Assessment of Chinese paddy-soil structure using X-ray computed tomography

Abstract

Soil structure is known to affect water and solute movement and to characterize soil quality. For paddy fields, the soil structure has not been explored nor discussed in detail with respect to rice crop nutrient uptake and root development. The objectives were to visualize the size and connectivity of structural pores including arrangement of aggregates and to quantify the vertical bulk density distribution in the upper soil horizons affected by cultivation. The experimental paddy fields, located in subtropical SE-China in the province Jiangxi, were 20-years (Y) and more than 100-years old (O). Undisturbed soil columns of 10 cm diameter (5–42 cm and 0–38 cm depth) including the plough pan transition down to the subsoil were scanned using a medical X-ray computed tomograph (CT) with spatial resolution of 0.25 mm in horizontal and 1 mm in vertical direction. Vertical bulk density profiles were calculated from Hounsfield Units (HU) and gravimetric water contents measured in 10 mm and particle densities in 20 to 50 mm vertical intervals. Secondary pores were separated using 2 HU-threshold values and described by 3D plots of ‘air-filled’ and ‘low-density’ regions; the matrix structures were analyzed by 2D CT-images. Vertical root channels reached throughout the compacted horizons down to the bottom of samples from both fields. Aggregated matrix structures were found in the subsoil of both samples and horizontal cracks were identified between the cultivated horizon and the pan for Y. The compacted horizon was significantly deeper and larger for the sample from field O as compared to that from field Y, and the bulk density for sample O decreased abruptly in contrast to Y where density decreased gradually with depth. The contrasting bulk density distributions suggest that plough pan compaction continues and did not reach a ‘steady-state’, especially for sample Y. The results indicate the importance of the management system on bulk density and for soil structure formation.