Impacts on surface and sub soil physical properties under minimum tillage through long-term compost application

Background: The application of composts to agricultural soil is a well-established practice with evidence showing multiple benefits within the field and beyond through changes in a number of soil functions. With soil health and function becoming increasingly important it is critical to understand the impact of soil management on function and changes in soil carbon, both within the root growth zone and, more importantly, within soil below the plough pan, an area of increasing interest.

Methods: A long-term compost application trial was established in 2004 under continuous spring barley with 3 differing compost application rates and a unamended control treatment. Following establishment in 2004 all treatments, except the control, received 50 t ha^-1 of municipal green compost, no amendments in 2005, low (35 t ha^-1), medium (100 t ha^-1), and high (200 t ha^-1), applications in 2006 and 2007 before continuous 35 t ha^-1 annual applications from 2008 to 2022. Plot structure is a randomised block design with soil being a sandy silt loam cultivated under minimum tillage practices. Intact soil cores were collected from both surface soils (~20mm) and subsoils (~300mm) for each plot in spring 2022 prior to compost application, cultivation, and sowing. Full water release data was collected including characterisation of the least limiting water range (LLWR), the available water beyond which mechanical impedance restricts root elongation (2.0 MPa). Additionally, soil resilience tests were performed to simulate trafficking with impacts on soil bulk density quantified and data on wet aggregate stability, visual evaluation of soil structure, and hydraulic conductivity were also collected.

Results: Within surface soils, medium and high compost application rates increased hydraulic conductivity when compared to control plots, the low compost application rate decreased hydraulic conductivity when compared to unamended plots. Surprisingly, within subsoil, compost application was found to significantly impact hydraulic conductivity (P<0.04) with hydraulic conductivity shown to be higher within the medium rate compost application treatment. A significant difference in water stable aggregates (WSA) within surface soils was observed between treatments (P<0.01) and a significant difference in soil bulk density (BD) between treatments (P<0.01) with BD decreasing with increasing compost levels. No significant differences in sub soil bulk density were observed between treatments (P=0.131) however WSA was found to be significantly different in sub soils between treatments (P<0.01). Data on carbon, soil water release characteristics, and nutrient status will also be presented highlighting the long-term benefits of compost application.

Conclusions: Results show that the surface application of compost under minimum tillage practice and the production of continuous spring barley can influence subsoil functions with wider ecosystem benefits.