Spatial heterogeneity of soil properties and solute transport characteristics and their correlations in degraded wetland soils

Soil properties have a significant influence on solutes redistribution in the soil vadose zones. The aim of this study was to assess the relevance of soil properties for solute transport characteristics in degraded wetland soils using 72 undisturbed soil columns from two experimental fields located in Robinia pseudoacacia (CH) and Tamarix chinensis (CL) communities. Combining soil column tracer experiments, all experiments were conducted under the same initial and boundary conditions using Brilliant Blue FCF as a conservative tracer. Solute transport characteristics were described by four measures of dye solution steady infiltration rate of effluents, dye solution concentration of effluents, soil column dye staining patterns, and cumulative dye solution leaching. Numerical modeling by the dual-permeability model in HYDRUS-1D was used to simulate the proportion of cumulative dye solution leaching from soil macropore flow. This study showed that basic soil properties exhibited a significant difference at CH site and at CL site. Dye solution steady infiltration rate of effluents at CH site decreased with soil depth, but increased at first and then decreased with soil depth at CL site. Dye solution concentration of effluents both at CH site and at CL site decreased nonlinearly with soil depth. Soil column dye staining patterns were significantly different among different soil locations, indicating the largest dark blue staining domains from soil depth of 0-10 cm at CH site and 20-40 cm at CL site. The proportion of cumulative dye solution leaching from soil macropore flow was from 37.6 to 61.1% at CH site, whereas from 0 to 99.9% at CL site. Basic soil properties played inconsistent roles in solute transport characteristics. The understanding of soil properties and its correlation with solute transport characteristics is the first step for degraded wetland restoration and development. Some alternative solutions of wetland restoration are proposed for managers.

The interaction between wheat roots and soil pores in structured field soil

Wheat (Triticum aestivum L.) root growth in the subsoil is usually constrained by soil strength, although roots can use macropores to elongate to deeper layers. The quantitative relationship between the elongation of wheat roots and the soil pore system, however, is still to be determined. We studied the depth distribution of roots of six wheat varieties and explored their relationship with soil macroporosity from samples with the field structure preserved. Undisturbed soil cores (to a depth of 100 cm) were collected from the field and then non-destructively imaged using X-ray computed tomography (at a spatial resolution of 90 µm) to quantify soil macropore structure and root number density (the number of roots cm–2 within a horizontal cross-section of a soil core). Soil macroporosity changed significantly with depth but not between the different wheat lines. There was no significant difference in root number density between wheat varieties. In the subsoil, wheat roots used macropores, especially biopores (i.e. former root or earthworm channels) to grow into deeper layers. Soil macroporosity explained 59% of the variance in root number density. Our data suggested that the development of the wheat root system in the field was more affected by the soil macropore system than by genotype. On this basis, management practices which enhance the porosity of the subsoil may therefore be an effective strategy to improve deep rooting of wheat.