Round shaped, continuous vertical pores (CVPs) in the soil are typically created by roots and earthworms. CVPs with diameters > 2 mm are abundant in many agricultural soils. We hypothesized that potential effects of CVPs on shoot growth of winter wheat ( Triticum aestivum L.) increase with: 1) decreasing availability of water and 2) decreasing availability of nutrients in the topsoil. We conducted a microcosm experiment with different irrigation regimes (Irr+/Irr-) and P concentrations (P+/P-), with or without artificially created continuous vertical pores (CVP+/CVP-). Winter wheat was cultivated for 16 weeks. In the bulk soil, presence of CVPs resulted in decreased root length in 20 - 40 cm but increased root length in 40 - 60 cm soil depth. In general, total root length of winter wheat in 20 - 60 cm soil depth was higher when CVPs were present or when P concentrations in the topsoil were elevated. Presence of CVPs generally had a positive effect on shoot dry matter and N uptake of wheat. In columns with high phosphorous concentrations but low soil moisture in the topsoil, presence of CVPs increased shoot dry matter by 66%; in contrast, the beneficial effect of CVPs on shoot dry matter was only 39% in columns with high nutrient concentrations and high soil moisture in the topsoil. In total numbers, however, the effect of CVPs on P uptake into the shoot was more pronounced when P concentrations in the topsoil were elevated. We conclude that CVPs can promote the exploration of the solid soil phase by high root-length densities, but adequate nutrient supply in the topsoil is essential.
Biopores are voids in the soil created by soil organisms including earthworms and plant roots [
On the other hand, CVPs may increase the subsoil’s contribution to crop nutrition: pore walls are often enriched in plant nutrients such as N and P [
The contribution of the subsoil to crop nutrition in general depends on water stress and availability of plant nutrients [
Our hypotheses could not be tested under field conditions, particularly because of uncertainties regarding the field heterogeneity of the soil structure and difficulties with controlling soil moisture. Hence we conducted a pot experiment under controlled conditions using three-stage columns with a top segment containing soil with different moisture and nutrient contents, a middle segment with or without artificial pores and a lower segment providing water saturated soil.
A multifactorial microcosm experiment was established with two different phosphorus concentrations in the topsoil (P+/P−), irrigation (Irr+/Irr−) and presence of continuous vertical pores in the subsoil (CVP+/CVP−) as factors. The experiment was conducted in PVC-pipes with an inner diameter of 19 cm and a total height of 85 cm (
Field triala | Treatment | Excavation depth (cm) | Sand (%)b | Silt (%)b | Clay (%)b | Ctot (g∙kg−1) | Ntot (g∙kg−1) | P (mg/100g) |
---|---|---|---|---|---|---|---|---|
DIK | P- | 0 - 20 | 25 | 60 | 15 | 0.93 | 0.10 | 4.63 |
CKA | P+ | 0 - 20 | 8 | 77 | 15 | 1.04 | 0.11 | 9.55 |
CKA | all | 45 - 75 | 4 | 69 | 27 | 0.49 | 0.06 | 1.72 |
aDIK: “Dikopshof”; CKA: “Campus Klein Altendorf”; bSoil texture according to: [
segments was moderately compacted to bulk densities of approximately 1.3 g/cm3. Soil in the middle segment was compacted to approximately 1.6 g/cm3. We used a modified hydraulic device originally designed to lift hey-bales. Soil was compressed with a round “stamp” neatly fitting into the tubes. In order to achieve a more evenly distributed compaction, we pressed each soil column in 3 steps. In half of the middle segments (CVP+) 6 artificial vertical pores were created with an electric drill (diameter: 8 mm).
A gauge was used to make sure that the pores were equally distributed in each of the columns. The density of vertical pores is equivalent to approximately 210 pores m−2, corresponding to a density of approximately 149 - 256 coarse pores m−2 previously reported for the soil under study [
12 seeds of winter wheat (Triticum aestivum L.) “Graziaro” were sown into each column corresponding to a seeding density of 422 grains m−2. In total, wheat was cultivated for 16 weeks. During the first 12 weeks, columns were set up in a greenhouse at ambient temperature. At seeding, columns from all treatments were irrigated with 30 ml tap water to allow equal emergence. Thereafter, Irr+ columns were irrigated twice a week with 30 ml tap water, whereas irrigation in the Irr− columns was ceased. During the last 4 weeks of the experiment, drought stress was induced using fan heaters and 400 W mercury-vapor lamps for 12 h/day. The temperature amplitude at plant height was 20˚C - 30˚C. Columns from the Irr+ treatment were daily irrigated with 30 ml tap water for two weeks. Since drought symptoms became visible, irrigation was increased to 100 ml tap water/d for the last two weeks.
