Water Retention and Pore Size Distribution of a Biopolymeric‐Amended Loam Soil

Abstract

Core Ideas Transversal relaxation time spectra were a good proxy to determine pore size distribution. Chitosan provides more degradable C to the soil microbial community than Arabic gum. The water retention curve showed that 10 g Arabic gum kg−1 soil decreased plant‐available water. The same amount of chitosan increased available water content compared with the reference soil. The pore size distribution (PSD) of biopolymeric‐amended soils is rarely investigated due to difficulties in its quantification using classical methods. In this study, we analyzed the impact of biopolymeric soil amendments on the PSD of a dryland loamy soil based on its physical and biological properties using a completely randomized design with four treatments consisting of two different dosages (10 and 5 g kg−1) of two different biopolymers, (chitosan [CH] and Arabic gum [AG]) plus a reference soil. To determine the effects of CH and AG on the PSD, nuclear magnetic resonance relaxometry (NMRR) measurements were used to determine the longitudinal (T1) and transversal (T2) relaxation times. A set of soil structure–related characteristics was also determined in the laboratory. The results revealed that T2 spectra provided a good proxy to determine the PSD, showing good agreement between the PSD from T2 spectra and that calculated from the water retention curve (WRC) (R2 > 0.78; RMSE <1.38 μm). The application of CH also increased the zeta potential of the soil to −18.5 mV, compared with −20 mV obtained for the reference soil. The WRC measurements revealed that AG decreased the available water content for plant use compared with the reference soil, whereas CH increased the available water in comparison to the reference soil. Considering the parameters of the van Genuchten model, the application of AG and CH mainly affected the parameter α, confirming the dominant changes in macropores. This finding was confirmed by NMRR relaxation spectra. Furthermore, the application of CH and AG stimulated the microbial activity of the amended soil, leading to an increase in soil respiration.