Porosity and water flow in soil at locations with biopores from decomposing corn roots

Studies evaluating the use of roots from different species to improve soil structure have been increasing in response to the promotion of regenerative agriculture (Embrapa, 2024). However, because studies with roots are laborious and time-consuming, research on this topic is not numerous and several knowledge gaps persist. The study reported in this article is very specific and aggregates information, analysis, and discussion on the contribution of biopores formed by the decomposition of corn roots to soil structure attributes and water flow.

Of all the corn roots, the nodal (or adventitious) roots create the largest biopores in the soil. Since the thickest portion of these roots is concentrated in the surface region of the soil, the benefits of biopores formed after the decomposition of adventitious roots would also be restricted to the surface part of the soil profile. Because bioporosity is limited to the surface portion of the soil, its contribution to increased water infiltration will only occur in the initial infiltration phase if the rest of the soil profile is poorly permeable. Furthermore, the contribution of bioporosity also depends on its horizontal extent, which is determined by the plant population established in the area.

Even though hydrodynamic theory (for example, Poiseuille's equation) indicates that there should be an increase in water flow in soil where there are biopores, discontinuities, constrictions, and obstructions in biopores reduce the flow rate compared to what would be predicted by the hydrodynamic equations when applied to regular pipes without impediments to flow.

In practice, characterizing biopores presents complexities and uncertainties, while measuring their effects is less difficult. However, the uncertainties in the measurements stem from the fact that it is uncertain whether the effects measured in one crop will be repeated in another, because possible discontinuities, constrictions, and blockages in the biopores are unknown in both.

Measurement is therefore a practical necessity not to test what type of effect to be expected from biopores. This already exists in the scientific literature. Measurement is important to reduce uncertainties about the knowledge of the magnitude of the effects, since they are dependent on varying conditions of the plant, soil, management, etc.

In this context, this study presents measurements of soil structure attributes and water flow in soil locations with and without biopores formed by the decomposition of adventitious maize roots, and discusses strategies to increase the scope of benefits and assess the risks of extrapolating the results.

Sampling and measurements

The research was carried out in a field in the municipality of Ivorá (RS), where regolithic neosols predominate (shallow soils, with stoniness and an approximate depth in this field between 20 cm and 80 cm).

The crop has been managed using no-till farming for at least 20 years. Soil sampling was carried out in April 2024, immediately after the soybean harvest (Figure 1). Twenty sampling points were randomly marked in the field. At each point, soil samples were collected from the 0-10 cm and 10-20 cm layers under two conditions: in the position of the rhizosphere of corn plants from the previous crop and in a position corresponding to the inter-row space of the corn from the previous crop. In both cases, care was taken not to collect samples where there were soybean rows and where the remnants of corn roots showed mechanical damage resulting from traffic or sowing and harvesting operations.

Based on the sampling positions, four treatments were defined: rhizosphere 0-10 cm, rhizosphere 10-20 cm, inter-row 0-10 cm, and inter-row 10-20 cm. In each treatment, 20 soil samples with preserved structure were collected (20 repetitions), totaling 80 samples. The determinations performed on the samples were: soil density (Ds), total porosity (Pt), macroporosity (Ma), microporosity (Mi), and saturated hydraulic conductivity (Ks), according to the methodology described in Teixeira (2017). The non-parametric Kruskal-Wallis statistical test was used for all variables (Ds, Pt, Ma, Mi, and Ks) to verify if there was a difference between the treatments, and Dunn's test was used to discriminate between different and similar treatments.

Differences detected

The results highlighted in green in Figure 2 show that only in the 0-10 cm rhizosphere position was there a decrease in Ds (1,12 g/cm3), an increase in Pt (0,56 cm3/cm3) and in Ma (0,14 cm3/cm3). These variables were statistically similar in the other sampling positions (results highlighted in light purple in Figure 2). Mi was similar in all four sampling positions, ranging between 0,39 cm3/cm3 and 0,41 cm3/cm3.

The effect related to the sampling position was greater on Ks (Figure 2). The difference in the average Ks between the sampled layers was 108 mm/h in the rhizosphere position and 80 mm/h in the inter-row, with the highest values ​​in the 0-10 cm layer. Therefore, in the 0-10 cm layer, Ks was significantly higher than in the 10-20 cm layer.

Figure 3 shows an exponential decrease in Ks with increasing Ds and an exponential increase in Ks with increasing macroporosity. This means that Ks is much more affected than density and porosity when there is a change in soil structure, which may result from compaction, scarification, biopore formation, etc.

In percentage terms, the value 1,12 g/cm³ is equivalent to 87% of the average of the remaining Ds values; the value 0,58 cm³/cm³ is equivalent to 114% of the remaining Pt values; and 0,14 cm³/cm is equivalent to 157% of the remaining Ma values. In contrast, the values ​​116 mm/h and 93 mm/h in the 0-10 cm layer are equivalent to 1.380% and 738% respectively in relation to the corresponding Ks values ​​in the 10-20 cm layer. This shows that adventitious corn roots, in addition to promoting soil decompaction, form large biopores (Figure 1), which is quite important for water infiltration into the soil.

Effects and risks

The results show that the effects resulting from the biopores created by decomposing corn roots were confined to a depth of 10 cm and to the rhizosphere (Figure 2). Therefore, the extent of the effect on the crop depends on the plant population.

Considering a population of 70 plants per hectare and assuming that the adventitious roots of each corn plant occupy a circular area with a diameter of 10 cm, it can be seen that only 5,5% of the soil surface would be occupied by biopores formed by adventitious roots (Figure 4).

Multiplying the total surface area occupied by roots in one hectare by a depth of 10 cm and relating the resulting volume to the total soil volume in one hectare up to 10 cm deep, it is also found that only 5,5% of the soil volume in the 0-10 cm layer would be occupied by biopores formed by adventitious roots (Figure 4).

The estimated percentages would be even lower if the calculation considered only the area and volume of the biopores, since the space occupied by them is smaller than the total space surrounding the rhizosphere (Figure 1 center).

Solutions for greater benefit

The small volume of soil occupied by adventitious roots indicates that the benefit of a single crop is quite limited. For the benefits of bioporosity to gain spatial scope in the field, it would be necessary to increase population density and repeat corn crops over the years.

Furthermore, crop management should also aim to preserve the bioporosity built up by each individual crop. In this respect, the benefits of bioporosity will be more prolonged if soil compaction and disturbance are minimized.

* By Beatris Tobin Schopf and Paulo Ivonir Gubiani (UFSM)

Article published in issue 320 of Cultivar Grandes Culturas Magazine