The pore size distribution (PSD) of biopolymeric-amended soils is rarely ever investigated due to difficulties in its quantification using classical methods. In this study we analyzed the impact of biopolymeric soil amendments on PSD of a dryland loamy soil being underpinned by 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, namely chitosan (CH) and Arabic gum (AG) plus a reference soil. To determine the effects of CH and AG on the pore size distribution (PSD), nuclear magnetic resonance relaxation (NMRR) measurements were used to determine the longitudinal (T1) and transversal (T2) relaxation times. Additionally, a set of soil structure-related characteristics were also determined in the laboratory. The results revealed that T2 relaxation time spectra provided a good proxy to determine PSD showing good agreement between the PSD from T2 spectra and that calculated from water retention (WRC) with R2 values higher than 0.78 and RMSE values lower than 1.38 μm. The application of CH also increased the zeta (ζ) potential of soil to -18.5 mV compared to -20 mV obtained for the reference soil. The WRC measurements revealed that AG decreased the available water (AW) content for plant use compared to the reference soil, while CH increased the AW 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 macro-pores. This finding was also 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.
