Effects of loosening combined with straw incorporation into the upper subsoil on soil properties and crop yield in a three-year field experiment

Subsoil management needs to be integrated into the current tillage regimes in order to access additional resources of water and nutrients and sustain crop production. However, arable subsoil is often deficient in nutrients and carbon, and it is compacted, affecting root growth and yield. In this study, crop yield and soil responses to loosening of the upper subsoil, without and with straw injection below the plough layer (25-34 cm), were studied during three crop cycles (2016-2018) in a field experiment near Uppsala, Sweden. Responses to straw injection after loosening were studied after single and triple consecutive applications of 24-30 Mg ha-1 during 2015-2017 to spring-sown barley and oats. Subsoil loosening combined with one-time or repeated straw addition (LS treatments) significantly reduced soil bulk density (BD) and increased porosity, soil organic carbon (SOC) and total nitrogen (N) compared with loosening (L) alone (one-time or repeated annually) and the control. In treatment L, the soil re-compacted over time to a similar level as in the control. Field inspections indicated higher abundance of earthworms and biopores in and close to straw incorporation strips. Aggregates readily crumbled/fragmented by hand and casts (fine crumbs) were frequently observed in earthworm burrows. The treatment LS improved soil properties (SOC and porosity) and water holding capacity, but had no significant influence on crop yield compared with the control. Crop yield in all treatments was 6.5-6.8 Mg ha-1 in 2017 and 3.8-4.0 Mg ha-1 in 2018, and differences were non-significant. Absence of yield effect due to treatments could be possibly due to other confounding factors buffering expression of treatment effects on yield. Lower relative chlorophyll content in leaves in the loosening with straw treatment during early growth stages, did not affect final crop yield. Subsoil loosening performed three times gave no further improvement in soil properties and grain yield compared with one-time loosening. There was no difference in yield between repeated subsoil loosening + straw and one-time treatment. It will be interesting to study the long-term effects of deep straw injection and evaluate its impact under other soil and weather conditions.