Effects of vegetation roots on the structure and hydraulic properties of soils: A perspective review

Effects of different irrigation amounts on soil structure in newly cultivated land on the Loess Plateau

It is imperative to promote water-saving irrigation technology and develop newly cultivated land in the Loess Plateau. This study focused on the interaction between roots and soil to examine the effects of different amounts of irrigation on soil structure of newly cultivated land. Herein, five irrigation levels were set, i.e., sufficient irrigation (W100), mild deficit irrigation (W80), moderate deficit irrigation (W60), severe deficit irrigation (W40), and rain-fed (RF). Physical properties and structural stability indexes of the rhizosphere soil were measured, and their relationship with plant root morphology were analyzed. The results showed that the soil structure under the high irrigation amount group (W80 and W100) was relatively stable. The average particle density of soil in each plot decreased significantly after the experiment, while the soil total porosity remained unchanged in W80 and W100 treatments. The proportion of large aggregates, the mean weight diameter, and the geometric mean diameter of soil significantly reduced in the low irrigation amount group (RF, W40, and W60). In contrast, the W100 and W80 treatments inhibited the decline in soil aggregate stability. Change in the generalized soil structure index (GSSI) and soil three-phase structure distance (STPSD) of W100 and W80 treatments were not significant, before and after the experiment. These results suggested that the soil in newly cultivated land treated with sufficient and mild deficit irrigation was closer to the ideal state for crops growing. Path analysis identified the average soil moisture content had the greatest negative effect on STPSD primarily through the root length, root surface area, and root dry weight. In conclusion, irrigation amount occupies a dominant position among all factors influencing soil structure considered in the study. And the mild deficit irrigation is suitable for agricultural cultivation in the Loess Plateau area, from the soil structure protection and water-saving perspectives.

Root Exudates in Soilless Culture Conditions

Root metabolite secretion plays a critical role in increasing nutrient acquisition, allelopathy, and shaping the root-associated microbiome. While much research has explored the ecological functions of root exudates, their relevance to horticultural practices, particularly soilless cultivation, remains underexplored. Steering root exudation could help growers enhance the effectiveness of plant growth-promoting bacteria. This review summarizes current knowledge on root exudation in soilless systems, examining its process and discussing environmental influences in the context of soilless cultivation. Plants in soilless systems exhibit higher total carbon exudation rates compared to those in natural soils, with exudation profiles varying across systems and species. Root exudation decreases with plant age, with most environmental adaptations occurring during early growth stages. Several environmental factors unique to soilless systems affect root exudation. For instance, nutrient availability has a major impact on root exudation. Light intensity reduces exudation rates, and light quality influences exudation profiles in a species- and environment-dependent manner. Elevated CO2 and temperature increase exudation. Factors related to the hydroponic nutrient solution and growing media composition remain insufficiently understood, necessitating further research.

The recent genetic modification techniques for improve soil conservation, nutrient uptake and utilization

Advances in genetic modification (GM) techniques have generated huge interest in improving nutrient utilization, maximizing nutrient uptake, and conserving soil in the pursuit of sustainable agriculture. Unfortunately, little is still known about the recent advancements in the application of GM tactics to enhance each of these areas. This review explores the latest GM strategies intended to support soil conservation, maximize nutrient uptake, and improve nutrient utilization in farming, highlighting the critical roles that soil health and nutrient management play in sustainable farming. GM strategies such as improving the efficiency of nutrient uptake through enhanced root systems and increased nutrient transport mechanisms are well discussed. This study suggests that addressing potential obstacles, such as ethical and regulatory concerns, is a necessity for long-term sustainability applications of GM technologies to raise agricultural yields.

