Alteration of soil biophysical properties after decomposition of contrasting root systems

Background: Increased water infiltration in the presence of vegetation has been reported in the literature for both woody and herbaceous plants. However, there is a lack of experimental data on macropores development after root decomposition, and consequent alteration of soil biophysical properties.

Methods: To test the effect of contrasting root systems on saturated hydraulic conductivity [K_s], individual plants of Daucus carota [F-DC] (Forb; coarse taproot with few small lateral roots); Deschampsia cespitosa [G-DC] (Grass; fibrous root system), Lotus corniculatus [L-LC] (Legume; thin taproot with several lateral roots) were grown in columns (50 mm diameter; 315 mm height) with sandy loam soil packed at 1.4 Mg/m³. Following 7-month plant-establishment, the columns were split into five sections (60 mm height each). K_s was tested in each section (i.e., down soil depth) using a constant-head permeameter. Fallow soil was also tested as control. Following the K_s tests, column sections (i.e., soil cores) were buried in soil and left for decomposition in a controlled environment. After 7-month decomposition, sections were excavated and re-tested for K_s. To measure the biophysical properties of soil in the root-channels, the same three species were also grown in the top-half of a soil column (300 mm height; 50 mm width; 1.2 Mg/m³) longitudinally divided by a 40-μm nylon-mesh. The columns were maintained at a 15-degree slope to facilitate root growth at the soil-mesh interface. Following plant establishment (5 months), plants were killed by herbicide. The soil columns (rooted and control fallow) were buried in soil and left for decomposition in a controlled environment for 7 months. After the decomposition period, the soil columns were split, and the mesh was removed to expose the developed root-channels. The soil in the exposed root-channels was tested for water sorptivity, water repellency, water retention, soil stability in water, hardness and elasticity.

Results: K_s after plant establishment did not differ notably from that of control soil. In contrast, an abrupt increase in K_s (up to 80-times in F-DC) was measured after decomposition in the vegetated soils (e.g., from 2.04e^-6 ± 9.20e^-7 to 1.48e^-4 ± 3.30e^-5 in F-DC at 3 – 63 mm depth). The increase in K_s in G-DC and L-LC was smaller (up to 20-times) compared to F-DC. No K_s change was observed in the control soil. Soil surrounding the root-channels showed greater stability and plant available water. However, we observed smaller sorptivity and greater water repellency in soil surrounding the root-channels of F-DC and G-DC, respectively.

Conclusions: Biophysical alteration of soil after root decomposition depends on plant species. Our findings show that it is possible to engineer soil biophysical properties and bio-pores using contrasting herbaceous species.