Analytical comparison of soil particle size distribution in different land uses/covers of the Vaz watershed using laser granulometry

Introduction

The distribution of soil particle sizes is a key physical characteristic that affects soil structure and is closely related to soil texture and behavior. This feature enhances the exchange of matter and energy in the soil environment. The distribution of soil particle sizes not only influences soil permeability and organic matter content but also affects soil fertility, erosion susceptibility, soil conservation, moisture and nutrient movement, vegetation productivity, environmental remediation, and land degradation. On the other hand, human interventions and the improper use of soil based on its inherent potential have led to changes in various land uses, resulting in floods, erosion, sediment yield, and soil transformation. Therefore, understanding the distribution of soil particle sizes and determining the sensitivity of a region's soil to erosion can help illustrate the pattern of erosion distribution and ultimately assist managers in making decisions regarding watershed management actions. Thus, in this study, we analyze soil particle size distribution in various land uses/covers in the Vaz watershed in Mazandaran province using laser particle size analysis and statistical indices.

Materials and Methods

In the present study, the land use map of the watershed was prepared using Landsat satellite images in remote sensing software (ENVI). Then, using this map, the area of land uses and access routes to the region were determined, resulting in 47 sampling points, including 25 points for forest, 13 points for rangeland, 4 points for agriculture, and 5 points for residential areas. Samples were then collected in the field. The samples were exposed to open air to dry their moisture; subsequently, approximately 50 grams of each sample were placed in an oven for final drying after removing straw, chaff, and other waste materials for 24 hours. Finally, 30 grams of each were placed in Falcon tubes and sent to the laboratory for laser analysis. After receiving the laser particle size distribution results, these results were examined in terms of clay, silt, sand, pebble, mean particle size, sorting, skewness, kurtosis, 10d, 50d (median), and 90d. To extract particle size statistics and percentages of particle fractions, the GRADISTAT macro written in Excel was used.

Results and Discussion

The results showed that the average particle size of clay, silt, sand, and pebble in forest land use was 1.03, 51.52, 45.39, and 2.06, respectively; in rangeland use, it was 0.62, 52.41, 45.21, and 1.76; in agricultural land use, it was 1.33, 75.2, 23.39, and 0.08; and in residential land use, it was 0.57, 36.96, 54.5, and 7.97. The particle size distribution in forest and rangeland uses indicates that silt and sand fractions have the highest percentages, reflecting a balanced soil structure. This may be due to the influence of vegetation and the accumulation of organic matter. Agricultural land use has a significant percentage of silt (75.2%), indicating a finer texture, likely resulting from agricultural practices such as tillage. Residential land use shows a higher proportion of sand (54.5%) and pebble (7.97%), which is associated with construction activities in this land use. The evaluation of grain size statistics reveals that the mean particle size in residential land use is 2.70 Φ, in forest land use 3.99 Φ, in rangeland use 4.04 Φ, and in agricultural land use 5.37 Φ, corresponding to fine sand, very fine sand, silt, and silt textures, respectively. The presence of sand and pebble mines in the area and construction activities are the main reasons for the coarser mean particle size in residential land use. The sorting (standard deviation) of soil particles in agricultural land use is 1.78 Φ, in residential land use 2.21 Φ, in forest land use 2.21 Φ, and in rangeland use 2.24 Φ. According to Folk's classification, residential, forest, and rangeland uses have very poor sorting, while agricultural land use has poor sorting. The skewness of particles in agricultural land use is -0.41, in rangeland use -0.07, in forest land use -0.03, and in residential land use 0.23. Based on Folk's classification, skewness is nearly symmetrical in forest and rangeland uses, skewed toward very coarse particles in agricultural land use, and skewed toward coarse particles in residential land use.

Conclusion

In general, analyzing soil particle size is crucial for identifying erosion-sensitive areas. In this study, we conducted soil particle size analysis in the Vaz watershed using sampling from various land uses/covers and laser technology. The predominance of silt and sand in the watershed indicates its sensitivity to erosion. Given that the Vaz watershed has a steep slope and considering the construction activities throughout the watershed, as well as the low presence of stable particles (coarse particle size), it is recommended that some watershed management actions be implemented in the upper part of the watershed. Also, the results obtained and the distribution the map of soil particles can assist in prioritizing soil conservation measures and controlling erosion in the Vaz watershed.

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