Soil infiltration mechanisms under plant root disturbance in arid and semi-arid grasslands and the response of solute transport in rhizosphere soil

The symbiotic relationship between plant roots and soil infiltration is of great significance for sustainable development of the agriculture and forestry. Through detailed summary of the relationship between root morphological parameters and soil infiltration rates in arid and semi-arid grasslands mainly with leguminous herbs, gramineous herbs and shrubs, the mechanisms that key parameters (root length density, surface area density, diameter, biomass density, architecture, secretion and decay rate) disturb soil infiltration through affecting soil structure such as porosity, soil bulk density and soil organic matter (SOM) are elucidated. Furthermore, the degree of root disturbance on soil structure and infiltration rates are partially clarified by constructing quantitatively structural equation modeling path diagrams. The results show roots have the most significant effect to increase soil infiltration rates through increasing non-capillary pores, contributing to >50 % of the positive effect. In contrast, the increased SOM influenced by roots can obstruct soil infiltration and offset about 25 % of the positive effects. In addition, the impact of root disturbance on transport of nutrients, pesticide and pathogenic microorganisms in rhizosphere soil is also discussed to analyze the potential influence on food and water environmental safety. The presence of roots reduces the amount of leachate-prone nutrients, but their disturbance increases the rate of soil infiltration thus accelerates transport of solutes into deeper soil. Meanwhile, the rhizosphere alters the environmental behavior of pesticides and pathogenic microorganisms, increasing risk of plant roots exposure to them. At present, systematically quantifying the interference of plant roots on soil structure and soil infiltration capacity remains a major challenge. It is necessary to further improve the research methodology and strengthen the study of root soil interaction mechanisms, providing scientific basis and technical support for sustainable agricultural development and ecological environment protection.

Melatonin-Mediated Modulation of Grapevine Resistance Physiology, Endogenous Hormonal Dynamics, and Fruit Quality Under Varying Irrigation Amounts

Grapevine, as a globally significant economic fruit tree, is highly sensitive to water stress, which not only damages its growth but also affects the formation of fruit quality. Melatonin (MT), acting as a signaling molecule, plays a crucial role in plant stress responses. However, the regulatory mechanisms of MT on grape leaf physiological characteristics and fruit quality under different irrigation amounts have not been fully elucidated. In this experiment, grape leaves were treated with a 150 μmol·L-1 MT solution at 0, 30, 60, and 90 days after flowering under different irrigation amounts (360, 300, 240, and 180 mm). It was found that MT significantly increased the contents of osmotic adjustment substances in leaves, reduced the level of reactive oxygen species, enhanced the activity of antioxidant enzymes, and promoted the metabolism of the ascorbic acid-glutathione cycle, thereby improving the antioxidant capacity of grapes and effectively alleviating the damage caused by a water deficit. At the same time, MT also maintains the dynamic balance of endogenous hormones by upregulating and downregulating the expression levels of related genes, thereby improving fruit quality. In summary, this study reveals the potential application value of MT in enhancing the drought resistance and fruit quality of grapes.

A 2-year pure biochar addition enhances soil carbon sequestration and reduces aggregate stability in understory conditions

The enhancement of soil aggregate size and stability is crucial for mitigating climate change and improving carbon sequestration in forest ecosystems. Biochar, derived from rice husks, has been suggested as an effective mean to increase soil carbon storage. However, isolating biochar's specific effects on soil aggregate formation and carbon sink capacity can be complex due to the overlapping influences of fertilization and understory vegetation cultivation. Our study circumvented these variables by incorporating different amounts of biochar into plantation soil without any additional cultivation or fertilization, conducting a detailed two-year field experiment. The findings revealed that biochar significantly increased the organic carbon content and density in the uncultivated under-forest Ferralsols, thus enhancing its carbon sink function. Intriguingly, while biochar raised the proportion of small soil aggregates (< 0.25 mm) and their organic carbon levels, it decreased the fraction of larger aggregates (> 0.25 mm), adversely affecting soil aggregate stability. These results suggest that biochar may compromise soil aggregate structure and stability in the absence of plant growth. The positive impact of biochar on soil carbon storage was found to depend more on its inherent inert carbon content than on soil type. Moreover, biochar alone was insufficient to increase the quantity of soil macroaggregates without the binding action of plant root exudates. Biochar's key function appears to be in enhancing the soil aggregate-forming processes facilitated by plant roots and microorganisms. Therefore, for optimal carbon sequestration in forest soils, integrating biochar application with appropriate agricultural practices is advisable.

