SR10 How does tillage intensity affect soil organic carbon (SOC)?
Farming practices can significantly affect the amount of carbon stored in soil. Reducing the intensity of tillage of arable land is a common environmental management practice for conserving carbon, and yet the quantitative impacts of these practices is not currently well understood. Continuing from a systematic map published in 2015, EviEM has undertaken a systematic review to estimate the effects of reduced tillage intensity on soil organic carbon. Overall there is an increase in soil carbon from reduced tillage, but this affects only the upper soil layers.
Impacts of farming on soil organic carbon
On farmland that is harvested annually, the carbon content of the soil declines as organic compounds are broken down, removed in the form of crops, leached out by run-off or lost by erosion. Globally, the top one meter of soil contains roughly three times as much carbon as the above-ground biomass of plants, and twice as much as the atmosphere. Changes in the stock of soil carbon can therefore cause significant changes in the atmospheric concentration of carbon dioxide. On a global scale, the losses of carbon from agricultural soils have been estimated at roughly 100 to 1,000 million tonnes of carbon every year.
However, there are methods that can reduce carbon losses or even increase the sequestration (capture) of carbon in arable soils, even though the land continues to be farmed. Increased carbon sequestration in arable soils offers major benefits: It improves yields owing to better soil structure and fertility, and it reduces greenhouse gas levels in the atmosphere, helping to mitigate the threat of global warming.
Tillage, including ploughing, has been undertaken traditionally to prepare the soil for growing crops and to remove weeds, which would otherwise compete with crops and reduce yields. Some research indicates that reduced intensity tillage has led to higher carbon levels in certain areas, although the results in Sweden have been mixed.
A review of the impacts of reduced tillage on soil organic carbon
A previously completed systematic map (SR4) identified 735 studies from across the world. Over 300 of these studies examined the effects of varying tillage intensity on carbon storage.
The studies describe a variety of tillage practices, including no tillage, shallow tillage, non-inversion tillage, and full-inversion tillage. This review investigated the effects of tillage of varying intensity on soil organic carbon to ask which practices are most effective at preserving and restoring carbon across a range of different systems.
The review supplemented the studies identified in the recently completed systematic map with studies found following an update of the searches to capture recently completed research. All relevant studies were subjected to critical appraisal of study reliability, and data were extracted and analysed using a series of meta-analyses.
Overall, the studies analysed in this review show that soils with no tillage practices have higher concentrations of soil organic carbon than in soils with high-intensity tillage. There were significantly higher concentrations in no tillage than at intensive tillage but also in comparison to reduced tillage cultivation. Even reduced tillage was a significantly better alternative than high-intensity tillage.
Most studies only reported carbon from the upper soil layer and only a third of all studies attempted to estimate the concentration or amount of carbon below 15 cm from the surface. Our analyses, however, show that it is precisely in the top soil layer that more carbon is accumulated under reduced or no tillage. According to earlier studies, we can expect that increased carbon storage provides better soil infiltration, gives the soil a higher water storing capacity, reduces the risk of soil erosion and promotes soil organisms. All of these factors are expected to increase the soil fertility and to contribute to increased yields.
Altered carbon storage due to land use change is a slow process. Therefore, in the systematic review, only studies that had lasted longer than ten years were included. The longer the studies (from ten to hundred years), the more carbon was stored in the top soil layer (0 to 15 cm deep). On the other hand, we found no clear increase in carbon storage over time in deeper soil layers. This is likely to depend on a large variation between different studies or that the rate in which carbon is stored is slower further down into the soil profile.
Implications of the findings
Our review and meta-analyses have shown that reduced tillage practices contribute to the storage of more carbon in the upper soil layer than in high-intensity tillage. Our analyses further show that today we cannot detect changes in the carbon stock from the full soil profile, but this could also depend on lack of evidence and a need of more data. The United Nations Food and Agriculture Organization (FAO) has launched the Global Soil Partnership, and the need for such activities is raised to an intergovernmental level. There is a strong interest among farmers to find ways to use the land more sustainable, both from an economic and environmental perspective.
Our review provides quantitative support to the claim that changes in soil organic carbon in arable land cannot be observed if the study lasts less than ten years. It is thus crucial for research funding to support longer-term experiments than this period. Furthermore, most studies in our review included only a small number of replicates of each trial, which means that calculated mean values are not as reliable as they might otherwise have been.
Katarina Hedlund (Chair), Centre for Environmental and Climate Research, Lund University, Sweden
Helene Bracht Jørgensen, Department of Biology, Lund University, Sweden
Louise E. Jackson, Department of Land, Air and Water Resources, University of California, Davis, USA
Thomas Kätterer, Department of Ecology, SLU Uppsala, Sweden
Emanuele Lugato, Joint Research Center (IES – Soil action), Ispra, Italy
Ingrid K. Thomsen, Department of Agroecology, University of Aarhus, Denmark
Per-Erik Isberg, Department of Statistics, Lund University, Sweden
Neal Haddaway (Project Manager), Stockholm Environment Institute, Stockholm, Sweden