Application of anaerobically digested cattle slurry and changes in SOM content and composition in organically managed grass-clover ley in Norway

Organically managed grass-clover leys are often claimed to improve soil quality and are considered as a cornerstone of sustainable agricultural practice. However, our earlier results from an organically managed farm in Tingvoll (NW Norway) demonstrated soil organic matter (SOM) content declined over the 35 years following the adoption of organic management, in parallel with increase in temperature and decrease in phosphor availability. In 2011, a field experiment was established on part of the grass-clover ley to study the effects of anaerobic digestion of dairy cattle slurry, i.e. waste product of a biogas reactor, on crop yield and soil characteristics. Slurry application, both treated and untreated, was normalized on the nitrogen content. As part of the global C-arouNd consortium, which aims to investigate how short- and long-term agricultural management practices affect SOM persistence, we are investigating how the long-term slurry applications have affected that and other nutrients.

Since spring 2024, greenhouse gas  emissions were recorded and plant material and soil were sampled. Preliminary results showed a higher SOM concentration with slurry application compared to the control, whereby anaerobic digested slurry let to significantly higher carbon and nitrogen contents in the soil than untreated slurry and unamended soil

More detailed analysis  of the soil, using an advanced density-thermal fractionation protocol, should give more insight into the long-term persistence of SOM. We expect that by first separating the Particulate from Mineral-Associated OM we will find better a more thermally stable Mineral-Associated OM fraction under anaerobically digested slurry application, highlighting changes in SOM composition.