Reducing drought vulnerability of forest soils using Xanthan gum-based soil conditioners

Climate change increases the frequency and severity of droughts in many parts of Europe, thereby affecting the availability of water resources. Therefore, preserving the soil water content is essential for maintaining forest diversity and plant vitality. To improve soil hydraulic properties and reduce drought vulnerability, three xanthan-gum-based soil conditioners (SC_R, SC_CG, and SC_ZZC) were developed under the European ONEforest project. These soil conditioners (SCs), including oxide ash and cellulose fibres of different lengths, differ in their filler properties. This study evaluated the performance of these soil conditioners in forest soils in Slovenia (S1), Spain (S2), and Germany (S3). Water absorption, water retention, hydraulic conductivity, seed germination, and plant growth in the untreated soils and mixtures were analysed. The results showed a 53 %-100 % increase in water absorption with high dosage of SCs (1.7 % of dry SC per dry soil mass). The addition of SCs also significantly improved the water retention capacity of the treated soils within a suction range of 0-100 kPa, which is advantageous for maintaining adequate water availability for plant growth. The effect of SCs on unsaturated hydraulic conductivity varies depending on the soil type. For example, at a similar suction, the unsaturated hydraulic conductivity of mixtures with soils S2 and S3 can be more than an order of magnitude lower than that of untreated soils S2 and S3. In contrast, mixtures with soil S1 showed unsaturated hydraulic conductivity similar to that of the untreated soil. In the seeding experiment, plants in treated soils survived for up to 8 d without watering compared to those in untreated soils, with survival linked to the initial water content of the mixtures. These findings suggest that soil conditioners reduce drought vulnerability by improving water retention and regulating water loss. However, the optimal dosage should be adjusted according to different soil types and local environmental conditions.

Migration and Accumulation Simulation Prediction of PPCPs in Urban Green Space Soil Irrigated with Recycled Water: A Review

The presence of pharmaceuticals and personal care products (PPCPs) in reclaimed water introduces an ongoing challenge as they infiltrate green space soils during irrigation, leading to a gradual buildup that poses considerable ecological risks. The simulation and forecasting of PPCPs accumulation in soil are pivotal for proactive ecological risk management. However, the majority of research efforts have predominantly concentrated on the vertical transport mechanisms of PPCPs in the soil, neglecting a holistic perspective that integrates both vertical and lateral transport phenomena, alongside accumulation dynamics. To address this gap, this study introduces a comprehensive conceptual model that encapsulates the dual processes of vertical and lateral transport, coupled with accumulation of PPCPs in the soil environment. Grounded in the distinctive properties of green space soils, we delve into the determinants governing the vertical and lateral migration of PPCPs. Furthermore, we consolidate existing simulation methodologies for contaminant transport, aiming to establish a comprehensive model that accurately predicts PPCPs accumulation in green space soils. This insight is critical for deducing the emission threshold of reclaimed water necessary for the protection of green space soils, informing the formulation of rational irrigation strategies, and anticipating future environmental risks. It provides a critical theoretical basis for more informed decision-making in the realm of urban water reuse and pollution control.

