Tillage effects on topsoil structural quality assessed using X-ray CT, soil cores and visual soil evaluation

Soil structure plays a key role in the ability of soil to fulfil essential functions and services in relation to, e.g., root growth, gas and water transport and organic matter turnover. The objective of this paper was: (1) To quantify tillage effects on soil structural quality in the entire topsoil layer (0–20 cm) using X-ray CT, visual evaluation and traditional core methods; and (2) To correlate pore network characteristics from X-ray CT imaging with the results from the visual evaluation and the core method. Samples were taken in May 2009 from a long-term rotation and tillage field experiment on a Danish sandy loam. The tillage treatments were direct drilling (D) and ploughing (P). For X-ray CT scanning, we sampled large soil cores (Ø = 20 cm, height = 20 cm) from the top layer. Small 100 cm3 samples were taken from the 4–8 and 12–16 cm layers for water content and bulk density measurements. Visual soil structure evaluation was carried out in the field at the same time as sampling. CT images (0.39 × 0.39 × 0.6 mm3 voxels) were produced using a medical X-ray CT scanner. The visual assessment showed a good structural quality in the top 5–8 cm for both treatments (Sq < 2). A poorer soil structure was observed in lower part or the topsoil where a firm structure (Sq = 2.9) was observed for D and relatively friable structure (Sq = 2.2) for P. Lower bulk density was found for P than for D in the 4–8 cm layer (1.34 and 1.52 g cm−3, respectively), whereas relatively high bulk density values were observed for both treatments in the 12–16 cm layers (1.50 and 1.56 g cm−3, respectively). The X-ray CT image analysis showed that the P soil had more networks, branches and junctions but a lower degree of anisotropy and shorter average branch length than the D soil. The image data also confirmed a clear stratification of the 0–20 cm topsoil layer for both tillage treatments. The stratification of the direct drilled soil was in accordance with our expectations whereas it was surprising for the ploughed soil. The dense lower topsoil layer for the ploughed soil was probably caused by compaction during secondary tillage and natural consolidation, and aggravated by a poor structural stability due to a low organic matter content. The visual soil evaluation scores were negatively correlated to soil porosity and number of pore networks estimated from X-ray CT imaging and positively correlated to the macropore characteristics of branch length and pore thickness.