Wildfires are hazards of increasing significance in recent decades. Our ability to forecast the evolution of fire regimes is inhibited by the lack of records of key fire parameters such as fire severity. Boron isotopes in the soil clay fraction have been shown to vary with fire severity, where increased B coincides with higher fire severity. To elucidate the relationship between the B isotope composition of soil clays and fire events, we performed adsorption experiments by reacting rainwater with combusted leaves to analyse how B isotopes are fractionated during processes leading to the imparting of boron from plants to clay minerals in soil during and following combustion. We find that 5% of B is volatilised during combustion of leaves and barks, where B is preferentially volatilised. No isotopic fractionation was detected during the leaching of leaves ash with rainwater, possibly due to the large water:clay ratio in our experiments. Adsorption of B from leaching solutions onto clay minerals shows isotopic fractionation, and hysteresis of the adsorbed B fraction. For experiments at pH between 7 and 9, the isotope fractionation between adsorbed and dissolved B (B) ranges from −8.8 to −14.5, indicating preferential adsorption of B onto clays compared to B. For experiments at pH 10, the B values range from +11.2 to +19.4, indicating a preferential adsorption of B over B. Irregardless of pH, clay fractions in all experiments show increases in B, as the leaching solutions have high B relative to the soil clay minerals prior to their interaction. Ash of leaves combusted at 550 °C (highest temperature in our experiments) induce the greatest increase in solution pH and B in clays. Our experiments suggests that the higher B isotope composition of clays following a high severity fire is likely imparted by solutions that leach isotopically heavy B from the combusted canopy.

中文翻译：

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野火中的硼：燃烧后挥发、浸出和吸附过程中硼同位素分馏的新见解

近几十年来，野火的危害日益严重。由于缺乏火灾严重程度等关键火灾参数的记录，我们预测火势演变的能力受到限制。土壤粘土部分中的硼同位素已被证明随火灾严重程度而变化，其中硼的增加与火灾严重程度较高一致。为了阐明土壤粘土的 B 同位素组成与火灾事件之间的关系，我们通过将雨水与燃烧的树叶反应进行吸附实验，以分析在导致硼从植物向土壤中的粘土矿物传递的过程中，B 同位素是如何分馏的。燃烧后。我们发现5%的B在树叶和树皮燃烧过程中挥发，其中B优先挥发。在用雨水淋滤叶灰的过程中没有检测到同位素分馏，这可能是由于我们的实验中水与粘土的比例较大。 B 从浸出溶液吸附到粘土矿物上显示出同位素分馏和吸附的 B 部分的滞后现象。对于 pH 值在 7 到 9 之间的实验，吸附和溶解的 B (B) 之间的同位素分馏范围为 -8.8 到 -14.5，表明与 B 相比，B 优先吸附在粘土上。对于 pH 10 的实验，B 值范围为+11.2 至 +19.4，表明 B 优先吸附 B。无论 pH 值如何，所有实验中的粘土部分都显示 B 增加，因为浸出溶液在相互作用之前相对于土壤粘土矿物具有较高的 B。在 550 °C（我们实验中的最高温度）下燃烧的树叶灰会导致粘土中溶液 pH 值和 B 的最大增加。我们的实验表明，在严重火灾后，粘土中较高的 B 同位素组成可能是由从燃烧的树冠中滤出重同位素 B 的溶液造成的。