Soil stores over 2500 Pg carbon (C), with the majority of C stored in deep soil layers (> 30 cm). Soil C can be lost to the atmosphere when organic compounds are mineralized to carbon dioxide (CO2, via oxidative decay or respiration) and moved upward through the soil profile (via diffusion). Soil moisture status can influence the balance between respiration and diffusion, thereby altering the soil CO2 concentration and flux. However, it is unclear how wetting and drying influence soil CO2 dynamics in surface vs. deep soil layers. Thus, we irrigated three soil profiles in Mediterranean arable land and continuously monitored soil CO2 concentration at 15, 30, 50, 70 and 90 cm during wetting and drying phases under ambient temperature conditions. We estimated gas diffusivity, CO2 flux, and temperature responses of soil CO2 concentration during the experiment. Decreases in gas diffusivity during the wetting period coincided with increases in soil CO2 concentrations. However, the negative gas diffusivity-soil CO2 concentration relationship did not hold true all the time, implying that CO2 production was the driving factor for the apparent soil CO2 concentration. We observed hysteretic responses of soil CO2 concentration to temperature as soil moisture varied, with deeper soil CO2 concentration being more sensitive to temperature than surface soil CO2 concentration, especially during the drying phase. The movement of CO2 was upward at all depths during the ambient phase, but the direction and the magnitude of CO2 fluxes varied across the depth gradient during the wetting and drying phases. This study highlights that the relative contribution of gas diffusivity vs. CO2 production to soil CO2 concentration changes with wetting and drying, and that the responses of soil CO2 concentration to temperature are dependent on the antecedent environmental conditions. Also, the downward movement of CO2 during the wetting and drying phases suggests that quantifying surface soil CO2 efflux may underestimate dynamic C processes in deeper soils.

中文翻译：

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干湿对近地表和深层土层土壤 CO2 浓度和通量的不同影响

土壤储存超过 2500 Pg 的碳 (C)，大部分碳储存在深层土层 (> 30 cm) 中。当有机化合物矿化为二氧化碳（CO2，通过氧化衰变或呼吸作用）并通过土壤剖面向上移动（通过扩散）时，土壤 C 会流失到大气中。土壤水分状况会影响呼吸和扩散之间的平衡，从而改变土壤 CO2 浓度和通量。然而，目前尚不清楚润湿和干燥如何影响表层与深层土壤中的土壤 CO2 动态。因此，我们灌溉了地中海耕地的三个土壤剖面，并在环境温度条件下，在润湿阶段和干燥阶段连续监测 15、30、50、70 和 90 厘米的土壤 CO2 浓度。我们估计了气体扩散率、CO2 通量、实验过程中土壤 CO2 浓度的温度响应。润湿期间气体扩散率的降低与土壤 CO2 浓度的增加同时发生。然而，负的气体扩散率-土壤 CO2 浓度关系并非一直成立，这意味着 CO2 产量是土壤 CO2 表观浓度的驱动因素。我们观察到土壤 CO2 浓度随土壤湿度变化对温度的滞后响应，更深的土壤 CO2 浓度比表层土壤 CO2 浓度对温度更敏感，尤其是在干燥阶段。在环境阶段，CO2 的运动在所有深度都是向上的，但在润湿和干燥阶段，CO2 通量的方向和大小随深度梯度而变化。本研究强调，气体扩散率与 CO2 产量对土壤 CO2 浓度的相对贡献随润湿和干燥而变化，并且土壤 CO2 浓度对温度的响应取决于先前的环境条件。此外，在湿润和干燥阶段 CO2 的向下运动表明，量化表层土壤 CO2 流出可能会低估深层土壤中的动态 C 过程。