Friday, May 30, 2008

Tree biodiversity increases release of carbon from soils

Source: European Commission, Environment DG
Published May 30, 2008

Soils play an important role in the carbon cycle, containing more than two thirds of the total carbon held in terrestrial ecosystems. Concern about climate change and rising CO2 levels is driving research to understand carbon cycling through soils. New research suggests that in tropical regions greater tree diversity increases soil respiration (the release of carbon) rates, but also improves overall ecosystem health, which ultimately leads to greater carbon absorption.

Soil respiration is the main pathway through which carbon is transferred from soil into the atmosphere, with an estimated 50-75 Gt carbon released each year. This is approximately 10 times the amount of carbon released from fossil fuel combustion. The large amount of carbon stored in soils means that small changes in soil respiration rate could have a huge impact on atmospheric CO2. Research undertaken in Panama explored the effects of monoculture, three and six species plantations on soil respiration.

Season has the largest influence on soil respiration rates, with higher rates observed during the wet season. Low soil moisture levels in the dry season are thought to limit respiration from both tree roots and microbes. Compared with monoculture, the three species plots had larger canopies and stronger growth, leading to lower soil temperatures and increased moisture during the wet season. The researchers suggest that these changes in soil climate are more suited to soil microbe activity, which may explain the increased soil respiration found in these plots.

Although higher soil respiration rates were associated with greater tree diversity, this appears to be linked to greater ecosystem health. The researchers also suggest that the larger tree canopies are associated with greater photosynthetic activity and consequently a greater supply of carbon to the tree roots. Greater photosynthetic activity would also mean that more carbon is removed from the atmosphere by the growing tree, outweighing any negative impact of increased soil respiration.

Tropical ecosystems contain more than half of the terrestrial biomass and one third of soil carbon and these ecosystems play an important role in the global carbon cycle. Over the past decade, estimates of carbon emissions from land-use change, such as deforestation to make way for pasture, range from 1-2 Gt C per year in tropical regions. Historical changes in land-use have released an estimated 40 Gt of soil carbon, primarily through soil respiration. Reforestation projects in tropical regions could become significant carbon sinks and this research suggests that plantations containing a diversity of species will be healthier and ultimately absorb more carbon than single species plantations.