Characterization of Soil Macropores Using X-ray Computed Tomography and Hydro-physical Properties Under Different Soil Management Practices

Abstract

The goal of this study was to quantify changes in soil macropore characteristics by X-ray Computed Tomography (CT) and soil hydro-physical properties as influenced by different soil management practices, such as, cover crops, crop rotations, and addition of biochar. The first objective quantified the impact of long-term cover crop (CC) vs. no cover crop (NC) treatment on soil planted with cotton. X-ray CT scanning of undisturbed soil cores collected from 400 mm deep soil profile indicated that CC improved macropore volume descriptors (macroporosity, macropore number density, and surface area density) in the surface soil layer (0-100 mm). Macropore geometry descriptors exhibited higher pore circularity, tortuosity, and lower degree of anisotropy under CC treatment than NC. Cover crops increased soil organic carbon levels (SOC), wet aggregated stability (WAS), water content at field capacity, plant available water, and reduced bulk density (ρb) in the surface soil layer. Enhanced root volume and root surface area exhibited higher root activity under the CC treatment. The results suggested that CC contributed to higher root activity in the soil and enhanced soil hydro-physical properties, and improved macropore characteristics. The second objective investigated the impact of long-term (> 125 years old) crop rotation treatments: continuous cotton (C) and 2-year cotton-corn rotation (CCR) on X-ray CT-derived macropore characteristics and soil hydro-physical properties in a 400 mm deep soil profile. Cotton-corn rotation had higher X-ray CT-derived macropore number density, macroporosity of medium-sized pores (pore diameter: 1-2 mm), and lower anisotropy values in the top 100 mm of the soil layer, indicating more dense and stable macropore networks under CCR as compared to C. The depth-wise distribution exhibited a reduction in macroporosity and interconnectivity values from 0-100 to 300-400 mm soil layer, indicating less disturbance at the lower depths by field and root activities. The investigation of soil hydro-physical properties (ρb, SOC, WAS, and water retention characteristics) revealed no significant difference among treatments due to long-term adoption of soil conservation practices (cover crops and conservation tillage). The third objective evaluated the short-term impact of biochar incorporation in the sandy-textured soil at different application rates: (i) BC0: 0-Mg/ha, (ii) BC7.5: 7.5-Mg/ha, (iii) BC15: 15-Mg/ha, and (iv) BC30: 30-Mg/ha. The biochar addition at the given application rates exhibited no significant impact on X-ray CT-derived pore indices, except on pore circularity and macroporosity fraction of different-sized macropores for the top 0-50 mm depth, where biochar was incorporated. The macroporosity fraction under the two size groups of macropores (1-10 and 10-100 mm3) was lower under BC15 at 0-50 mm and 50-100 mm depth classes. Lower macropore density under BC15 at the 50-100 mm soil layer indicates preferential movement of biochar particles below their application depth. The reduction in macropore indices with increase in application rates indicates the occupying effect of biochar. The higher application rate (BC30) showed comparable results with BC0, demonstrating that the higher biochar application rates can shift its effect from occupying to expansion. Soil hydro-physical properties revealed no significant differences among biochar treatments at both depth classes, indicating that the higher amount of sand particles and low organic matter may have diminished any improvement in soil hydro-physical properties by biochar addition. The results of this study will contribute to the understanding of soil macropore characteristics and their relationship with soil hydro-physical properties, providing meaningful insights for sustainable agricultural production and various structural and hydrological dynamics controlling pollutant leaching and greenhouse gas emissions.