Laboratory mesocosm data measuring the impact of bioturbation frequency on greenhouse gas emissions from reservoir sediments

Inland aquatic systems are major global contributors to the atmospheric carbon budget through greenhouse gas (GHG) emissions, although the amount and form of carbon released varies widely across and within systems. Bioturbation of aquatic sediments can impact biogeochemical conditions and physically release sediment-bound bubbles containing GHGs, but variation in the frequency of such disturbance may modify the rate and composition of resulting GHG emissions. We hypothesized that an intermediate bioturbation frequency would result in the greatest methane (CH4) releases due to mechanical release of trapped bubbles, while frequent disturbance would result in greater diffusive carbon dioxide (CO2) releases relative to CH4, due to increased aeration of the sediment. We tested this bioturbation frequency hypothesis using laboratory mesocosms containing homogenized reservoir sediment. We used mechanical disturbance to simulate bioturbation at 3, 7, 14, or 21-day intervals; a control treatment was undisturbed for the duration of the experiment. We measured GHG emission (ebullition and diffusion) rates. An intermediate frequency of disturbance (7 days) produced the highest total GHG emission rate, while the most frequent disturbance interval (3 days) and least frequent interval (0 days) reduced overall GHG emissions relative to weekly disturbance by 24% and 15%, respectively. These patterns were primarily driven by differences in CH4 ebullition. Contrary to our hypothesis, there was no relationship between disturbance frequency and diffusive CO2 emissions. For all disturbance treatments, the majority of ebullition occurred during disturbance events, suggesting mechanical release of entrapped bubbles is an important emission mechanism. The frequency of disturbance has variable effects on GHG emissions and may explain conflicting results in prior studies of bioturbation. Our study provides insight into bioturbation as a driver of within-system variation in GHG emissions and highlights that variable bioturbation frequency results in non-linear responses in CH4 emissions, a globally important GHG, from reservoir sediments.内陆水生生态系统是全球大气碳收支的重要贡献源，其通过温室气体（GHG）排放实现这一作用，尽管不同系统间及系统内部的碳释放量与碳形态差异极大。水生沉积物的生物扰动（bioturbation）可改变沉积物的生物地球化学环境，并物理释放结合在沉积物中的含温室气体气泡，而这类扰动的频率差异可能改变后续温室气体排放的速率与组成。我们提出如下假说：中等频率的生物扰动会因被困气泡的机械释放而产生最高的甲烷（CH4）排放；而相较于甲烷，高频扰动会因沉积物通气量提升，导致更多的扩散性二氧化碳（CO2）排放。我们利用包含均质化水库沉积物的实验室中型实验生态系统（mesocosms）对该生物扰动频率假说进行了验证。我们以3、7、14或21天为间隔施加机械扰动以模拟生物扰动，并设置了全程未受扰动的对照组。我们测定了温室气体的排放速率，包括冒泡排放（ebullition）与扩散排放（diffusion）。实验结果显示，中等频率的扰动（7天间隔）产生了最高的总温室气体排放速率；相较于每周扰动组，最频繁的扰动间隔（3天）与最不频繁的间隔（0天，即对照组）分别使总温室气体排放降低了24%与15%。这些排放模式主要由甲烷冒泡排放的差异所驱动。与我们的假说相悖的是，扰动频率与扩散性二氧化碳排放之间并未呈现显著关联。在所有扰动处理组中，绝大多数冒泡排放均发生在扰动事件期间，这表明被困气泡的机械释放是一种重要的温室气体排放机制。扰动频率对温室气体排放具有可变影响，这或可解释此前生物扰动相关研究中出现的矛盾结果。本研究揭示了生物扰动作为温室气体排放系统内变异驱动因子的作用，并指出扰动频率的差异会使水库沉积物排放的全球重要温室气体——甲烷的排放呈现非线性响应。