More Root Carbon in Extensive Cultivation Systems - the Greenhouse Gas Inventory Is Optimised

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Soils possess great potential for sequestering CO2 from the atmosphere in the form of carbon, and thus for mitigating global warming. Agroscope researchers have now shown that in extensive cultivation systems, significantly more carbon enters the soil via plant roots than previously assumed. Based on these findings, the estimated values for the National Greenhouse Gas Inventory were adjusted.

The Swiss Federal Government uses the Greenhouse Gas Inventory to check whether Switzerland is on track with its CO2 reduction targets. Soils play an important role here, since they absorb CO2 from the atmosphere. The roots of crops constitute the main source of soil organic carbon in arable soils. These inputs are estimated in the Greenhouse Gas Inventory on the basis of the above-ground biomass. Previously, it was assumed that the more above-ground biomass that is formed, the greater the increase in carbon input via the roots. This means that higher carbon inputs were assumed for high-yield, intensive cultivation systems than for extensive systems such as organic arable farming.

Long-Term Field Trials Refute Previous Assumptions

Agroscope has now refuted these assumptions. In two of Agroscope’s most important long-term trials, the DOC Trial* and the ZOFE*, researchers examined the below-ground carbon inputs of maize and winter wheat. The carbon inputs are the sum of the carbon supplied to the soil both via the roots themselves and via root exudates.  Surprisingly, the below-ground carbon inputs in both crops remained constant, although the above-ground biomass varied by up to a factor of 3.5. With maize, the organic system in the DOC trial actually had one-third higher carbon inputs via root exudates than did conventional cropping systems, with 20% less above-ground biomass.

In a nutshell, therefore, we could say that in organic cultivation systems, compared to the above-ground biomass, more carbon is supplied to the soil via root biomass and root exudates than in intensive arable cropping systems.

Field Studies on Commercial Farms Corroborate Model Studies

The results of a further comprehensive study on 24 farms in Switzerland corroborate the findings from the long-term trials. Although the wheat yields on the organic farms were one-third lower than under conventional management, root biomass was 40% higher. The findings underscore the potential of extensive systems such as organic farming to increase carbon input into the soil via the roots, and thereby mitigate climate change.          

Adjustment in the Greenhouse Gas Inventory

Based on the findings of the Agroscope researchers, the estimated values in the model for calculating the Greenhouse Gas Inventory were adjusted. The Greenhouse Gas Inventory is used to check whether Switzerland is on course with the international and national reduction targets according to the Kyoto Protocol and CO2 legislation. CO2 emissions and soil CO2 uptake also play an important role here. Both can be influenced by agricultural use. Since carbon inputs via the roots cannot be measured over a large area, they were previously estimated for cereal crops on the basis of the above-ground biomass yields.

Publication of the Results

The Agroscope researchers’ results were compiled as part of the National Research Programme ’Sustainable Use of the Resource Soil, NRP68’, and published in the following prestigious scientific journals:

·         Agriculture Ecosystems and Environment
 (DOI: 10.1016/j.agee.2018.07.010, Link­ee.2018.07.010)

·         «Science of the Total Environment»
(DOI: 10.1016/j.scitotenv.2020.143551, Link­itotenv.2020.143551)

*Background Information on the Trials

Agroscope examined the below-ground carbon inputs of maize and winter wheat in two long-term field trials with differing management intensities. The trials represent different Swiss arable sites:

·         The DOC [= ’Dynamic, Organic, Conventional’] long-term system comparison has been comparing organic and conventional cultivation systems since 1978  (Therwil, canton of Basil Country)

·         The Zurich Organic Fertilisation Experiment (ZOFE) was launched in 1949, and is the oldest long-term trial in Switzerland (Zurich, Reckenholz)

The comprehensive practical study determined the root inputs of winter wheat on 24 farms in the Greater Zurich area. The farms were divided into three groups:

·         Conventional mixed farms

·         Conventional farms with conservation soil management

·         Organic farms