Do agricultural management operations and seasonal dynamics in soil structure affect soil hydraulic properties?

In agroecosystems, various factors, from soil management to bioturbation, affect the structure of soils throughout the season. It has been hypothesised that soil structure mainly affects the water retention curve (WRC) and the hydraulic conductivity curve (HCC) close to saturation. Still, the existing experimental data on the magnitude of changes, the potential range affected by soil structures, and their seasonal dynamics are limited.

In this study, we collected undisturbed soil cores at six different times over two growth seasons (mid-season, post-harvest, and after seedbed preparation) from the topsoil (5–10 cm depth) and the subsoil (35-40 cm depth) in a 30-year-long-term field trial run by the Bayerische Landesanstalt für Landwirtschaft. The field trial aims to study the long-term effects of various tillage systems on yield and soil properties. We sampled plots managed with conventional ploughing and direct seeding, all of which had the same loamy soil texture and crop rotation. To quantify the soil hydraulic properties from saturation to oven dryness, four laboratory methods were employed for the same soil core: the Falling Head method, the Multistep Flux method, the evaporation method and the dewpoint method.

Under conventional ploughing, we found a decrease in total porosity of 5 vol.-% after harvest, which was recovered after seedbed preparation and did not change throughout the growing season.  In addition, the WRC changed the shape of the curves after tillage, indicating soil settlement and significant changes in the soils pore size distribution. In contrast, we found that the WRC of the direct seeding maintain the shape but were scaled with seasonal changes in total porosity. For both treatments, seasonal changes in WRC were observed  from saturation up to pF 3, and greatest changes were observed after harvest and after tillage/seedbed preparation. For the HCC, we observed a highly bimodal behaviour and seasonal dynamics near-saturation (0-1.5 pF) indicating the effect of tillage voids and biopores on near-saturation conductivity.

We conclude that agricultural management operations and seasonal dynamics in soil structure controls the wet range of HCC and the WRC from saturation up to pF3. Overall, this study presents experimental evidence on the effect of soil structure on SHP and its potential effect on soil water dynamics.