Impact of long-term reduced tillage on water and gas transport in a sandy-loam soil and linkages to biological indicators

Conservation tillage practices, such as reduced tillage, are often considered beneficial regarding soil fertility and sustainability. However, a risk of developing a shallow compact hardpan is associated with these practices that can hinder optimal water and gas transport within the root zone and thus impact soil health and productivity. To explore this risk, we compared conventional mouldboard ploughing (to 30 cm depth; MP) and reduced tillage (5-7 cm; RT) in a long-term experiment (approximately 15 years) on sandy loam soil. The field was uniformly tilled to a depth of 15-20 cm depth after the termination of the experiment. We evaluated the soil water and gas flow variables (saturated hydraulic conductivity, gas diffusion, and effective air-filled porosity), and biological soil properties for the 20-30 cm layer for the MP and RT treatments.

Soil was more compact in the reduced tillage treatment compared to conventional tillage, especially in the 20-30 cm soil layer. Soil bulk densities in the 20-30 cm soil layer were 1.62 and 1.80 g cm^-3 in MP and RT treatments, respectively. There was no difference between the conventional and reduced tillage for effective air-filled porosity or gas diffusivity measured at -100 hPa water potential. Similarly, saturated hydraulic conductivity measured in the field was not different under the two tillage practices. The conventional tillage had 61% more earthworm abundance than the reduced tillage. The results indicated that despite the formation of a compact hardpan at 20-30 cm soil layer, as characterized by a higher bulk density, there were little to no effects of tillage on the soil functions related to water and gas transport. Moreover, the linkages between the soil physical quality indicators and microbial indicators (enzyme activity, microbial biomass carbon and nitrogen) were also explored.