Variability of in situ soil water retention curves under different tillage systems and growing seasons

Soil pore space can change over time due to tillage, frost, wetting and drying cycles, plant roots, and compaction. However, a soil water retention curve (SWRC) is usually considered to be static and is determinedin a laboratory on a limited number of small soil samples. Field SWRCs were determined at a depth of 20 cm on Mollic Gleysol. Measurements were made in two growing seasons at different spatial locations on plots with different tillage systems: no-tillage (NT) and conventional tillage (CT). The unimodal van Genuchten retention function (VG) was fitted to each SWRC and model parameters were estimated. We evaluated the variability of the curves within growing seasons, predicted soilwater contents at field capacity, and evaluated the differences in the estimated parameters of the VG model between different growing seasons and tillage systems considering intraseasonal and spatial variability. The shape of the curves depended more on the environmental conditions during the growing season than on the tillage system. In the wetter and cooler growing season of 2020, when soybeans were grown, the soil retained water in a narrower range. In the drier growing season of 2021, when maize was grown, intraseasonal variability in SWRCs was greater than in 2020. Regardless of the growing season, estimated α values were more intraseasonally variable under the CT than under the NT. Based on the SWRCs, predicted water contents at field capacity varied intraseasonally, seasonally, and spatially. Accounting for spatial and intraseasonal variability, the estimated VG parameter θr differed between years, regardless of tillage system. In 2020, the parameter α was higher under the NT system than under the CT system. Under the CT, α differed between years, while under NT it did not. For the parameter n, there were no significant differences between tillage systems or growing seasons. Seasonal, intraseasonal, and spatial variability in soil hydraulic properties should be considered in soil water modelling studies.