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

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.