THERMAL ACCLIMATION OF SOIL MICROBIAL RESPIRATION: CONSEQUENCES FOR GLOBAL WARMING-INDUCED CARBON LOSSES?

植物目前正在通过吸收人类通过燃烧化石燃料释放到大气中的大量二氧化碳来降低21世纪世纪气候变化的速度。然而，随着植物物质的分解（称为土壤呼吸），土壤产生二氧化碳的速率在较高的温度下增加。因此，随着全球气温的上升，人们担心目前吸收二氧化碳的生态系统可能会开始释放二氧化碳，模型预测这可能会使气候变化的速度增加40%。这一预测主要是基于土壤呼吸对短期温度变化的反应。然而，在长期的变暖实验中，在最初的活动刺激之后，呼吸速率往往会下降到变暖前的水平。这导致了这样的建议，即负责分解有机物质的微生物可能正在适应以补偿温度升高，这种现象可能会保护世界土壤中的碳储存。对于长期变暖实验中观察到的模式，还有另一种解释。最初的活动刺激可能会导致土壤碳储存的耗尽，使微生物分解得更少，从而降低呼吸速率。虽然驯化可以保护种群，但碳耗竭的解释意味着呼吸速率的降低只是土壤中碳不断流失到大气中的结果。因此，区分这两种可能的解释至关重要。在此之前，方法论的局限性使我们无法确定哪种解释是正确的。问题是，当土壤升温时，驯化和碳损失都将降低呼吸速率，因此无法区分它们。我们已经证明，这个问题可以通过使用土壤冷却来克服。当土壤冷却时，最初的活动会下降，但如果发生驯化以补偿温度的降低，呼吸速率随后会增加。另一方面，由于碳损失在较低温度下继续，尽管速率降低，但它们不涉及呼吸速率的任何恢复。因此，碳损失和热适应现在正朝着相反的方向工作，使我们能够区分它们。这一逻辑被应用于确定是否从北极瑞典采取的土壤中的微生物活性适应温度的变化。冷却后，呼吸率没有恢复的迹象。相反，许多天后，温度降低，呼吸速率在冷却的土壤继续急剧下降，没有这样的反应被观察到在土壤保持在较温暖的温度。所以冷却的效果随着时间的推移而放大。看来，土壤微生物通过进一步减少活动来应对较低的温度。反过来看，一个更活跃的微生物群落在更高的温度下存活下来;所以微生物群落的反应增强了温度对分解速率的影响。这种现象以前没有观察到，我们不知道它有多普遍。通过将我们的工作扩展到从不同生态系统和从北极到地中海的土壤采样，我们的资助计划旨在研究土壤微生物群落对温度的反应在控制欧洲土壤分解速率方面的重要性。我们将确定是否发生驯化或微生物群落的反应是否普遍增强呼吸系统对温度的反应。我们还将研究如何控制总体响应。我们的项目将提高对全球变暖将如何影响土壤分解速率的理解，并允许对21世纪世纪气候变化速率进行更准确的预测。

Plants are currently reducing the rate of 21st Century climate change by absorbing a substantial amount of the carbon dioxide that Humankind releases to the atmosphere through the burning of fossil fuels. However, the rate of carbon dioxide production by soils as plant material decomposes (known as soil respiration) increases at higher temperatures. Therefore, as global temperatures rise, it is feared that ecosystems which are currently absorbing carbon dioxide may begin to release it, with models predicting that this could increase the rate of climate change by 40 %. This prediction is based largely on knowledge of how soil respiration responds to short-term changes in temperature. However, in long-term warming experiments, following the initial stimulation of activity, rates of respiration tend to decline back towards pre-warming levels. This has led to the suggestion that the micro-organisms responsible for breaking down organic matter may be acclimating to compensate for the warmer temperatures, and that this phenomenon may preserve carbon stocks in the world's soils. There is an alternative explanation for the patterns observed in long-term warming experiments. The initial stimulation of activity may result in the depletion of soil carbon stores, leaving microbes with less to break down, and so reducing rates of respiration. While acclimation could preserve stocks, the carbon depletion explanation implies that the reduction in respiration rates is simply a consequence of the continuing loss of carbon from soils to the atmosphere. Therefore, it is critical to distinguish between these two possible explanations. Previously, methodological limitations have prevented us from determining which explanation is correct. The problem was that when soils are warmed up, acclimation and carbon loss are both expected to reduce respiration rates, making it impossible to distinguish between them. We have shown that this problem can be overcome by using soil cooling. When soils are cooled, initially activity will decline but if acclimation occurs to compensate for the lowering of temperature, rates of respiration should subsequently increase. On the other hand, as carbon losses continue at the lower temperature, albeit at a reduced rate, they cannot be implicated in any recovery of respiration rates. So carbon loss and thermal acclimation are now working in opposite directions, allowing us to distinguish between them. This logic was applied to determine whether microbial activity in soils taken from arctic Sweden acclimates to changes in temperature. After cooling, respiration rates showed no signs of recovery. Rather, many days after temperatures were reduced, respiration rates in the cooled soils continued to decline steeply, with no such response being observed in soils maintained at a warmer temperature. So the effect of cooling was amplified over time. It appears that the soil microbes were responding to the colder temperatures by further reducing activity. Looking at this in reverse, a more active microbial community survived at higher temperatures; so microbial community responses enhanced the effect of temperature on decomposition rates. This phenomenon has not been observed before, and we do not know how prevalent it might be. By extending our work to soils sampled from different ecosystems and at sites ranging from the high Arctic to the Mediterranean, our grant proposal aims to investigate how important soil microbial community responses to temperature are in controlling decomposition rates in European soils. We will determine whether acclimation occurs or whether microbial community responses generally enhance respiratory responses to temperature. We will also investigate how the overall response is controlled. Our project will improve understanding of how global warming will affect decomposition rates in soils, and allow more accurate predictions of rates of 21st century climate change to be made.