The role of grass-clover leys in soil aggregation and carbon storage: a multi-omics approach

Intensive conventional agricultural practices, including conventional tillage and short rotations of annual crops, degrades soil organic carbon (SOC) stores, harms soil organisms and degrades macroaggregates important for soil structure and vital ecosystem services. More sustainable regenerative farming practices, for example reintroducing grass-clover leys into arable rotations, need to be adopted urgently to reduce non-renewable inputs and disturbance, increase organic matter inputs, and allow beneficial soil organisms to recover. The main aims of this thesis were to elucidate the mechanistic basis of soil changes caused by grass-clover leys in arable rotations by characterising changes in soil microbial communities, metabolomes and their impacts on soil aggregation and C storage. Introducing a three-year ley into conventionally managed arable fields increased proportions of soil macroaggregates (>2000 μm) by 5.4-fold, achieving similar proportions as the undisturbed adjacent hedgerow. Bulk SOC rose from 20.3 to 22.6 Mg ha-1 (0.77 Mg C ha-1 yr-1) but was not statistically significant. Macroaggregate-associated and protected OC increased from 2.0 Mg ha-1 to 9.6 Mg ha-1. Simultaneously, fungal communities significantly changed under ley, with increased prevalence and diversity in the Ascomycota and Glomeromycota phyla, but no significant changes identified in soil bacteria. Biosurfactants, lignans and flavonoids were upregulated in the soil metabolome, possibly associated with aggregate stability. Finally, a one-year earthworm manipulation experiment revealed that soil macroaggregates, and associated OC and N, all increase with increased earthworm numbers, being particularly driven by endogeics. This thesis affirms the benefits of grass-clover leys in arable rotations, evidencing their involvement in improving soil structure and quality associated with changes to the soil metabolome, microbiome and earthworm populations. These results provide justification for the rewarding of farmers for including leys in arable rotations for the biological, chemical and physical benefits to soil health which are in the interest of society.