严重扰乱了土壤生化循环


冻融 （FT） 事件深刻扰乱了中高纬度地区土壤和水的生化过程，尤其是河岸带，这些地区通常被认为是土壤-水相互作用的热点，因此是对未来气候变化最敏感的生态系统之一。然而，异质组成和逐渐排放的融水如何影响邻近土壤的生化循环仍然在很大程度上是未知的。本研究将黄土高原一个山谷的溪水在 –10°C 下冻结 12 小时，并分别加入三个阶段 （T1 ∼ T3） 逐步排放的融水 （在 +10°C） 以重新润湿从同一河床收集的土壤 （土壤 + T1 ∼ 土壤 + T3）。结果表明：（1）在第一阶段T1优先排放了约65%的总溶解有机碳和53%的总NO3--N，富集率为1.60 ∼ 1.94。（2） T1 排放的溶解有机物明显更易生物降解，SUVA254 显著降低但 HIX 较高，并且以腐殖酸样、溶解微生物代谢物样和黄腐酸样成分为主。（3） 添加到土壤中后，在 T1 排放的融水（例如土壤+T1）显着加速了土壤有机碳的矿化，在拟合到一级动力学方程后，k 因子增加了 2.4 ∼ 8.07 倍，引发了 125 ∼ 152% 的总 CO2 排放量增加。孵育 15 天后，添加 T1 还显著促进了土壤微生物量碳的积累，尤其是在 FT 土壤上。 总体而言，在初始熔化阶段，富含营养的融水与更多可生物降解的 DOM 成分优先排放，显着促进了受体土壤中的微生物生长和呼吸活动，并引发了相当大的 CO2 排放脉冲。这揭示了寒冷河岸区一个常见但长期被忽视的现象，在那里，渐进式冻融可以分割并因此随着融化时间改变溪流中的 DOM 成分，进而深刻扰乱相邻土壤体的生化循环。