Soil structural stability following decades of straw incorporation and use of ryegrass cover crops

Abstract

Maintaining good soil structural stability is an important element in agricultural sustainability. Incorporation of cereal straw and use of cover crops improve soil structural stability but the long-term individual and interactive effects of these management practices is poorly understood. We examined the impact of four rates of straw incorporation in spring barley (0, 4, 8 and 12 Mg ha−1, annually) combined with a ryegrass cover crop undersown in the barley in spring. Soil was sampled after four decades of treatments in the Askov (Denmark) straw incorporation experiment situated on a sandy loam with 12% clay. We assessed clay dispersibility measured on two macro-aggregate size fractions (ClayDis 1–2 mm and ClayDis 8–16 mm), wet-stability of aggregates, and clay dispersibility of < 8 mm field-moist soil and aggregate strength. Soil structural stability and strength were related to soil organic carbon (SOC), SOC/Clay and root biomass. The SOC content increased with straw rate and when ryegrass cover crops were grown. Root biomass was marginally greater (P = 0.068), when cover crops were included. The soils receiving 8 and 12 Mg straw ha−1 had a significantly lower ClayDis 1–2 mm and ClayDis 8–16 mm than soil with straw removal. Inclusion of a ryegrass cover crop did not affect ClayDis 1–2 mm, but decreased ClayDis 8–16 mm marginally (P = 0.054) and decreased clay dispersibility of field-moist soil. The results suggest, that binding agents from cover crops such as roots increase stabilization of large macro-aggregates, while the increased stability due to straw incorporation was related to SOC irrespective of aggregate size.