Soil pore characteristics and gas transport properties of a no-tillage system in a subtropical climate

No-tillage system (NT) has been widely adopted around the world to restore the quality of soil structure in previously tilled soils, especially in tropical and subtropical climates. Our objective was to assess the legacy of chiseling in a NT in a subtropical climate by means of gas transport and pore morphology. The experiment was conducted in a split-plot design with two factors and four replications. The main plot was divided into a part under 18 years of no-tillage (18 NT) and a part with 5 years of no-tillage after receiving 13 years of chiseling tillage down to 25 cm depth (5 NT). The subplots were soybean and maize crops. Sixteen soil samples of 567 cm(3) (large cores) were sampled from the 4-12 cm depth and scanned to detect pore diameter > 120 mu m. Then, small subsamples of 100 cm(3) (small cores) were extracted from the large ones to detect pore diameter > 74 mu m using X-ray computed tomography (CT). A range of CT-derived soil pore characteristics were quantified for both sample sizes. In the small samples, air permeability (k(a)) and relative gas diffusivity (D-s/D-o) were determined at -60, -100, and -300 hPa matric potentials. Classical laboratory results showed no significant treatment effects on bulk density (Bd), total porosity (TP), macroporosity (epsilon(a>30)), k(a) and D-s/D-o. There was no significant treatment effect on CT-macroporosity, although 5 NT tended to be higher than 18 NT. CT images showed a significant tillage effect on CT-maximum connectivity of macropores, where for 5 NT was higher (0.75) than in 18 NT (0.65) on the large samples. Therefore, a slight residual effect from chiseling was detected after 5 years of no-tillage. TP and epsilon(a>30) decreased and Bd increased significantly after subsampling. D-s/D-o and k(a) were significantly correlated to CT-derived pore characteristics. This suggests that CT image analysis has high capacity for predicting gas transport.