Soil carbon sequestration – when aspirations and reality collide

Written by

Rolf Sommer

Research ThemeRestoring Degraded Landscapes

Solutions

Topics

Land

This piece celebrates December 5, World Soils Day, and will be cross-posted on the International Center for Tropical Agriculture (CIAT) Blog.

Just a few weeks ago, we published a study stating that farm land soils, if managed properly, have the potential to sequester up to 7 billion tons of carbon dioxide; as much carbon as is emitted by the US transport sector.

These estimates, published in Scientific Reports, were based on recent global soil and land use maps, as a basis for scaling rational soil carbon sequestration rates at field-scale - tons of carbon sequestered per hectare per year - to the entire globe. Quite a simplification, of course, but good enough for showcasing the magnitude, and shedding light onto the considerably overlooked potential – in the soil!

However, to trap or “sequester”, carbon in the soil, the way we manage soils needs to change. Conventional agriculture is depleting soil health and fertility at an alarming rate, with either too little (sub-Saharan Africa) or overdosed (Europe, North America) inputs, excessive soil tillage and mono-cropping.

Two, rather well-known, improved management practices to halt such a trend are Conservation Agriculture (CA) and Integrated Soil Fertility Management (ISFM). CA combines practices that promote minimal soil disturbance, covering the soil through retention of plant residues, and crop diversification such as by planting two or more crops next to, or after, each other.

ISFM, meanwhile, enables plants to utilize resources more efficiently, and includes the judicious use of mineral fertilizers in combination with organic matter like manure, and improved crop varieties. These two management practices were - and are - also tipped as ways to increase carbon in soils. At least, that is what we thought - until we learned otherwise very recently!

Soil organic carbon (SOC) in the top 30 cm, currently (T0), on all available cropland soils globally (i.e. those not excluded from the analysis as high SOC soils or sandy soils). Maps were produced based upon a geospatial analysis of datasets from the SoilsGrids250 database19, using ESRI ArcGIS software (version 10.3; www.esri.com).

Annual increase in soil organic carbon (SOC) in the top 30 cm, on all available cropland soils globally (i.e. those not excluded from the analysis as high SOC or sandy soils) under the medium scenario (i.e. an increase in percent SOC of 0.27 over 20 years). Maps were produced based upon a geospatial analysis of datasets from the SoilsGrids250 database19, using ESRI ArcGIS software (version 10.3; www.esri.com).

Our recent publication Reducing losses but failing to sequester carbon in soils has provoked some questions. As it seems, soils in the humid tropics of Western Kenya under CA and ISFM for over a decade did not sequester carbon.

Colleagues asked me: Don’t those two studies contradict each other? At first glance, it might seem to be that way. But if we look closer, that’s really not the case.

The first global study is a desktop exercise, in which we describe aspirations and quantify a technical potential. Needless to say, projected global soil carbon sinks are not yet reality. Our recent publication, on the other hand, summarizes observations from our long-term trials in Western Kenya. Through on-the-ground measurements, we found that, despite practicing CA and ISFM which are known to enhance soil fertility and boost yields, soil carbon decreased over time in all tested treatments.

Given the confusion over these two studies, it’s imperative that we precisely define what carbon sequestration means. Namely: “the process of removing carbon from the atmosphere and depositing it in a reservoir.” To be sure that that sequestration really happens, one snapshot in time is not sufficient; what monitoring this process involves is a series of data over time.

Yet most literature uses the term carbon sequestration loosely. Rather than observing trends over time, many published studies rely on snapshots, merely comparing one-time differences with respect to a control. In this respect, they point to the marginal benefit of adopting a comparatively improved practice. We also do this in our recent paper, but call such benefits ‘avoided losses’, in the absence of true sequestration.

Take the example of fuel efficient cars. They avoid carbon dioxide emissions, but they don’t actually remove carbon dioxide from the atmosphere. In the same way, avoiding loss of carbon from the soil is not the same as removing carbon from the atmosphere and then trapping it as well. But, both avoiding emissions, and carbon sequestration, intend to reduce the amount of carbon dioxide in the atmosphere and mitigate climate change.

That being said, the practices used in our trials have helped preserve carbon in soils. It’s just that soils in the humid tropics of Africa are fragile, in the sense that decomposition of organic matter and loss of carbon is incredibly fast given ample rainfall and warm temperatures.

What the study in Kenya tells us is that we are not there yet. To make farmland soil an effective carbon sink, we need to find better solutions.

The message from our recent study is this: While some soils currently do not sequester carbon, it doesn’t mean they can’t in the future. All soils have the potential to sequester carbon if we can establish the right practices to do so within a given context. We, the scientists here at CIAT and our partners, are working to do that. Stay tuned!