Soil moisture influences soil carbon dynamics, including microbial growth and respiration. The response of such ‘soil respiration’ to moisture changes is generally assumed to be linear and reversible, i.e. to depend only on the current moisture state. Current models thus do not account for antecedent soil moisture conditions when determining soil respiration or the available substrate pool. We conducted a laboratory incubation to determine how the antecedent conditions of drought and flood influenced soil organic matter (SOM) chemistry, bioavailability, and respiration. We sampled soils from an upland coastal forest, Beaver Creek, WA USA, and subjected them to drying and rewetting treatments. For the drying treatment, field moist soils were saturated and then dried to 75, 50, 35, and 5% saturation. In the rewetting treatment, field moist soils were air-dried and then rewet to 35, 50, 75, and 100% saturation. We measured respiration and water extractable organic carbon (WEOC) concentrations and used ¹H-NMR and FT-ICR-MS to characterize the WEOC pool across the treatments. The drying vs. wetting treatment strongly influenced SOM bioavailability, as rewet soils (with antecedent drought) had greater WEOC concentrations and respiration fluxes compared to the drying soils (with antecedent flood). In addition, air-dry soils had the highest WEOC concentrations, and the NMR-resolved peaks showed a strong contribution of protein groups in these soils. Both NMR and FT-ICR-MS analyses indicated increased contribution of complex aromatic groups/molecules in the rewet soils, compared to the drying soils. We suggest that drying introduced organic matter into the WEOC pool via desorption of aromatic molecules and/or by microbial cell lysis, and this stimulated microbial mineralization rates. Our work indicates that even short-term shifts in antecedent moisture conditions can strongly influence soil C dynamics at the core scale. The predictive uncertainties in current soil models may be reduced by a more accurate representation of soil water and C persistence that includes a mechanistic and quantitative understanding of the impact of antecedent moisture conditions.

土壤水分会影响土壤碳动态，包括微生物的生长和呼吸。通常认为这种“土壤呼吸”对水分变化的响应是线性和可逆的，即仅取决于当前的水分状态。因此，当前的模型在确定土壤呼吸或可用的底物池时并未考虑到先前的土壤湿度条件。我们进行了实验室孵化，以确定干旱和洪水的前期条件如何影响土壤有机质（SOM）的化学，生物利用度和呼吸作用。我们从美国华盛顿州比弗克里克（Beaver Creek）的山地沿海森林中取样土壤，并对其进行干燥和重新润湿处理。对于干燥处理，将田间潮湿的土壤饱和，然后干燥至75％，50％，35％和5％的饱和度。在再润湿处理中 将田间潮湿的土壤风干，然后再润湿至35％，50％，75％和100％饱和度。我们测量了呼吸和水可提取的有机碳（WEOC）浓度，并使用了^1个H-NMR和FT-ICR-MS表征了整个处理过程中的WEOC池。干燥与湿润处理强烈影响SOM的生物利用度，因为再湿润土壤（干旱前期）比干燥土壤（干旱前期）具有更高的WEOC浓度和呼吸通量。此外，风干土壤的WEOC浓度最高，而NMR解析峰表明这些土壤中蛋白质基团的贡献很大。NMR和FT-ICR-MS分析均表明，与干燥土壤相比，在再润湿土壤中复杂芳香族基团/分子的贡献增加。我们建议干燥通过芳香分子的解吸和/或微生物细胞裂解将有机物引入WEOC池中，这会刺激微生物的矿化速率。我们的工作表明，即使前期水分条件发生短期变化，也会在核心尺度上强烈影响土壤碳的动态变化。当前土壤模型中的预测不确定性可以通过更准确地表示土壤水和碳的持久性来减少，其中包括对先前水分条件的影响的机械和定量理解。