Drought stress is a key environmental factor limiting the function of sandy grassland ecosystems.γ-Polyglutamic acid (γ-PGA), as a natural anionic polymer produced by microbial fermentation, has shown significant potential in alleviating plant drought stress due to its unique chelating properties, water retention and biodegradability. Based on field control experiments carried out in Polish sandy grassland, this article systematically elaborates on the regulatory effects of γ-PGA on sandy soil ion availability, nitrogen transformation, and water retention properties, as well as its promotion mechanism on plant root elongation, aboveground biomass, regermination ability, and dicotyledonous plant regeneration. The appropriate application concentration and principle of action are also discussed.
Introduction
Due to its low water-holding capacity, low cation exchange capacity and drought-prone characteristics, sandy soil poses a serious threat to the sustainability of grassland ecosystems in the context of climate change. In Poland, more than 60% of mineral soils are developed in sandy parent material. Semi-natural grasslands with Nardus stricta as the dominant species are particularly vulnerable to drought. Its degradation will trigger a chain effect of reduced pasture productivity and loss of agricultural landscape biodiversity.γ-Polyglutamic acid (γ-PGA) is a key metabolite produced by microorganisms such as Lactobacillus in the extracellular matrix of biofilms. It is polymerized by D- or L-glutamic acid through amide bonds. It is water-soluble, biodegradable, non-toxic and environmentally friendly. Agricultural application research in recent years has shown that γ-PGA can be used as a soil conditioner and biostimulant, and has multiple functions in increasing crop yields and enhancing stress resistance.
Overview of research methods
Skalski et al. conducted field experiments in the typical Nardus stricta grassland near Staszów, Holy Cross Province, Poland (50°35′N, 21°11′E) from June to August 2019. During the experiment, according to the Selyaninov hydrothermal coefficient (HTC), four ten-day periods were extremely dry (HTC=0.0–0.25), three ten-day periods were dry or extremely dry, and only one ten-day period was wet, constituting a real drought stress condition.
The experiment adopted a randomized block design, with 3 water treatments (no irrigation, weekly irrigation, monthly irrigation) and 4 PGA concentration treatments (0%, 0.3%, 1%, 2%), for a total of 12 treatment combinations. The soil tested was loamy sand (81% sand, 16% silt, 3% clay), which was highly acidic and nutrient-poor, especially lacking in Ca²⁺, Mg²⁺, K⁺, Zn²⁺, Mn²⁺ and Fe²⁺/³⁺.
The regulatory effect of γ-polyglutamic acid on the properties of sandy soil
1.Cation exchange and improved effectiveness of nutritional elements
The chelating properties of PGA enable it to form fully water-soluble salts with alkaline cations such as K⁺, Na⁺, Ca²⁺, Mg²⁺, etc., which is of special value for light soils with inherently low cation exchange capacity. The experimental results showed that the Ca²⁺ and Mg²⁺ contents increased with the increase of PGA concentration, indicating that PGA enhanced the retention ability of these ions in sandy soil.
In terms of phosphorus, the available phosphorus content was significantly higher than the control under lower PGA concentration (0.3% and 1%) treatments. This is different from the findings of Zhang et al. and Xue and Zhang, which observed a decrease in available phosphorus after PGA application and attributed it to microbial immobilization. The microbial biomass (C and N) in sandy soil is lower than that in silt soil or clay, so the activation effect of PGA on phosphorus is more prominent in sandy soil.（1）
2.Regulation of nitrogen conversion and nitrification processes
Mineral nitrogen (N-NH₄⁺ + N-NO₃⁻) content was significantly increased under PGA treatment. What deserves more attention is the regulation of the nitrification process by PGA: under 2% PGA treatment, the NH₄⁺:NO₃⁻ ratio reached 2.95, while the control was only 1.30, indicating that high concentration of PGA more than doubled the nitrification rate. At the lowest concentration (0.3% PGA), the nitrification process is the most intense and the NH₄⁺:NO₃⁻ ratio drops to 0.65.
This finding has important practical significance: applying higher concentrations of PGA to sandy soil can retain more NH₄⁺ ions and reduce NO₃⁻ losses due to leaching. For sandy grasslands with poor nutrient retention capacity, this means a substantial improvement in nitrogen use efficiency.
3.Promotion of plant absorption of potassium
Different from the trends of Ca²⁺, Mg²⁺ and P, soil K⁺ content under PGA treatment was lower than the control, and further decreased as the PGA concentration decreased. This “abnormal” phenomenon just proves that PGA promotes the absorption of potassium by plants – the decrease in potassium content in the soil reflects the increase in the intensity of plant absorption. Potassium plays a key role in plant drought resistance by regulating osmotic pressure and maintaining cell turgor pressure, and its mobility and bioavailability will decrease when soil moisture content decreases. The addition of PGA allows plants to effectively obtain potassium under drought conditions, which has direct physiological significance for alleviating drought stress.
