High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe(III) reduction by electron shuttling
High-pH and anoxic conditions during soil organic matter extraction increases its electron-exchange capacity and ability to stimulate microbial Fe(III) reduction by electron shuttling
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DOI：
10.5194/bg-17-683-2020
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发表时间：
2020-02
期刊：
影响因子：
4.9
通讯作者：
Yuge Bai;Edisson Subdiaga;S. Haderlein;H. Knicker;A. Kappler
中科院分区：
文献类型：
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作者：
Yuge Bai;Edisson Subdiaga;S. Haderlein;H. Knicker;A. Kappler
抽象的。土壤有机质（SOM）具有氧化还原活性，可以被微生物还原，并在非生物反应中将电子转移到Fe（III）矿物，从而充当电子穿梭。从土壤中分离有机质（OM）的标准程序包括使用碱性和酸性溶液以及分离腐殖酸（HA）和富里酸（FA）。该过程可能导致SOM化学和氧化还原性质的不希望的变化。为了确定提取条件对SOM提取物的氧化还原和电子穿梭性质的影响，我们从底土收集的土壤样品中制备了HA、FA和水可提取的有机物（OM）提取物，应用0.1M NaOH和6 M HCl的组合或超纯水（pH 7）（0-15厘米，A层，pH 6.5-6.8）在Schönbuch森林，巴登-符滕贝格，德国。化学提取（NaOH scinHCl）和水提取在单独的实验中进行，无论是好氧或缺氧条件下。此外，我们将NaOH scinHCl处理应用于水可提取OM的子样品，以将HA和FA与水可提取OM分离。当比较不同提取方法从土壤中提取的碳量时，我们发现从土壤中化学提取的FA和HA可占土壤有机碳库的34%-40%，而水提取的OM仅占0.41%-2.74%土壤有机碳总量。较高的提取效率的化学提取可能是由于去质子化的羧基和苯酚官能团在高pH值下。阳极氧化提取条件下也导致了更多的提取碳。对于水可提取的OM，7倍以上的C缺氧条件下提取相比，好氧条件。这种差异可能是由于发生微生物还原和溶解的Fe（III）矿物在土壤中的缺氧水提取过程中，因此伴随释放的Fe（III）矿物结合的有机物。为了比较不同SOM提取物的氧化还原活性，分析了所有提取的HA、FA和水可提取的OM的电子交换容量（EEC），并且我们的结果表明，在缺氧提取条件下，从水可提取的OM化学分离的HA具有比水可提取的OM本身高2倍的EEC值，这表明在缺氧条件下用NaOH萃取期间，通过氨基酸、醛和含羟基和儿茶酚的分子之间的缩合反应，可能形成氧化还原活性芳族官能团。我们还用所有提取物进行了微生物Fe（III）还原实验，发现提取物的更高EEC反过来导致通过电子穿梭对微生物Fe（III）矿物还原的更高刺激，即，更快的初始Fe（III）还原速率，并且在大多数情况下还具有更高的还原程度。我们的研究结果表明，在中性pH值的水提取的OM应被用来更好地反映环境SOM氧化还原过程中的实验室实验和潜在的文物的化学提取方法和缺氧提取条件时，需要考虑评估和比较非生物和微生物SOM氧化还原过程。
Abstract. Soil organic matter (SOM) is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals, thus serving as an electron shuttle. The standard procedure to isolate organic matter (OM) from soil involves the use of alkaline and acidic solutions and the separation of humic acids (HAs) and fulvic acids (FAs). This process potentially leads to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron-shuttling properties of SOM extracts, we prepared HA, FA, and water-extractable organic matter (OM) extracts, applying either a combination of 0.1 M NaOH and 6 M HCl or ultrapure water (pH 7), from soil samples collected from the subsoil (0–15 cm, A horizon, pH 6.5–6.8) in Schönbuch forest, Baden-Württemberg, Germany. Both chemical extractions (NaOH∕HCl) and water extractions were done in separate experiments under either oxic or anoxic conditions. Furthermore, we applied the NaOH∕HCl treatment to a subsample of the water-extractable OM to separate HA and FA from the water-extractable OM. When comparing the amount of carbon extracted from soil by different extraction methods, we found that FA and HA chemically extracted from the soil can make up to 34 %–40 % of the soil organic carbon pool while the water-extractable OM only represents 0.41 %–2.74 % of the total soil organic carbon. The higher extraction efficiency of the chemical extraction is probably due to the deprotonation of carboxyl and phenol functional groups under high pH. Anoxic extraction conditions also led to more extracted carbon. For water-extractable OM, 7 times more C was extracted under anoxic conditions compared to oxic conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the anoxic water extraction and thus the concomitant release of Fe(III) mineral-bound organic matter. To compare the redox activity of different SOM extracts, the electron-exchange capacity (EEC) of all extracted HA, FA, and water-extractable OM was analyzed and our results showed that, under anoxic extraction conditions, the HA chemically isolated from the water-extractable OM had 2 times higher EEC values compare to the water-extractable OM itself, suggesting the potential formation of redox-active aromatic functional groups during the extraction with NaOH under anoxic conditions by condensation reactions between amino acids, aldehydes, and hydroxyl- and catechol-containing molecules. We also performed a microbial Fe(III) reduction experiment with all extracts and found that higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e., faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that OM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.