Effects of poly-γ-glutamic acid on water use efficiency, cotton yield, and fiber quality in the sandy soil of southern Xinjiang, China

Poly-γ-glutamic acid (γ-PGA) is a promising, environmentally friendly, non-toxic, biodegradable, and super water-absorbable polymer. Using γ-PGA in agriculture is of great significance to promote crop growth, save irrigation water, and cope with water scarcity problems. The objectives of this study were to investigate the effects of application of different γ-PGA amounts on soil water storage, cotton growth, water use efficiency (WUE), cotton yield, and fiber quality under plastic mulched drip irrigation conditions. A 2-year potted experiment in a cotton field were conducted in the sandy soil of southern Xinjiang, China. Five different amounts of γ-PGA consisted of 0 kg ha−1 (P0), 20 kg ha−1 (P20), 40 kg ha−1 (P40), 80 kg ha−1 (P80), and 160 kg ha−1 (P160) were applied to these potted plots. All treatments were arranged in a randomized block designed with three replicates. The result showed that using γ-PGA significantly increased soil water storage and soil profile water content in the 0–40 cm soil layers compared to P0. γ-PGA significantly increased cotton stem diameter, boll numbers per plant, plant height, and leaf area index compared to the P0. Furthermore, cotton yield was significantly increased as well as WUE and fiber length; however, no influence was found on the fiber quality of cotton. Quadratic relationships were found between the different amount of γ-PGA applied and WUE and between the different applied amounts of γ-PGA and seed cotton yield. High WUE (7.48–8.19 kg ha−1 mm−1) and cotton yield (4376–4682.5 kg ha−1) were obtained under the P80 treatment. The study suggested that γ-PGA improved soil water-holding capacity, promoted cotton growth, and enhanced WUE and cotton yield. Therefore, these results recommend using γ-PGA at a dose of 80 kg ha−1 for dealing with drought conditions in the sandy soil of southern Xinjiang, China.