How does mucilage alter soil mechanical properties and soil penetration resistance to root growth?
Final Report Abstract
Plants benefit from a vital root system that anchors them in the soil and provides adequate nutrient and water supply. The establishment of a root system depends on the physical, chemical, and biological properties of the soil surrounding the root (‘rhizosphere’) and the bulk soil. Plants are known to modify soil properties through root exudates that contain mucilage, a polymeric gel consisting of polysaccharides and lipids. However, it is not yet well understood how mucilage type and concentration affect root penetration, and how the effects depend on other soil (e.g. water content, soil texture) and root (e.g. tip geometry) properties. Therefore, the overall aim of this research project was to better understand how plant root exudates alter soil mechanical properties in general and soil penetration resistance in particular. To address this aim, experimental investigations were carried where penetration resistance was determined for a large variety of samples using a rheometer with a steel needle as a root proxy. In a first step, the combined effects of root tip geometry, water content, and mucilage concentration on penetration resistance to root growth were investigated. The results showed that the penetration resistance decreased significantly at high water content for both needles with different tip geometry. The needle with a blunt tip resulted in a significantly higher penetration resistance than a sharp needle at low water content, but a lower penetration resistance at high water content. Addition of mucilage significantly reduced the penetration resistance for both needle types, especially at low water contents. However, different mucilage concentrations showed no significant effect on the penetration resistance. In a second step, the effect of soil texture, mucilage type, and mucilage concentration on soil penetration resistance was investigated. Loam and sand were used as representative soil textures. Samples with different gravimetric water content and flax and chia seed mucilage at different mass concentrations of 0.1% and 0.5% were prepared for both soil types. The results showed that loam consistently required significantly more energy than sand for penetration. The presence of mucilage significantly reduced the required energy for penetration, with a more substantial effect in loam. No significant difference was observed between mucilage types, except in sand at 9% water content where flax seed mucilage was more effective than chia seed mucilage in reducing penetration resistance. In a third and final step, the effect of different mucilage application methods on soil penetration resistance was investigated. Most previous studies used mixtures of soil and mucilage on a mass basis to investigate how a homogeneous distribution of mucilage affects soil properties. This approach fails to reflect natural conditions where mucilage is secreted locally at the root tip. A novel droplet application was therefore used in the first two steps that more closely mimics the localized secretion of mucilage at the root tip. Within this project, an even more realistic application method was developed using a custom-made hollow needle that features a precision micro-opening on the side near the tip that can be used to inject mucilage with a syringe pump during needle insertion. The penetration resistance obtained with the three mucilage application methods was compared for a sandy soil with two different water content using both chia and flax seed mucilage at two concentrations. The results of this comparison showed that the application method strongly influenced the penetration resistance. The injection method consistently resulted in the lowest required energy both for full cone and shaft insertion for chia seed mucilage, indicating that forces required for deformation and to overcome friction were both considerably lower compared to the other application methods. The results for flax seed mucilage were qualitatively similar, but the differences between the droplet and mixing method were less pronounced. Overall, this project documented the complex interactions between mucilage type, concentration, soil water content, and application method, and stresses the multifaceted nature of mucilage effects on the physical properties of soil. Future research should carefully consider the mucilage application technique, and continue to investigate the link between the mechanical properties of soil-mucilage mixtures and the physical and chemical properties of pure mucilage.
Publications
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How do soil mechanical properties and mucilage affect the penetration resistance to root growth? Poster presentation at EGU General Assembly 2023, Vienna, Austria
Kumar Mysore Janakiram, R., J. Vanderborght & J.A. Huisman
