Constraints on Organic Matter Stability in Pyrenean Subalpine Grassland Soils: Physical Protection, Biochemical Quality, and the Role of Free Iron Forms

Pere Rovira, T. Sauras-Yera, R. Poch
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Abstract

The stability of soil organic matter (SOM) depends on its degree of physical protection, biochemical quality (q), and mineralogical features such as the abundance of iron or aluminum oxyhydroxides: All constraints stabilize SOM, but the relevance of each is herein discussed. We studied from this point of view the stability of SOM in four grassland soils. The SOM in these profiles was characterized for its physical protection (ultrasonic dispersion + size fractionation) and its q (acid hydrolysis, carbohydrates, phenolics, and unhydrolyzable carbon). The profiles were also analyzed for free iron forms extracted with several chemicals: dithionite-citrate-bicarbonate, citric acid, oxalic-oxalate (Tamm’s solution), and DTPA. Soil horizons were incubated under optimal conditions to obtain the C lost after 33 days (Cresp33) and basal respiration rate (BRR). The microbial C was obtained at the end of the incubation. The microbial activity rate (MAR: mg C respired per g microbial C per day) was obtained from these measures. The sum soluble + microbial C was taken as the active C pool. As expected, the stability of SOM depends on its distribution between the size fractions: The higher the proportion of particulate organic matter (POM: >20 µm size), the higher the soil respiration rate. In contrast, q barely affects SOM decomposition. Both physical availability (size fractionation) and q (acid hydrolysis) affect the size of the microbial C pool, but they barely affect MAR. The effects of free iron on SOM stability are complex: While dithionite-extracted Fe negatively affected Cresp33, BRR, and MAR, the Fe extracted by smoother methods (Tamm’s reagent and DTPA) positively relates to Cresp33, BRR, and MAR. Free iron apparently modulates soil microbial metabolism because it is the only studied parameter that significantly affected MAR; however, the precise effect depends on the precise free Fe fraction. From our data, SOM stability relies on a net of constraints, including physical availability and free Fe forms, with q being of minor relevance. Our dataset suggests a role for free iron as a modulator of microbial activity, deserving future research.
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比利牛斯亚高山草地土壤中有机物稳定性的制约因素:物理保护、生化质量和游离铁的作用
土壤有机质(SOM)的稳定性取决于其物理保护程度、生化质量(q)以及铁或铝氧氢氧化物的丰度等矿物学特征:所有制约因素都能稳定 SOM,但本文讨论的是每个制约因素的相关性。从这个角度出发,我们研究了四种草地土壤中 SOM 的稳定性。我们对这些剖面中的 SOM 进行了物理保护(超声波分散+粒度分馏)和 q(酸水解、碳水化合物、酚类和不可水解碳)鉴定。此外,还分析了用以下几种化学物质提取的游离铁:二亚硫酸盐-柠檬酸-碳酸氢盐、柠檬酸、草酸-草酸盐(塔姆溶液)和 DTPA。在最佳条件下对土壤层进行培养,以获得 33 天后损失的碳量(Cresp33)和基础呼吸率(BRR)。微生物 C 是在培养结束时获得的。微生物活性率(MAR:每克微生物 C 每天呼吸的毫克 C)由这些指标得出。可溶性碳和微生物碳的总和被视为活性碳池。正如预期的那样,SOM 的稳定性取决于其在不同粒径组分之间的分布:颗粒有机物(POM:>20 µm)的比例越高,土壤呼吸速率就越高。相比之下,q 几乎不影响 SOM 的分解。物理可用性(粒度分馏)和 q(酸水解)都会影响微生物 C 池的大小,但几乎不会影响 MAR。游离铁对 SOM 稳定性的影响是复杂的:虽然提取的铁对 Cresp33、BRR 和 MAR 有负面影响,但用更平滑的方法(塔姆试剂和 DTPA)提取的铁与 Cresp33、BRR 和 MAR 有正面关系。游离铁显然会调节土壤微生物的新陈代谢,因为游离铁是唯一对 MAR 有显著影响的研究参数;但是,确切的影响取决于游离铁的确切含量。从我们的数据来看,SOM 的稳定性取决于一系列限制因素,包括物理可用性和游离铁的形式,而 q 的影响较小。我们的数据集表明,游离铁是微生物活动的调节剂,值得在未来进行研究。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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