Analysis of the effects of water erosion on soil properties and microbial structure in the Sarsaro Watershed, Khash County

Introduction and Goal
Water erosion, as one of the most important factors of soil degradation, has a significant impact on soil quality and performance in various ecosystems. The Sarsaro watershed in Khash County is considered a vulnerable area to water erosion due to specific climatic conditions and human interventions such as over grazing and loss of vegetation cover. Despite the importance of the issue, comprehensive studies that simultaneously show the physical, chemical, and biological properties of the soil in this region have not been conducted. Therefore, this study aimed to evaluate the impact of water erosion on soil changes and microbial structure in the Sarsaro watershed in order to provide strategies for sustainable natural resource management through better understanding of erosion effects.
Materials and Methods
In this study, soil sampling was carried out in a completely randomized design. First, erosion and sediment map available in the region were used to classify soil erosion intensities, including: no erosion, low erosion, moderate erosion, and severe erosion. Then, in each erosion region, four homogeneous areas with almost similar physiographic conditions were selected, and in each of them, five soil samples were taken (one sample in the center and four samples in the form of a plus sign around it). Soil sampling was carried out from a depth of 0 to 30 cm. For each erosion site, samples taken from homogeneous areas were mixed to prepare a composite sample. Immediately after collection, the soil samples were divided into two parts. Some of the samples intended for measuring biological properties were transported to the laboratory in sealed containers in the vicinity of dry ice without sieving and while maintain the initial humidity conditions, and were stored in the refrigerator until the experiments were performed. The other part of the samples was used to measure physical and chemical properties after drying in the open air and passing through a 2 mm sieve. One-way analysis of variance (ANOVA) of soil properties data was performed using SPSS software. Comparison of means was performed using Duncan's test with a 95% confidence level. Also, the correlation analysis between the studied investigated measures was performed in the R software.
Results and Discussion
The results of the analysis of variance showed that intensity of erosion effects on soil physical properties was significant (p < 0.01). The sand size increased significantly from 25% in non-eroded soils. The correlation of silt and clay with erosion intensity was negative (-0.96) and (-0.95), respectively, and the amount of both decreased. Soil moisture in non-eroded lands was 12.5%, which decreased to 6.09% in severely erosion (p < 0.01). Also, porosity decreased (negative correlation -0.97) and apparent specific gravity increased (positive correlation). Soil chemical parameters such as organic carbon from 2.23% to 0.93%, total nitrogen from 0.31% to 0.08%, absorbable potassium and absorbable phosphorus decreased significantly (p < 0.01). Electrical conductivity increased significantly (positive correlation of 0.92). Changes in pH were not significant (p > 0.05). Soil biological properties were severely reduced: catalase enzyme activity decreased from 28.05 units in non-eroded soils to 3.57 units in severely eroded soils (p < 0.01). Microbial biomass carbon decreased from 223.6 mg to 93.3 mg, microbial biomass nitrogen from 31.6 mg to 7.8 mg, and the population of microorganisms and basal microbial respiration also showed significant decreases (p < 0.01). The soil microbial contribution also decreased and its correlation with erosion intensity was high and negative (greater than -0.96). These data indicate the severe and negative effects of water erosion in the physical, chemical, and biological quality of the soil, and the reduction in the health and productive capacity of ecosystem.
Conclusion and Suggestions 
The results of this study demonstrated that the effect of water erosion on the physical, chemical, and biological quality of soils in the Sarsaru watershed were severe and negative. Erosion, through the removal of surface layers, reduction of organic matter, degradation of soil structure, depletion of essential nutrients, and weakening microbial communities, jeopardized the health and function of soil ecosystems and reduces their productive capacity. Therefore, to prevent further soil destruction and reduction in its biological function, controlling erosion and comprehensive management of soil resources are important priorities in vulnerable areas such as the Sarsaru watershed. Based on the results of this study, it is suggested that biological indicators be considered as effective tools in monitoring the health of eroded soils to provide a suitable basis for management decisions.