Root Influences Rhizosphere Hydraulic Properties through Soil Organic Carbon and Microbial Activity

Processes of water retention and movement and the hydraulic conductivity are altered in the rhizosphere. The aim of this study was to investigate the physical-hydric properties of soil aggregates in the rhizosphere of annual ryegrass (Lolium multiflorum) cropped in a Kandiudalfic Eutrudox, taking into account aspects related to soil aggregate stability. Soil aggregates from rhizosphere soil (RZS) and soil between plant rows (SBP) were used to determine soil water retention curves (SWRCs) and saturated hydraulic conductivity (Ksat). In addition, properties related to soil aggregate stability, such as water-dispersible clay, soil organic carbon (SOC), and microbial activity, were also assessed. The higher microbial activity observed in the RZS was facilitated by increased SOC and microbial activity, resulting in improved soil aggregation (less water-dispersible clay). For nearly all measured matric potentials, RZS had a higher water content than SBP. This was attributed to the stability of aggregates, increase in SOC content, and the root exudates, which improved soil water retention. The increase in total porosity in RZS was associated with improved soil aggregation, which prevents deterioration of the soil pore space and results in higher Ksat and hydraulic conductivity as a function of the effective relative saturation in RZS compared to SBP.

Effects of train vibration load on the structure and hydraulic properties of soils

Investigating the impact of train-induced vibration loads on soil hydraulic properties, this study conducted experiments using a self-designed indoor soil seepage platform that incorporates vibration loads. The experiments were complemented with scanning electron microscopy to analyze the influence of train-induced vibration loads on soil hydraulic conductivity and its evolutionary characteristics under different vibration frequencies. The experimental results indicated that as the vibration frequency increases from no vibration (0 Hz) to 20 Hz, the time required for the soil volumetric moisture content to reach its peak and stabilize decreases rapidly. However, after the vibration frequency exceeds 20 Hz, the rate at which the time required for the volumetric moisture content to reach its peak and stabilize decreases slows down. Furthermore, the soil pore water pressure increases with the increase in vibration frequency. At a vibration frequency of 80 Hz, the peak value of pore water pressure increases by 105% compared to the non-vibration state, suggesting that higher vibration frequencies promote the development and acceleration of soil pore moisture migration. Additionally, as the vibration frequency increases, the soil hydraulic conductivity initially experiences a rapid increase, with a growth rate ranging from 40.1 to 47.4%. However, after the frequency exceeds 20 Hz, this growth rate significantly decreases, settling to only 18.6% to 7.8%. When the soil was subjected to a vibration load, the scanning electron microscopy test revealed alterations in its pore structure. Micropores and small pores transformed into macropores and mesopores. Additionally, the microstructural parameters indicated that vibration load decreased the complexity of soil pores, thereby speeding up the hydraulic conduction process. This, in turn, affected the hydraulic properties of the soil and established a relationship between pore structure complexity and soil hydraulic properties.

Interactive Effects of Microbial Fertilizer and Soil Salinity on the Hydraulic Properties of Salt-Affected Soil

Significant research has been conducted on the effects of fertilizers or agents on the sustainable development of agriculture in salinization areas. By contrast, limited consideration has been given to the interactive effects of microbial fertilizer (MF) and salinity on hydraulic properties in secondary salinization soil (SS) and coastal saline soil (CS). An incubation experiment was conducted to investigate the effects of saline soil types, salinity levels (non-saline, low-salinity, and high-salinity soils), and MF amounts (32.89 g kg-1 and 0 g kg-1) on soil hydraulic properties. Applied MF improved soil water holding capacity in each saline soil compared with that in CK, and SS was higher than CS. Applied MF increased saturated moisture, field capacity, capillary fracture moisture, the wilting coefficient, and the hygroscopic coefficient by 0.02-18.91% in SS, while it was increased by 11.62-181.88% in CS. It increased soil water supply capacity in SS (except for high-salinity soil) and CS by 0.02-14.53% and 0.04-2.34%, respectively, compared with that in CK. Soil available, readily available, and unavailable water were positively correlated with MF, while soil gravity and readily available and unavailable water were positively correlated with salinity in SS. Therefore, a potential fertilization program with MF should be developed to increase hydraulic properties or mitigate the adverse effects of salinity on plants in similar SS or CS areas.