An international team of researchers have suggested that greenhouse gas emissions from land use change are greater than has previously been stated due to negative impacts on plants and crops.

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The researchers said previous studies into the effects of land use change did not assess the effect of nitrogen on plants, which are assumed to have helped sequester carbon, but may in fact have been affected by emissions, hampering their growth.  

A modelling study by researchers at the University of Illinois, reputedly one of the first to take nitrogen into account, found that carbon emissions from human activities on land were 40 percent higher in the 1990s than studies that did not account for nitrogen have suggested.

Study leader Atul Jain explained the project's relevance, "One nutrient can make a huge impact on the carbon cycle and net emissions of the greenhouse gas carbon dioxide. We know that climate is changing, but the question is how much? To understand that, we have to understand interactive feedback processes – the interactions of climate with the land, but also interactions between nutrients within the land."

Study examines effects of nitrogen on the carbon cycle

The carbon cycle, the way in which carbon emissions into the atmosphere are then absorbed by oceans and ecosystems on land before being released again, is affected by a large number of variables, some of which are only just being studied.

Carbon is absorbed by plants during photosynthesis and by the oceans through sea-air gas exchange. On the other side of the cycle, carbon is released by burning fossil fuels and by changes in land use, such as deforestation to expand croplands. While fossil fuel emissions are well-known, there are large uncertainties in estimated emissions from land use change due to factors such as nutrient interactions.

Environmentalists have expressed concerns over the conversion of forests into land used for agriculture or managed as plantations, which is happening across the world. WWF has said deforestation is having a range of ecological and social impacts, which have the potential to worsen.

Demand for products associated with forest clearance is set to rise in the near future, according to the charity; this includes increased demand for soy to 300 million tonnes by 2020, a doubling of palm oil production over the same period and increases in the production of tea, coffee, rice and other crops, as well as deeply controversial biofuels.

Professor Jain said, "When humans disturb the land, the carbon stored in the plants and the soil goes back into the atmosphere. But when plants regrow, they absorb carbon through photosynthesis. Absorption or release of carbon can be enhanced or dampened depending on environmental conditions, such as climate and nutrient availability."

As nitrogen is an essential mineral nutrient for plants, plant growth – and therefore plants' ability to photosynthesize and absorb carbon – is limited by nitrogen availability.

Prasanth Meiyappan, one of Jain's co-authors added, "Most models used to estimate global land use change emissions to date do not have the capability to model this nitrogen limitation on plant regrowth following land use change. This means… they overestimate regrowth and they underestimate net emissions from the harvest-regrowth cycle."

The US-based researchers worked in collaboration with Dr Joanna House at the University of Bristol's Cabot Institute. Dr House concluded that by not accounting for nitrogen as a limiting nutrient for plant growth, other models might have underestimated the carbon emissions from land use change by as much as 70 percent in nontropical regions (those areas of the Earth away from the equator, outside of the tropics of Cancer and Capricorn) and by 40 percent globally.

Dr House warned, "This gross underestimation has great implications for international policy. If emissions from land-use change are higher than we thought, or the land sink (plants' ability to grow back) is more limited, then future emissions cuts would have to be deeper to meet the same mitigation targets."

The researchers said their study also raises questions about the effects of other nutrients on the carbon cycle, such as phosphorous. They have also called for further research into the release of carbon from soil when it is disturbed.

Data from the researchers' study will go to inform the Intergovernmental Panel on Climate Change.