Trees grown on agricultural land significantly contribute to global carbon budgets, say authors in this recent study.

If carbon from trees grown on agricultural land was well accounted for, total carbon estimates for agricultural land would be more than four times higher than they currently are, they add.

This is good news, and getting better: between 2000 and 2010, tree cover on agricultural land increased –three percent, resulting in a 4.6 percent increase in biomass carbon globally.

Yet while the importance of carbon stored by forests is widely recognized, carbon stored by trees on agricultural land has been much ignored, authors say.

Total carbon estimates for agricultural land could be more than four times higher

“What’s happening below ground when we plant trees on farms is arguably more important than what we see above ground.”

Deborah Bossio

Co-author of the study and CIAT’s Director of Soil Research., CIAT

Soil carbon: benefits of sequestration

The soil organic carbon pool is enormous – estimated to be two to three times higher than in the atmosphere. The additional carbon that can be stored as soil organic matter is also huge – up to 1.2 Gigatons per year in top soils on agricultural lands alone – another unexploited, under-appreciated carbon sink.

But until recently, with the launch of the “4 per 1000” initiative – promoting carbon sequestration in soils for food security and climate – the mitigation potential of sequestering carbon in soils was rarely discussed in policy circles.

This despite the fact that the potential to sequester carbon in agricultural soils is estimated to be almost twice that sequestered by trees on agricultural lands annually over the last 10 years, according to this new study.

“What’s happening below ground when we plant trees on farms is arguably more important than what we see above ground,” says Deborah Bossio, co-author of the study and CIAT’s Director of Soil Research.

“Through their roots and litter, trees increase soil organic matter and improve soil health, making farming systems more resilient to climate extremes. In the landscape, trees protect soils from erosion, preserving fertile topsoil, and farm productivity.”

Therefore the benefits of increasing tree cover on agricultural land go far beyond carbon sequestration – particularly because of the potential positive impacts on soil health.

Improved soil carbon; improved soil health

Soil conditions such as moisture content, temperature and nutrient levels – all improved by tree cover – are major controlling factors influencing agricultural productivity, and regional and household food security.

Because trees are also relatively permanent, their biomass contributes to a build-up of carbon over the long-term, which has other beneficial impacts on soil fertility, stabilizing wider ecosystem health, especially important during changing climatic conditions.

In general, tree cover – and biomass carbon on agricultural land – are higher in humid regions, like coastal West Africa at around 40 percent. Areas with less than 10 percent of tree cover include eastern China, northwest India and the southern border of the Sahara.

Given the large amount of agricultural land suitable for growing trees, growing more trees on farm land could be a relatively fast route to increasing soil carbon sequestration with a myriad of other benefits, authors point out.

Swimming against the tide

“However, we should not underestimate the task at hand to sequester more carbon in our soils,” adds Bossio. “In most agricultural systems, we are swimming against the tide: we are currently losing carbon every year, especially from peat soils.

“And as with all climate mitigation strategies on agricultural land, it’s important to remember that there may be downsides to planting trees, or other carbon sequestration interventions – which is why farmers are not doing it already.

“For example, if trees replace food production, or soil carbon storage locks up nutrients rather than cycling them, farmers may be losers rather than winners in the effort.”

There are different ways to increase carbon stocks in agricultural lands: by changing farm management within a system, or by changing from one land use to another – for example from grass land to forest.

Each can affect the amount of carbon released or sequestered, and the products from the land, and different practices need to be tested for site-specific relevance.

Further research is needed to identify drivers of change at regional, national, and sub-national scales, and to develop policies that enhance carbon sequestration on agricultural land which benefit farmers and society as a whole, authors say.

Call to action:

  • Landscape mapping, using site-specific data, can guide decision makers about where to invest in certain management practices over others to increase soil carbon.
  • Site-specific soil profiles and land use data can help decision makers get the bigger picture of where soils are most degraded, and which areas should be prioritized for investment to improve soil carbon stocks.

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