Relationship between soil properties and gum tragacanth production in Astragalus gossypinus Fischer habitats under animal grazing

Background and objectives: Continuous grazing and treading of animal in non-forage plant species habitats e.g. Astragalus gossypinus and Ferula assa-foetida can considerably affect plant growth and non-forage products (by-products) such as gum tragacanth yield through alteration of soil characteristics. Most studies in rangelands have addressed the relationship between different soil characteristics and forage production, and there is little information on the relationship between changes in soil properties and the amount of non-forage products (such as gum, essential oil, tangerine, assa-foetida, etc.) in rangelands. Knowing the effect of grazing on soil properties in these habitats and the relationship between soil changes and the amount of by-products is important in determining the optimal grazing intensity for soil and plant conservation and the production of maximum products. This study aimed to investigate the relationship between soil physico-chemical characteristics and gum tragacanth production in Astragalus gossypinus habitats.
Materials and methods: This study was carried out in A. gossypinus habitats at the rangelands of Golpayegan, Isfahan Province, Iran. Treatments included lenient, moderate and heavy grazing intensities in A. gossypinus habitats. Soil and plant (gum tragacanth) sampling were carried out on transect with 100 m length and 100m space from each other in summer 2013. The quantity of gum tragacanth was measured on five randomly selected individual plants on transect lines. Soil physico-chemical variables, i.e., bulk density, porosity, infiltration rate, pH, EC, OC, available P and N were measured with standard methods. Relationship between soil variables and gum tragacanth production was investigated with the Pearson correlation coefficient.
Results: The highest and lowest amount of gum production was observed in habitat with light and heavy grazing, respectively. Soil infiltration rate in moderate and lenient grazing intensities was 40.15 and 32.17 percent, respectively, lower than that in the habitats under heavy grazing intensity (P < 0.05). Mean value of soil EC in habitat with light grazing was lower than that in the habitats with moderate and heavy grazing intensities (P < 0.05). Mean value of soil organic carbon in moderately grazed habitat was 38 percent greater than that in lightly and heavily grazed habitats (P < 0.05). The highest and the lowest values of P were in heavily and moderately grazed habitats, respectively. Available N in moderately grazed habitat was greater that in lightly and heavily grazed habitats (P < 0.05). In different grazing intensities, there was a negative correlation between soil bulk density and gum production, but there was a positive correlation between soil porosity and infiltration rate with gum production (P > 0.05). All soil chemical variables, except EC, had a direct relation to gum production. Among soil chemical variables, the strongest positive correlation was found between the amount of organic carbon and soil available nitrogen with gum production in moderate grazing (P < 0.05) and the lowest correlation between the amount of available phosphorus and gum production in the light grazing (P > 0.05). In moderately grazed habitats, soil chemical properties had the strongest correlation with the gum production in comparison with light and heavy grazing intensities. In moderately grazed habitats, the amount of soil organic carbon and available nitrogen were significantly and positively correlated with gum production (r = 0.74; P < 0.05).
Conclusion: The correlation between soil chemical properties and gum production was stronger than the correlation between soil physical properties and gum production in A. gossypinus habitats. This study reports that if the livestock grazing is inevitable in A. gossypinus habitats, the stocking rate should not be higher than moderate intensity to obtain more gum as well as maintain soil and plant health.