Comparative assessment of soil aggregate stability in treated and untreated watersheds

Introduction

Soil aggregate stability measures the resistance of soil structure to mechanical or physicochemical forces and strongly influences soil erodibility, water retention, nutrient availability, and root growth (Hashimi et al., 2019) [6] . Higher aggregate stability reduces soil loss, improves macro-aggregate formation, enhances porosity, and limits the loss of carbon, nitrogen, and phosphorus, thereby supporting overall soil fertility and plant growth (Dorji et al., 2020) [3] . Bless (2022) [1] reported that forage cropping increased macro-aggregates (>1 mm) and organic carbon, enhancing aggregate stability compared to fallow land. Soil erosion disrupts aggregate formation by removing fine particles and cementing agents. Soil aggregate stability is a key determinant of soil erodibility, influencing dispersion, surface sealing, and degradation (Fulpagare et al., 2023) [4] . Measuring aggregate stability offers a costeffective alternative to field erosion studies for assessing soil vulnerability. Aggregate stability also plays a crucial role in protecting soil organic carbon and linking it to erosion processes across landscapes (Kalhoro et al., 2017) [7] . Soil aggregation and carbon storage are difficult to measure due to spatial and temporal heterogeneity (Han et al., 2010) [5] . Water-stable aggregate tests provide a simple, rapid way to assess the effects of land management on aggregation and carbon (Mbanjwa et al., 2022) [14] . Aggregate stability improves with higher organic carbon, especially in clay-rich soils, enhancing structure and resistance to water erosion (Mohanty et al., 2012) [16] . Several studies have used ASTER, SRTM and Cartosat DEM with 30 to 90 m resolution in predicting soil parameters for small and large area (Liu et al., 2019) [12] . Soil aggregates are formed by physical, chemical, and biological processes and are held together by different binding materials (Panet et al., 2015) [18] . However, they can break down due to natural forces like rainfall and erosion, and human activities such as tillage and machine traffic (Lawal et al., 2019) [9] . When aggregates break, the organic matter stored inside them is exposed and decomposes. Macroaggregates go through cycles of formation and breakdown depending on biological activity and the addition of fresh plant residues (Kumar et al., 2023) [8] . In monsoon regions, water stability of aggregates is very important because heavy rainfall can cause slaking and rapid breakdown (Han et al., 2010) [5] . Water-stable aggregates are considered an important indicator of soil quality, as they are closely related to erosion control, carbon storage, and nutrient movement (Mandal et al., 2017) [13] . Soil and water conservation measures help improve soil aggregate stability (Lee et al., 2012) [10] . Practices such as contour farming, terracing, mulching, cover crops, reduced tillage, and vegetative barriers reduce soil disturbance and protect the soil surface from heavy rainfall. These practices increase organic matter and root growth, which help bind soil particles together (Mesfin et al., 2018) [15] . Less disturbance also allows aggregates to remain stable for longer periods. Improved aggregate stability leads to better soil structure, increased water infiltration, reduced erosion, and improved soil fertility (Singh et al., 2019) [19] . A comparative assessment of soil aggregate stability in treated and untreated watersheds is necessary to understand the impact of conservation measures. Treated watersheds are expected to have better soil structure and more water-stable aggregates than untreated areas (Nandgude et al., 2015) [17] . Such a study helps measure the benefits of conservation practices and provides useful information for improving watershed management. This is especially important in monsoon regions where soils are highly vulnerable to erosion. The present study was aimed to investigate the soil aggregate stability in different land use/ land cover as well as to assess the soil aggregate stability in relation to soil properties and terrain attributes of the watershed.

Materials and Methods

Study area

The study area includes the Hiware Bazar and nearby Daithane Gunjal watersheds located in Ahilyanagar taluka, Maharashtra, India. The Hiware Bazar watershed covers an area of 1287.60 ha and is situated in a drought-prone rain-shadow region of the Western Ghats. It has undulating terrain with elevations ranging from 600 to 720 m above mean sea level (MSL), slopes between 3 and 25 percent, and receives less than 400 mm of annual rainfall. The Daithane Gunjal watershed covers 116.63 ha and has similar climatic and topographical conditions but remains untreated, with no soil and water conservation measures implemented.

