Macropores in a compacted soil impact maize growth at the seedling stage: Effects of pore diameter and density

Abstract

Macropores are often regarded as preferential paths for root growth to bypass high-strength soil. However, the influence of macropore characteristics on root growth in compacted soils is not clearly understood. In this study, we investigated the effects of macropore diameter, density, wall surface area and porosity in a compacted soil on maize growth and N uptake at the seedling stage. Different diameters (0.5-, 1- and 2-mm) and densities (0, 1750, 3500, 7000 and 28,000 m−2 indicated by 0, 5, 10, 20, 80 numbers, respectively) of artificial macropores were created in compacted (1.6 g cm−3) soil columns (150 mm height × 60 mm internal diameter). The absence of macropores was treated as a Control. After 20 days of maize growth, the 3D root architecture and the interaction between roots and macropores were visualized and quantified using X-ray computed tomography (CT). The nitrogen uptake by plants from the compacted subsoil was determined using 15N tracer. Our results showed that the presence of 0.5- or 1-mm macropores enhanced maize growth and 15N uptake relative to the Control, but not the presence of 2-mm macropores. At a given macropore diameter of 0.5-mm, the effect of macropore density on maize growth and 15N uptake was in the order of 7000 m−2 > 28,000 m−2 and 1750 m−2. Under the same macropore wall surface area conditions (i.e., 2513 mm2 in this study), maize growth and 15N uptake decreased with increasing macropore diameter. Under the same macroporosity (i.e., 0.342% in this study), maize growth and 15N uptake were more promoted by macropore characteristics of 1-mm diameter with 7000 m−2 density and 0.5-mm diameter with 28,000 m−2 density compared with 2-mm diameter with 1750 m−2 density. CT images showed that the number of macropores colonized by roots and the intersecting area between roots and macropores both increased with increasing macropore density. Our results demonstrate that the presence of macropores can improve crop growth, but its effect depends on macropore characteristics created by appropriate macropore diameter and density (e.g., 0.5 mm diameter and 7000 m−2 density in this study).