4.Enhancement of trace element absorption
The soil contents of Mn²⁺ and Zn²⁺ under PGA treatment were lower than those in the control, indicating enhanced plant uptake. The absorption of Zn is closely related to the supply of NO₃⁻, and RDA analysis also confirmed this correlation. The availability of Fe is related to the absorption intensity of NO₃⁻ and H⁺ – absorption of NO₃⁻ by the roots causes an increase in rhizosphere pH, which may affect the solubility of iron.
Effects of γ-polyglutamic acid on plant functional traits
1.Root elongation and morphological development
PGA treatment significantly promoted the longitudinal elongation of plant roots. This effect is particularly important under drought stress: deeper roots are able to access water deeper in the soil profile, thereby maintaining plant water supply when the surface soil is dry. Research on corn also confirmed that PGA treatment not only increased the fresh weight of roots, but also significantly alleviated the inhibition of root growth caused by PEG-simulated drought. The decrease in fresh weight of roots in the PGA treatment group (31.84%) was much lower than that in the control group (52.50%).（2）
2.Aboveground biomass accumulation
As a biostimulant, PGA significantly increases the above-ground biomass of plants. In the corn drought experiment, 50 mg/L PGA treatment caused the dry weight after 7 days of drought to reach 0.96 g, while the control was only 0.39 g.（2） The mechanism by which PGA promotes biomass accumulation may involve multiple levels: enhancing nutrient absorption, improving water status, and regulating stomatal behavior and the accumulation of osmotic adjustment substances through the ABA signaling pathway.
3.Improvement of re-germination ability of dominant grass species
As the dominant species in the experimental grassland, Nardus stricta’s regermination ability was significantly improved under PGA treatment. Re-germination ability is a key trait for grassland plants to resume growth after drought, and is directly related to grassland productivity and ecological recovery speed. The improving effect of PGA on this trait provides a feasible management intervention for the rapid recovery of sandy grassland after drought events.
4.Promotion of dicotyledonous plant regeneration
PGA treatment also improved the regeneration capacity of dicotyledonous plants in grassland. It is worth noting that on the specific indicator of the ratio of dicots to monocots, the effect of irrigation is better than that of PGA treatment, indicating that the impact of water supply on community structure cannot be completely replaced in some dimensions. However, for most other indicators, the improvement effect of PGA is equivalent to or even better than that of irrigation, which highlights the practical value of PGA as a water-saving soil amendment.
Suitable application concentration and mechanism of action
1.Nonlinear Characteristics of Concentration Effect
An important finding of the experiment is that low concentrations of PGA (0.3% or 1%) have the best effect on improving macro- and trace element availability and plant functional traits. Although high concentration (2%) is outstanding in nitrogen retention (with stronger nitrification inhibition), it is not optimal in terms of nutrient availability and plant response. This nonlinear dose effect suggests that agricultural applications of PGA require careful concentration optimization.
2.Multiplicity of mechanisms of action
The mechanism by which PGA alleviates drought stress can be summarized into the following levels:
Soil level: Improve cation exchange and retention capabilities through chelation; reduce nitrogen leaching by regulating the nitrification process; improve the moisture status of sandy soil through water retention properties.
Rhizosphere level: As a microbial metabolite, PGA can regulate the rhizosphere microbial community structure, enrich plant growth-promoting bacteria (such as Bacillus, Pseudomonas, Burkholderia), and reduce the relative abundance of pathogenic fungi.
Plant physiological level: PGA promotes the accumulation of ABA and induces stomata to close to reduce water loss; promotes the accumulation of osmotic adjustment substances such as proline and soluble sugar to maintain cell turgor; protects chlorophyll content and maintains the photosynthetic rate under drought conditions.
Molecular level: RNA-seq analysis shows that PGA treatment affects the expression of ABA biosynthesis, signal transduction and photosynthesis-related genes, indicating that its regulatory effect is deep at the transcription level.
Conclusion and outlook
As an environmentally friendly biopolymer, γ-polyglutamic acid has shown multi-faceted positive effects in alleviating drought stress in sandy grasslands: in terms of soil properties, it improves the availability of nutrient elements, regulates nitrogen transformation, and enhances cation retention capabilities; in terms of plant functional traits, it promotes root elongation, increases biomass accumulation, and improves the re-germination ability of dominant grass species and the regeneration of dicotyledonous plants.
Field experimental evidence shows that low-concentration PGA (0.3%–1%) has the best comprehensive effect in sandy grassland management, and its improvement effect is comparable to that of irrigation in most indicators, providing a water-saving biological intervention strategy for grassland management in arid and semi-arid areas. Future research should further focus on the dose effects of PGA under different soil types and climate conditions, the ecological safety of long-term application, and the specific molecular mechanisms of the interaction between PGA and microbial communities.