|
Soil carbon dioxide emissions may continue for longer than previously thought |
|
|
Current models of soil carbon suggest CO2 emissions caused by rising temperatures will decrease over time. However, a recent study, which deliberately warmed soil for ten years, found no evidence of such a decline. The results indicate that our understanding of soil carbon dynamics may need revisiting.
Current models of soil carbon suggest CO2 emissions caused by rising temperatures will decrease over time. However, a recent study, which deliberately warmed soil for ten years, found no evidence of such a decline. The results indicate that our understanding of soil carbon dynamics may need revisiting.
Soil is one of the most important carbon stores in the world. Just 0.1 per cent of carbon emitted into the atmosphere from European soils is the equivalent to the carbon emissions of 100 million extra cars on our roads. The EU has taken steps to protect this important resource in its Soil Thematic Strategy1 and it recently published a report and a study on the links between soil and climate change2.
Soil respiration, or loss of organic carbon as CO2 emissions, is encouraged by higher temperatures, which means that climate change may increase respiration. The emitted CO2 would then exacerbate climate warming, creating an ongoing cycle. It is therefore important to fully understand the long-term effects of temperature on soil carbon.
Studies of forest and grassland indicate that temperature-induced soil respiration ceases after just a few years, but longer-term effects have not been studied.
The researchers simulated a 3°C warming of soil by placing a heating wire on the soil's surface. They measured CO2 emissions from the soil and the CO2 concentration in air found in tiny pockets of the soil at different depths, ranging from 0.3 to 1.9 metres. After ten years of warming, the soil was compared to soil samples which had not been heated.
In the unheated soil, CO2 concentrations in the soil air were greater at depths between 0.8-1.55 metres compared to soil shallower than 0.8 metres and deeper than 1.55 metres. In contrast, in the warmer soil, higher CO2 concentrations were found at the full range of depths studied. The most striking increase in soil air CO2was in the upper half of the heated soil. In addition, soil warming increased the cumulative CO2emissions from the soil; emission levels were 38 per cent higher than from the unheated soil.
These results contradict current theories of soil carbon dynamics, which predict that temperature-induced soil respiration will drop over time. The theories propose that higher temperatures cause greater activity of fungi and bacteria in the soil. Initially this increases respiration but as the microbes use up their source of sugars in the soil their activity lessens and so does respiration. However, these theoretical estimates of respiration are lower than the respiration levels found by this study.
The authors suggest that warming might trigger additional processes that mobilise carbon and which are not part of current models. For example, warming could change the activity of fungi and bacteria so that they can feed off other material in the soil, other than sugars. This would increase decomposition of soil organic matter and therefore respiration.
The research highlights the complexity of soil processes, particularly when climate change is part of the equation. It also highlights the need to review current modelling approaches.
1. See http://ec.europa.eu/environment/soil/three_en.htm
2. See http://ec.europa.eu/environment/soil/review_en.htm |