Shoot biomass was measured as dry matter with oven drying (65˚C). C/N and P contents of soil and plant samples were measured using dry combustion with a Fisons NA-1500 elemental analyzer and atomic absorption spectrometry (AAS), respectively. Leaf area index was determined with a LI-3100 Area Meter (LI-COR Inc., USA). Middle segments of columns were horizontally dissected into 4 slices, representing soil depths of 20 - 30, 30 - 40, 40 - 50 and 50 - 60 cm. Slices containing artificial pores were sampled as follows.
Each pore was laterally opened using sharp knives. Roots growing inside of the pore lumen were carefully collected with a pair of tweezers. If necessary, roots were separated from the pore wall with a scalpel. The remaining samples were soaked in tap water and washed over a sieve (mesh size: 0.5 mm) in order to separate roots from mineral particles an organic debris. The samples were further sorted to remove any non-root material. Immersed roots were carefully placed on trays in order to avoid overlapping and scanned with 800 dpi using a Perfection V700 Photo scanner (Epson Corp.). Root-length density (cm roots cm−3 soil) was determined with the software package WhinRhizoPro 2016 (Regent Instruments Inc.). Soil from columns without artificial pores was directly soaked in tap water. Roots were washed and scanned as described above.
Data were checked for normality with Shapiro-Wilk tests. Since normality was generally given, means were compared without previous transformation by multifactorial ANOVA with continuous vertical pores (CVP), P supply in the topsoil (P) and Irrigation (Irr) as fixed factors and block effect as random factor. Means of shoot parameters were additionally compared on single treatment level by one factorial ANOVA with Tukey tests. All statistical analyses were done with IBM SPSS Statistics Version 25.
Total root length of winter wheat in 20 - 60 cm soil depth was higher when continuous vertical pores were present (
Variable | CVP | P | Irr | CVP × P | CVP × Irr | P × Irr | CVP × P × Irr | Block |
---|---|---|---|---|---|---|---|---|
Shoot dry mass | 0.000 | 0.000 | 0.000 | 0.000 | 0.126 | 0.630 | 0.880 | 0.416 |
N uptake | 0.000 | 0.000 | 0.001 | 0.007 | 0.240 | 0.293 | 0.451 | 0.409 |
P uptake | 0.000 | 0.000 | 0.000 | 0.003 | 0.351 | 0.001 | 0.659 | 0.881 |
Leaf area index | 0.000 | 0.000 | 0.000 | 0.001 | 0.493 | 0.981 | 0.957 | 0.776 |
RLD (20 - 30 cm) | 0.000 | 0.000 | 0.660 | 0.134 | 0.841 | 0.435 | 0.463 | 0.894 |
RLD (30 - 40 cm) | 0.000 | 0.000 | 0.196 | 0.009 | 0.075 | 0.312 | 0.819 | 0.101 |
RLD (40 - 50 cm) | 0.000 | 0.000 | 0.573 | 0.000 | 0.562 | 0.685 | 0.196 | 0.089 |
RLD (50 - 60 cm) | 0.000 | 0.003 | 0.235 | 0.001 | 0.239 | 0.909 | 0.549 | 0.902 |
Shoot: root ratio | 0.000 | 0.000 | 0.000 | 0.001 | 0.493 | 0.981 | 0.957 | 0.776 |
effect gradually increased with increasing soil depth. Irrigation did not change root length significantly, thus
The positive effect of continuous pores on the root length of wheat in our study was much more evident in treatments with high P concentrations in the topsoil, indicating that suitable growing conditions in the topsoil along with adequate structure of the subsoil are advantageous for extensive exploration of subsoil resources.
