Effects of integrating grass-clover leys with livestock into arable crop rotations on soil carbon stocks and particulate and mineral-associated soil organic matter fractions in conventional and organic systems

\ua9 2021 British Society of Soil Science. Organic systems, integrated crop–livestock systems (ICL) and leys are posited as strategies to increase soil carbon (C) stocks. However, previous studies have: (i) only considered one driver of change; (ii) evaluated soil C content instead of stocks; (iii) been limited to the 0.20 m depth; (iv) used short-term leys; and (v) rarely assessed the distribution of C among soil organic matter (SOM) fractions, which relates to C stabilization. The aim of this study was to investigate the impact of conventional vs. organic agricultural systems, grazing regimes (non-grazed vs. grazed) and different proportions of temporary grass-clover leys in crop rotations (ley time proportion – LTP) on soil C stocks and C distribution among SOM fractions down to 0.60 m soil depth. SOM fractions assessed were particulate organic matter (POM > 53 μm), heavy fraction (HF > 53 μm) and mineral-associated silt and clay fraction (SC < 53 μm). There were no differences in soil C stocks between the conventional and the organic system, but the former had higher SC-C in the 0.15 m depth. Increasing the LTP associated with livestock grazing increased: (a) soil C stocks in the topsoil (0.30 m); and (b) POM-C and HF-C in the 0.15 m and POM-C in the 0.30–0.60 m depth under both agricultural systems. The inclusion of the longer grazed grass-clover leys was especially important for topsoil POM-C and HF-C under the conventional system. These results suggest that ICL systems with increasing LTP in crop rotations can play an important role in achieving a net C benefit