Soil macroaggregation drives sequestration of organic carbon and nitrogen with three-year grass-clover leys in arable rotations.

Conventional arable cropping with annual crops established by ploughing and harrowing degrades larger soil aggregates that contribute to storing soil organic carbon (SOC). The urgent need to increase SOC content of arable soils to improve their functioning and sequester atmospheric CO2 has motivated studies into the effects of reintroducing leys into long-term conventional arable fields. However, effects of short-term leys on total SOC accumulation have been equivocal. As soil aggregation may be important for carbon storage, we investigated the effects of arable-to-ley conversion on cambisol soil after three years of ley, on concentrations and stocks of SOC, nitrogen and their distributions in different sized water-stable aggregates. These values were benchmarked against soil from beneath hedgerow margins. SOC stocks (0-7 cm depth) rose from 20.3 to 22.6 Mg ha-1 in the arable-to-ley conversion, compared to 30 Mg ha-1 in hedgerows, but this 2.3 Mg ha-1 difference (or 0.77 Mg C ha-1 yr-1) was not significant). However, the proportion of large macroaggregates (> 2000 µm) increased 5.4-fold in the arable-to-ley conversion, recovering to similar abundance as hedgerow soils, driving near parallel increases in SOC and nitrogen within large macroaggregates (5.1 and 5.7-fold respectively). The total SOC (0-7 cm depth) stored in large macroaggregates increased from 2.0 to 9.6 Mg ha-1 in the arable-to-ley conversion, which no longer differed significantly from the 12.1 Mg ha-1 under hedgerows. The carbon therefore accumulated three times faster, at 2.53 Mg C ha-1 yr-1, in the large macroaggregates compared to the bulk soil. These findings highlight the value of monitoring large macroaggregate-bound SOC as a key early indicator of shifts in soil quality in response to change in field management, and the benefits of leys in soil aggregation, carbon accumulation, and soil functioning, providing justification for fiscal incentives that encourage wider use of leys in arable rotations.