Further analysis of the treatments with continuous vertical pores revealed that many wheat roots had reached the lower segment by growing through the large vertical pores in the middle segment. The share of roots growing inside the pore increased with soil depth from 38% in 20 - 30 cm to 64% in 50 - 60 cm (
length of wheat. Merely in the columns with continuous vertical pores, slightly but in significantly increased root length at drought was observed in the deeper soil layers (data not shown). Higher root length under deficit irrigation was reported previously [
Presence of continuous vertical pores generally had a positive effect on shoot dry matter and N uptake of wheat, but the magnitude of the effect was different between treatments (
Treatment | CVP+/P+/Irr+ | CVP+/P+/Irr− | CVP+/P−/Irr+ | CVP+/P−/Irr− | CVP/P+/Irr+ | CVP/P+/Irr− | CVP−/P−/Irr+ | CVP−/P−/Irr− |
---|---|---|---|---|---|---|---|---|
shoot dry matter | ||||||||
CVP+/P+/Irr+ | 0.0 | 13.1 | 104.3 | 158.4 | 38.9 | 87.4 | 142.0 | 295.8 |
CVP+/P+/Irr− | −11.6 | 0.0 | 80.6 | 128.4 | 22.8 | 65.7 | 113.9 | 249.8 |
CVP+/P−/Irr+ | −51.1 | −44.6 | 0.0 | 26.5 | −32.0 | −8.3 | 18.5 | 93.7 |
CVP+/P−/Irr− | −61.3 | −56.2 | −20.9 | 0.0 | −46.2 | −27.5 | −6.3 | 53.1 |
CVP−/P+/Irr+ | −28.0 | −18.6 | 47.1 | 86.0 | 0.0 | 34.9 | 74.2 | 184.9 |
CVP−/P+/Irr− | −46.6 | −39.6 | 9.0 | 37.9 | −25.9 | 0.0 | 29.1 | 111.2 |
CVP−/P−/Irr+ | −58.7 | −53.3 | −15.6 | 6.8 | −42.6 | −22.6 | 0.0 | 63.5 |
CVP−/P−/Irr− | −74.7 | −71.4 | −48.4 | −34.7 | −64.9 | −52.6 | −38.8 | 0.0 |
N uptake | ||||||||
CVP+/P+/Irr+ | 0.0 | 14.6 | 88.0 | 176.5 | 61.6 | 130.4 | 169.9 | 460.9 |
CVP+/P+/Irr− | −12.8 | 0.0 | 64.0 | 141.2 | 41.0 | 101.1 | 135.5 | 389.3 |
CVP+/P−/Irr+ | −46.8 | −39.0 | 0.0 | 47.1 | −14.0 | 22.6 | 43.6 | 198.3 |
CVP+/P−/Irr− | −63.8 | −58.5 | −32.0 | 0.0 | −41.5 | −16.7 | −2.4 | 102.8 |
CVP−/P+/Irr+ | −38.1 | −29.1 | 16.3 | 71.1 | 0.0 | 42.6 | 67.0 | 247.0 |
CVP−/P+/Irr− | −56.6 | −50.3 | −18.4 | 20.0 | −29.9 | 0.0 | 17.1 | 143.4 |
CVP−/P−/Irr+ | −63.0 | −57.5 | −30.3 | 2.4 | −40.1 | −14.6 | 0.0 | 107.8 |
CVP−/P−/Irr− | −82.2 | −79.6 | −66.5 | −50.7 | −71.2 | −58.9 | −51.9 | 0.0 |
P uptake | ||||||||
CVP+/P+/Irr+ | 0.0 | 41.7 | 232.2 | 410.0 | 32.7 | 158.8 | 382.2 | 1213.2 |
CVP+/P+/Irr− | −29.4 | 0.0 | 134.4 | 260.0 | −6.3 | 82.7 | 240.3 | 826.8 |
CVP+/P−/Irr+ | −69.9 | −57.3 | 0.0 | 53.5 | −60.0 | −22.1 | 45.2 | 295.3 |
CVP+/P−/Irr− | −80.4 | −72.2 | −34.9 | 0.0 | −74.0 | −49.3 | −5.5 | 157.5 |
CVP−/P+/Irr+ | −24.7 | 6.7 | 150.2 | 284.2 | 0.0 | 95.0 | 263.3 | 889.3 |
CVP−/P+/Irr− | −61.4 | −45.3 | 28.3 | 97.1 | −48.7 | 0.0 | 86.3 | 407.4 |
CVP−/P−/Irr+ | −79.3 | −70.6 | −31.1 | 5.8 | −72.5 | −46.3 | 0.0 | 172.3 |
CVP−/P−/Irr− | −92.4 | −89.2 | −74.7 | −61.2 | −89.9 | −80.3 | −63.3 | 0.0 |
Leaf area index | ||||||||
CVP+/P+/Irr+ | 0.0 | 30.4 | 73.0 | 192.3 | 86.8 | 326.5 | 128.1 | 591.3 |
CVP+/P+/Irr− | −23.3 | 0.0 | 32.6 | 124.1 | 43.2 | 227.0 | 74.9 | 430.0 |
CVP+/P−/Irr+ | −42.2 | −24.6 | 0.0 | 69.0 | 8.0 | 146.5 | 31.8 | 299.6 |
CVP+/P−/Irr− | −65.8 | −55.4 | −40.8 | 0.0 | −36.1 | 45.9 | −22.0 | 136.5 |
CVP−/P+/Irr+ | −46.5 | −30.2 | −7.4 | 56.5 | 0.0 | 128.3 | 22.1 | 270.1 |
CVP−/P+/Irr− | −76.6 | −69.4 | −59.4 | −31.5 | −56.2 | 0.0 | −46.5 | 62.1 |
CVP−/P−/Irr+ | −56.2 | −42.8 | −24.2 | 28.2 | −18.1 | 87.0 | 0.0 | 203.1 |
CVP−/P−/Irr− | −85.5 | −81.1 | −75.0 | −57.7 | −73.0 | −38.3 | −67.0 | 0.0 |
P uptake (83% vs. 33%). Thus, the effect of continuous vertical pores in our experiment can be explained primarily with improved accessibility to water stored in deep soil layers. An earlier study [