Soil sampling

Soil samples from the watershed were collected from different land cover patterns. A total 122 soil samples were collected in the treated and untreated areas from the watershed, and their location was recorded using a GPS device. Soil samples were collected using soil auger at two different depths, one at 0-15 cm depth and another at 15-30 cm depth.

Land use and Land cover mapping of the Watershed

In this study, a land use land cover (LULC) map was created using cloud-free Sentinel-2A satellite imagery downloaded from the Copernicus portal. A supervised image classification method, employing the maximum likelihood classifier, was used to categorize the imagery into seven land cover classes. The image processing was performed using ArcGIS 10.8.2 software.

Soil Aggregate Stability analysis

In the present study, soil aggregate stability was assessed using the wet sieving method with Yoder's (1936) [20] apparatus, and results were expressed as Mean Weight Diameter (MWD), calculated from the proportion of soil retained on successive sieves. MWD was determined for all soil samples collected from the Hiware Bazar and Daithane Gunjal watersheds. Soil aggregate stability was determined using the wet sieving method with a Yoder apparatus (Yoder, 1936) [20] . Air-dried and sieved soil samples were placed on nested sieves with mesh sizes of 5 mm, 3 mm, 1 mm, 0.5 mm, 0.25 mm, 0.05 mm, and 0.01 mm, and immersed in water. The soil was mechanically agitated to separate aggregates by size, after which aggregates retained on each sieve were collected, dried, weighed, and sand particles removed. The mean weight diameter (MWD) was calculated by multiplying the weight of aggregates on each sieve by their mean diameter, summing across all sieves, and dividing by the total soil weight. MWD represents the average size of soil aggregates and serves as an indicator of soil structural stability, influencing water infiltration, erosion resistance, and response to management practices (Zhou et al., 2020; Li, 2023; Chaudhari et al, 2013) [21,11,2] .

Results and Discussion

Land use land cover (LULC) mapping of the watersheds

The LULC analysis indicates that the Hiware Bazar watershed is characterized by a structurally balanced and intensively managed land-use system, dominated by current fallow (45.36%) and cropland (20.14%), reflecting regulated agricultural practices and effective soil-water management. Substantial forest cover (19.75%) and minimal barren land (11.18%) further signify successful watershed interventions, ecological stabilization, and higher land productivity. In contrast, the Daithane Gunjal watershed is overwhelmingly dominated by barren land (62.10%), with limited agricultural area (crop land + fallow ≈ 32.25%) and sparse forest cover, indicating degraded land conditions, low cultivation intensity, and suboptimal resource utilization. Hiware Bazar demonstrates a mature, integrated watershed management framework, whereas Daithane Gunjal exhibits pronounced land degradation but considerable potential for rehabilitation through afforestation and soil conservation measures. The figure 4

Aggregate stability of soil Aggregate stability in various Land use/land cover

The Mean Weight Diameter (MWD) of soil aggregates was measured at two depths (0-15 cm and 15-30 cm) under different land use/land cover (LULC) classes in Hiware Bazar and Daithane Gunjal watersheds. MWD values are expressed in millimeters (mm). Higher MWD values indicate better soil aggregation, improved soil structure, and greater resistance to erosion (Table 3). MWD was consistently higher in the Hiware Bazar watershed than in Daithane Gunjal across all LULC classes and at both depths. This shows better soil structural stability in the treated watershed. Forest land recorded the highest MWD values in both watersheds. In Hiware Bazar, forest MWD ranged from 0.79 to 0.67 mm, while in Daithane Gunjal it ranged from 0.42 to 0.34 mm. The higher values under forest indicate wellaggregated and stable soils due to greater organic matter and minimum disturbance. Cropland in Hiware Bazar showed clear improvement in MWD (0.84-0.71 mm) compared to Daithane Gunjal (0.55-0.39 mm). This suggests that watershed treatments and improved land management practices enhanced soil aggregation. Current fallow land also had higher MWD in Hiware Bazar (0.50-0.42 mm) than in Daithane Gunjal (0.35-0.30 mm), indicating better soil condition in the treated area. Barren land recorded the lowest MWD values in both watersheds, with particularly low values in Daithane Gunjal. This reflects poor soil structure and higher degradation due to the absence of vegetation cover and conservation measures. The results show that watershed interventions in Hiware Bazar significantly improved soil aggregation and structural stability across different land uses and soil depths.

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