The use of grass-clover leys in rotation with arable crops has the potential to maintain and increase soil carbon stocks in northern Europe’s agricultural landscapes, while improving soil’s physical properties and fertility.
With pressures on nitrogen fertiliser supply, the use of leys in both arable and mixed farming is attractive as an option for biological nitrogen fixation, which can in turn reduce costs.
In a presentation at the Symposium of the European Grassland Federation, researchers from Norway and Sweden said that results from long-term studies in their countries show links between the frequency of such leys in rotations and key soil qualities.
A case for inclusion
The work tested a range of rotations, as well as approaches to fertiliser and tillage, but in particular, field experiments in Sweden showed that ley-cereal rotations increased soil organic carbon (SOC) stocks compared to simple cereal rotations, regardless of whether crop residues were removed or left to incorporate into the soil.
“Based on the findings from these long-term studies in Sweden, we can repeatably conclude that leys have positive effects on SOC and should be included as much as possible in crop rotations,” the researchers wrote in a peer-reviewed paper accompanying the Symposium.
Tailoring rotations for impact
Norwegian work underlined the impact of leys on soil characteristics. Rotations including ley had higher aggregate stability and earthworm density compared to programmes using conventional tillage — but it was also apparent that the period covered by ley in rotations is crucial in bringing about desirable qualities and stable carbon concentrations in soil. The researchers recommended a recommendation of minimum 50% ley inclusion in rotations.
“The inclusion of leys in crop rotations had a positive impact on soil structure, SOC and soil biology. SOC concentrations and stocks generally increase with the frequency of leys in rotations,” the scientists said.
“However, 1 or 2 years of ley in rotations may not be sufficient to maintain SOC over time under Nordic conditions in soil with relatively high SOC content.”
Soil starting point is key
They also pointed to evidence from Norway suggesting that it is easier to raise soil organic carbon levels in land with low initial carbon levels than with high—a note of caution against unrealistic expectations for ‘carbon farming’ land management options.
“Site history…determines whether a certain rotation or management will lead to increasing or decreasing soil organic carbon stocks in absolute terms,” they stressed.
“This means that, although leys will accumulate more carbon in soils than cereal-dominated rotations, even ley-dominated systems may result in decreasing soil organic carbon stocks.”
