Poly-γ-glutamic acid application rate effects on hydraulic properties of sandy soil under wetting-drying cycles conditions

When amending soil to increase sandy soil water holding capacity, amendment application rate and environment need consideration. Recently, poly-gamma-glutamic acid (gamma-PGA) as a soil amendment has been increasingly applied in agricultural development. The purpose of this study was to determine the effect of gamma-PGA on soil water retention and soil hydraulic conductivity under wetting-drying (W-D) conditions. Specifically, this study investigated the influence of different gamma-PGA application rates (wt%: 0, 0.4, 0.8, 1.2, and 1.6%) on the soil water characteristic curve, saturated hydraulic conductivity (K-s), and the other hydraulic characteristic parameters (saturated water content [theta(s)], field capacity [FC], permanent wilting point [PWP], and available water content [AWC]). Results showed that gamma-PGA significantly increased sandy soil water retention by increasing theta(s), FC, PWP, and AWC. Poly-gamma-glutamic acid could decrease the proportion of large pore size (2-15 mu m) and K-s and enhance the proportion of particles with small pore size (0.2-2 mu m). Nevertheless, the effect of gamma-PGA on hydraulic properties of sandy soil depended on gamma-PGA application rate and the number of W-D cycles. Under 8-W-D and 12-W-D, there was no significant difference on soil water retention among all treatments. Increasing the application rate of gamma-PGA was not more effective; P3 (1.2%) and P4 (1.6%) treatments had little difference in soil water holding capacity. This study quantitatively investigated effects of gamma-PGA on soil water retention under different W-D cycles. Our results suggest that 1.2% might be recommended as an appropriate gamma-PGA application amount.