Soil organic carbon (SOC) constitutes an important reservoir in the global carbon cycle that is vulnerable to transformation and loss from land use and climate change. Anoxic conditions protect SOC from microbial degradation through limiting the energetics of respiration and inhibiting extracellular oxidative enzymes. Given growing evidence of prevalent anaerobic microsites in upland soils, we designed an experiment testing the development of dissolved organic carbon (DOC) signatures of energetic and enzymatic limitations on microbial carbon utilization across simulated soil aggregates or peds. Reactors comprised a soil column “aggregate” underlying an advective “macropore” channel. Soils received downward diffusive inputs of aerated porewater media with added nitrate, sulfate, or no amendment—where native ferrihydrite served as dominant anaerobic terminal electron acceptor (TEA). After 40 days, added nitrate resulted in highest bulk respiration and DOC production while sulfate did not differ from the control. Nominal oxidation state of carbon (NOSC) was higher (more favorable) with added TEAs at soil surfaces and decreased with depth, while NOSC in the non-amended soil remained lower and constant with depth. DOC generally increased with depth, which along with decreasing NOSC values indicates joint electron-donor and acceptor control over respiration energetics. Of all organic compound classes, only the relative abundance of phenolics increased between 0 and 0.5 cm depth, which aligns with the oxic-anoxic transition and suggests oxidative enzyme inhibition. Our results suggest that oxygen limitation within upland soil aggregates may preserve SOC via both energetic and enzymatic C protection mechanisms, which are vulnerable upon exposure to oxygen.

土壤有机碳（SOC）是全球碳循环中的重要储集层，易受土地利用和气候变化的影响而遭受转化和损失。缺氧条件通过限制呼吸的能量和抑制细胞外氧化酶来保护SOC免受微生物降解。鉴于越来越多的证据表明旱地土壤中普遍存在厌氧微生物，我们设计了一个实验，用于测试在模拟土壤团聚体或peds上微生物碳利用的能量和酶学限制的溶解有机碳（DOC）特征的发展。反应堆包括在对流“大孔”通道下方的土壤柱“聚集体”。土壤接受了充气孔隙水介质的向下扩散输入，并添加了硝酸盐，硫酸盐，或不做任何修改-天然亚铁酸盐作为主要的厌​​氧末端电子受体（TEA）。40天后，添加硝酸盐导致最高的整体呼吸和DOC产生，而硫酸盐与对照无差异。在土壤表面添加TEA时，名义碳的氧化态（NOSC）较高（更有利），并且随深度而降低，而未改良土壤中的NOSC则保持较低且随深度恒定。DOC通常随着深度的增加而增加，这与NOSC值的降低一起表明电子给体和受体对呼吸能的联合控制。在所有有机化合物类别中，只有酚类的相对丰度在0到0.5 cm深度之间增加，这与氧-氧转移相吻合，并暗示了氧化酶的抑制作用。