Soil macropore characteristics and aggregate stability with poly--glutamic acid amendment under wetting-drying cycles

Soils are typically subjected to multiple wetting-drying (WD) cycles due to irrigation and seasonal climate cycles, which directly impact soil pore structure and soil aggregate stability. Poly-gamma-glutamic acid (gamma-PGA) is a polymer used to improve soil water holding capacity and plant growth. However, the impact of gamma-PGA on soil pore structure requires further research, particularly under WD cycles. Therefore, we investigated the different amounts of gamma-PGA on soil structure, including soil aggregate stability, macropore (>100 mu m) structure characteristics and the relationship between macropore characteristics (equivalent pore diameter, pore shape factor, soil porosity, fractal dimension (FD), soil connectivity and the percentage of aggregate content with particle size larger than 0.25 mm) and soil aggregate stability by structural equation modelling (SEM) under WD cycles. A sandy soil and a loam soil were studied, and amended with gamma-PGA at three different concentrations: 0 (P0), 0.4% (P4) and 0.8% (P8) (w/w, %). Results showed that gamma-PGA amendment increased the mean weight diameter (MWD) and the percentage of aggregate content with particle size larger than 0.25 mm (R-0.25) under WD cycles in both sandy and loam soils, while the MWD between P4 and P8 did not differ significantly. As the number of WD cycles increased, soil porosity (TP) increased due to an increase in pores of 100-500 mu m. With gamma-PGA added to soil, large microporosity (>1000 mu m) increased in sandy soil, but decreased in loam soil. In addition, 8WD cycles also increased the FD (2.6%-4.2%) and pore connectivity (Con) compared with 4WD. Structural equation modelling (SEM) revealed that soil pore characteristics accounted for 74% and 98% of the variation in sandy and loam soils, respectively. TP, FD, Con and R-0.25 directly contributed to MWD, according to the SEM. These findings improve our understanding of pore characteristics and aggregate stability, which are key factors influencing soil quality when amended with gamma-PGA during the seasonal WD period.