When irrigation was ceased, presence of vertical pores in the middle segment increased P uptake from 8.1 to 14.8 mg/pot (+83%) when the topsoil was rich in nutrients, but from 1.6 to 4.1 mg/pot (+158%) when P concentrations in the topsoil were low (
Whereas P uptake from CVP coatings has not yet been quantified, P uptake from the subsoil largely depends on soil properties and growing conditions and varies between 3% - 4% [
It is important to note that the beneficial effect of continuous vertical pores on shoot dry-matter of wheat, as seen in the optimum treatment (Irr+/P+), increased at drought (Irr−/P+) but decreased at P deficiency (Irr+/P−). Hence, the deeper root system in the columns with continuous vertical pores presumably facilitated water uptake from the deep soil, whereas it was less favorable for P uptake, which primarily takes place in the topsoil. In general, wheat cultivated in topsoil with elevated P concentrations yielded approximately twice as much shoot dry matter than wheat cultivated in topsoil with low P concentrations. P uptake into the shoot more than tripled on average with elevated P concentrations in the topsoil.
Water stress during the last stage of the experiment reduced shoot dry matter of wheat by between 13% and 63%, depending on presence of continuous vertical pores and P concentrations in the topsoil (
Enhanced nutrient concentrations in the topsoil and irrigation increased the shoot: root ratio (
Vertical pores in the subsoil generally decreased the ratio of shoot mass to root length (
Roots are a major sink for carbon and unnecessarily large root systems can reduce water efficiency and even grain yield [
It has been suggested that increasing CVP densities by cultivating taprooted crops or promoting anecic earthworms can be an element of strategies for soil fertility building [
In organic agriculture, it is a general aim to promote the exploration of the solid soil phase by high root-length densities. Concerning this matter, our study indicates that vertical pores can support this goal, but adequate nutrient supply to the topsoil is essential. Crops growing under severe nutrient deficiency in early development stages obviously lack the resources to build extensive root systems. Such circumstances cannot be compensated by beneficial growing conditions in the subsoil. Accordingly, in contrast to our hypothesis, our study provides no evidence that the beneficial effect of CVPs on shoot growth of winter wheat increases with decreasing availability of nutrients in the topsoil.
It should be noted that nutrient supply from the drilosphere was probably underestimated in this study. Although the vertical pores were incubated with earthworms before cultivation, the pore wall conditions in a pot experiment do not entirely reflect the pore properties in field, where pores were modified by earthworms and roots probably over decades. The significance of drilosphere properties for root growth and crop performance needs to be addressed in the future.
This study was supported by the BonaRes framework of the German Federal Ministry of Education (Bundesministerium für Bildung und Forschung, BMBF) within the project soil³. We thank the staff of the Institute of Organic Agriculture in Bonn for assistance. The last author is grateful for funding by the German Research Foundation (Heisenberg programme, KA 2703/2-1, KA 2703-3-1).
Dresemann, T., Athmann, M., Heringer, L. and Kautz, T. (2018) Effects of Continuous Vertical Soil Pores on Root and Shoot Growth of Winter Wheat: A Microcosm Study. Agricultural Sciences, 9, 750-764. https://doi.org/10.4236/as.2018.96053