Researchers study how soil breathes to understand carbon-cycling

Soil respiration (SR) plays a major role in moving carbon from the ecosystem to the atmosphere. Converting land for agricultural use accelerates CO2 emissions via SR. Planting trees (afforestation) has been heralded as a potential climate change mitigation approach. However, new research suggests that the effects of agricultural practices on peatland remain for decades and can continue to influence CO2 emissions even 30 years after afforestation.

Interactions between plant roots and soil microbial processes contribute to plant growth and carbon cycling in terrestrial ecosystems. At present, soil is considered to be a net sink for carbon, absorbing more from the atmosphere than it releases. However, land-use changes have an impact on soil properties and change the rate at which CO2 is emitted.

The Northern peatlands of Europe occupy about 3.5 million km2 and hold nearly one-third of the world's soil carbon. Using this land for agricultural purposes requires fertiliser applications, which helps accelerate the decomposition of soil organic matter, such as peat. In turn, this increases CO2 emissions via SR. Mitigation of this effect is thought to be possible by planting trees (which absorb CO2). As a result of afforestation, the soil is no longer fertilised and may become less favourable for microbes, which leads to a slower decomposition rate and reduces soil CO2 emissions.

In this study, SR was separated into three components originating from: peat (SRP) and aboveground litter (SRL) decomposition and root respiration (SRR). The study focused on six afforested organic soil croplands in Finland. Measurements of CO2 release from soil revealed that temperature was the main factor in total SR, whereas water levels had little effect on the decomposition rates of organic material and only minor effect on SRR. SRP contributed 42 per cent of the total SR, whereas SRR contributed 41 per cent and SRL 17 per cent. Afforestation lowered peat decomposition rates. Nevertheless it was estimated that the effect of previous agricultural practices would lead to higher SR in these areas than undisturbed forested peatland for decades after forests were introduced.

A global plan to increase forest land-cover was initiated with the Statement of Forest Principles1 adopted at the United Nations Conference on Environment and Development (UNCED)2. In 2000, the United Nations Forum on Forests (UNFF)3 was established to promote sustainable forest development, underpinned by policy. In June this year members of the European Parliament will meet to discuss the roles that soil and agricultural and forestry practices can play in mitigating the effects of climate change4. This work suggests that investigating SR will enable researchers to more accurately model cycling of carbon in soils. Key questions for the future are: how will soil biodiversity change if atmospheric CO2 continues to rise, and what impact will this have on the ability of soil to retain carbon?

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