Effects of γ-polyglutamic acid on grassland sandy soil properties and plant functional traits exposed to drought stress

Abstract

The current study provides field experimental data that support the use of γ-polyglutamic acid (γ-PGA) in drought stress and proposes its application in grassland management. We hypothesized that water treatment combined with PGA application to sandy soil would reduce drought stress in grasslands more effectively than watering alone. A randomized block design was used, with three replicate watering blocks (no watering, weekly watering, and monthly watering) and PGA treatments at four different concentrations (0%, 0.3%, 1%, and 2% PGA). The results showed that PGA acts as a biostimulant, alleviating the effects of stress in plants by: (1) increasing the availability of ions, especially K⁺, Zn²⁺, Mn²⁺, Fe^2+/3+, Ca²⁺, and Mg²⁺, as well as N-NH₄⁺, and N-NO₃^-, (2) elongating plant roots, (3) increasing the aboveground biomass, (4) improving the resprouting capacity of the dominant grass Nardus stricta, and (5) improving the regeneration of dicotyledons. In the case of meadows on sandy soils, the use of low PGA concentrations (0.3% or 1%) was the most beneficial for the availability of macro- and microelements and improving the functional traits of plants. Irrigation had a greater effect than using PGA only for the dicotyledon to monocotyledon ratio.

Figure 1

Mean daily temperature (orange line) and total daily rainfall (mm) during the period 1 June–19 August 2019 in 10-day sequences classified according to Selyaninov's Hydrothermal Coefficient (IMGW: https://danepubliczne.imgw.pl/data/dane_pomiarowo_obserwacyjne/: access August 2020).

Figure 2

Study site design and spatial arrangement of 24 watering blocks and four PGA treatments in each block. Description of abbreviations in the text.

Figure 3

Mean comparison of the different concentrations of (a) available phosphorus (P), (b) ammonium nitrogen (N-NH4⁺) and (c) nitrate nitrogen (N-NO₃⁻) compounds with respect to PGA treatments by Duncan tests (means with the same letter did not show significant differences).

Figure 4

Mean comparison of the different concentrations of metal compounds with respect to PGA treatments by Duncan tests (means with the same letter did not show significant differences).

Figure 5

Mean soil pH (calculated on the basis of hydrogen ion concentration in mmol dm⁻³) (a) and electrical conductivity (EC) (b) with multiple comparison Duncan test (means with the same letter did not show significant differences).

Figure 6

Mean and 95% of confidence interval values of grassland life traits: (a) Nardus stricta root length and (b) aboveground biomass; (c) maximum shoot height of dicotyledons; (d) DFR index; (e) DMR index per sample unit. Blue lines—reference watering (RW), green lines—monthly watering (MW), red lines—weekly watering (WW).

Figure 7

The ordination diagram for the first two axes for redundancy analysis of treatments/experimental variables (arrows), soil properties (dots), and plant life traits (squares).