Effects of tillage practices and cover crops on X-ray computed tomography derived soil pore characteristics

Abstract

Soil management practices such as conservation tillage and cover crops improve soil health and crop yield. However, these practices may create connected soil pore networks that can cause preferential transport of contaminants to the groundwater or surface water via subsurface flow pathways. The goal of this study was to quantify the impact of tillage and cover crops on the soil macropore characteristics at different resolutions of X-ray computed tomography scanning. The study site was at the E.V. Smith Research Center, Shorter, Alabama, USA. Intact-undisturbed soil cores were collected (fall 2021 and spring 2022) from an agricultural field planted with cotton (Gossypium hirsutum L.). Treatments included strip tillage with cover crops, strip tillage without cover crops, conventional tillage with cover crops, and conventional tillage without cover crops. The cover crop treatment consisted of a mixture of cereal rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.), and tillage treatments consisted of strip tillage as conservation tillage and conventional tillage with a disk harrow, chisel plow, and disk harrow again. Six intact undisturbed soil cores (150 mm diameter and 640 mm deep) were collected from each treatment class and subjected to non-invasive X-ray computed tomography (CT) scanning at different resolutions. Additional soil columns (76 mm diameter and 640 mm deep) were collected and scanned at 0.1875 mm image resolution. All the images were analyzed using the ImageJ image processing software to determine the soil pore characteristics. The study was divided into three main objectives: (1) cover crop effects on X-ray computed tomography-derived soil pore characteristics; (2) effects of conventional and strip tillage on X-ray computed tomography-derived soil pore networks and their temporal dynamics; (3) effect of image resolution and soil core diameter on soil pore characteristics quantified using X-ray computed tomography. The results from the first objective show that soil columns under cover crop treatments had higher values of macroporosity and macropore number density for the top 100 mm of the soil profile, whereas the properties such as connection probability were significant in the sub-surface depth of 200-400 mm. Significant correlations were also observed between larger root volumes and complex, and irregularly shaped pore characteristics. The findings from the second objective suggest that soil columns under the strip tillage treatment had significantly greater number of macroporosity, macropore number density, macropore length density, and node density for the 0-200 mm depth class in season 1 as compared to soil columns collected from the conventional tillage treatment. This was attributed to the fact that season 1 was sampled in a relatively drier month of the year, which could have led to crack development. All the pore characteristics also showed significant temporal variability in values from season 1 to season 2. Findings for the third objective indicate that scanning soil cores at higher resolution resulted in a greater number of isolated pores with greater anisotropy values. Further, the number of pores decreased significantly in the 76 mm diameter cores compared to the 150 mm diameter cores. Overall, image resolution and soil core diameter impacted the values of pore properties detected from X-ray CT images.