Comparative assessment of the distribution of soil fertility indictators under different land use covers in Tarikere taluk of Karnataka

Introduction

Soil is a vital component of the earth's terrestrial ecocover and serves as the foundation for agricultural productivity, biodiversity maintenance and ecological stability. It functions as a natural medium for plant growth, regulates the hydrological cycle, recycles essential nutrients and supports countless microorganisms. As the global population continues to rise, pressure on soil resources has intensified, leading to land degradation, loss of soil fertility and reduced ecological services (Lal, 2020) [8] . Land use and management practices have influence on available status of nutrients especially major nutrients. Land use covers play an important role in nutrient availability and transformation. Change in land use affects soil properties, which may alter the availability and forms of nutrients in soil. Soil physical and chemical properties are vital indicators of soil quality as they directly regulate water retention, aeration, nutrient availability and biological activity. In Indian agro-ecocovers, these properties vary considerably with land use and management practices, as forest and plantation soils generally maintain higher soil organic carbon, CEC and nutrient status compared to continuously cultivated lands, where intensive cropping often leads to nutrient depletion and soil degradation (Bhattacharyya et al., 2015) [3] The southern transition zone (STZ) of Karnataka represents a geographically and ecologically significant region within the state's agro-climatic classification. Located between the humid Western Ghats and the arid interior plateau, the STZ is characterized by varied soil types including red loam and black clay, moderate rainfall and diverse cropping patterns. The heterogeneity of land uses ranging from forests and plantation crops to annual agricultural fields makes it an ideal setting to study how land use changes affect soil properties. However, this transition zone is ecologically sensitive and land use intensification may pose risks to its soil health and long-term sustainability (Reddy et al., 2015) [17] . Tarikere taluk, located in the Chikkamagaluru district of Karnataka, forms an important part of the southern transition zone (STZ). The region, with red loamy and clay soils and annual rainfall of 950-1200 mm, supports diverse land uses including paddy, arecanut, maize, natural forest and barren land, where such heterogeneity provides an ideal setting to study the impact of land use changes on soil properties and long-term agro-ecocover sustainability.

Materials and Methods

The selected study area lies in the 13°48′58″N and 13.816244°N latitude and 76°02′00″E and 76.033287°E longitude of southern transition zone of Karnataka, specifically within Tarikere taluk of Chikkamagaluru district as shown in fig. 1. This region is characterized by a diverse of land use covers encompassing natural forests, plantations and agricultural fields, reflecting varied management practices.

Collection of soil samples

The soil samples were collected from different land use cover viz., natural forest, arecanut, paddy, maize and barren land. Twenty (20) surface soil samples (0-20 cm) collected from each land use which makes to 100 surface samples across five land use cover as shown in fig. 2. The soil samples were brought to the laboratory, dried under shade, powdered using wooden pestle and mortar, passed through 2 mm sieve and preserved for analysis of physical and chemical parameters.

Preparation of soil samples

Particle size distribution of soil samples was determined by International pipette method as described by Piper (1966) [14] . Soil pH was determined in 1:2.5 soil to water suspension by dipping the combined electrode (glass electrode plus calomel electrode) of digital pH meter as outlined by Jackson (1973). The electrical conductivity (EC) of soils was measured in 1:2 soil to water extract using conductivity bridge (Jackson, 1973). The results of EC were expressed in terms of dS m -1 at 25 °C. Soil organic carbon in soil samples was determined by treating a known weight (0.50 g) of 0.2 mm sieved soil with known and excess quantity of chromic acid (Sulphuric acid and potassium dichromate) to oxidize the organic carbon present in the soil to carbon dioxide. After oxidation, the untreated potassium dichromate left in the contents was back titrated against standard ferrous ammonium sulphate using diphenylamine indicator (Walkely and Black, 1934) [23] . The cation exchange capacity (CEC) of soils was estimated by taking a known quantity of soil which was leached several times with neutral normal ammonium acetate solution and washed with alcohol to remove the excess electrolyte. Potassium ions displaced the adsorbed NH4 + ion by leaching the soil with a 10 per cent KCl solution. Finally, the NH4 + ion present in the leachate was determined by the distillation method as described by Page et al. (1982) [13] . Soil microbial biomass carbon was measured by the chloroform fumigation extraction method (Vance et al., 1987) [22] . 10 g of fresh soil samples (maintained at 4°C) was fumigated for 24 hours under vacuum in vacuum desiccators using ethanol free chloroform. After fumigation chloroform fumes were removed by excavation. Unfumigated soils (10 g) and fumigated soil were extracted using 50 ml 0.5 M K2SO4 and the extract from unfumigated soils (10 g) and fumigated soil were subjected to wet oxidation separately with potassium persulphate and H2SO4 by heating the content on a digestion block at 120°C for 30 minutes. The solution was allowed to cool and 20-25 ml distilled water was added, 2-3 drops of ferroin indicator solution and then titrated with ferrous ammonium sulphate solution. Available nitrogen in soil was determined by the alkaline potassium permanganate method as described by Subbaiah and Asija (1956) [19] . Available phosphorus in soil samples was extracted by Brays method for soils having pH less than 6.5 (0.03 N NH4F + 0.025 N HCl) and Olsen's method for soils having pH more than 6.5 (0.5 M NaHCO3) as described by Jackson (1973) [5] . The extract was treated with chloromolybdic acid and stannous chloride and the intensity of colour developed was measured by spectrophotometer at 660 nm. Available potassium was extracted from the soils using neutral N ammonium acetate and the concentration of potassium present in the extract was determined by the flame photometric method (Jackson, 1973) [5] . DTPA extractable Fe, Mn, Cu and Zn of soil were extracted with DTPA extractant (0.005 M Diethylene Triamine Penta Acetic acid and 0.01 M CaCl2 + 0.1 N Tri Ethanol Amine with pH 7.3) and concentrations of Zn, Fe, Mn and Cu in the extract was measured by using Atomic Absorption Spectrophotometer (Lindsay and Norvell, 1978) [9] .

Result and Discussion Soil Physical and Chemical Properties of Soil Under Different Land Use Covers Particle Size Distribution

Data depicted in Table 1 indicates the proportions of sand, silt and clay in soils under different land use covers in Tarikere taluk of Chikkamagaluru district. Across the different land use covers, the soils exhibited distinct variations in their textural classes. Soils under natural forest were classified from loam to clay loam, while those under arecanut ranged from sandy loam to clay loam. Similarly, soils under paddy were grouped between sandy loam and clay loam. In contrast, maize and barren land soils exhibited relatively coarser textures, varying from sandy loam to sandy clay loam. This variation in texture can be attributed to the combined influence of parent material, land use practices and topographic position. Similar textural variability across land use covers was also reported by (Anand et al., 2025; Ravikumar and Govindaraju, 2019) [1,16] . [16,4] . Overall, soils in the study area were slightly acidic to slightly alkaline in nature, which is favorable for nutrient availability.

Electrical Conductivity (EC)

Electrical conductivity values across land use covers ranged between 0.12 and 0.95 dS m⁻¹ at 25 ℃, well within the permissible limit (<1.0 dS m⁻¹). [4,16] .

Soil Organic Carbon (SOC)

SOC content across land use covers varied from 3.22 to 8.97 g kg⁻¹, reflecting differences in vegetation type and management. Natural forest soils recorded the highest mean SOC (7.12±1.05 g kg⁻¹), followed by arecanut plantations (6.48±1.11 g kg⁻¹) (fig. 3). This enrichment can be linked to continuous litter deposition, minimal soil disturbance and high microbial activity that enhance organic matter turnover (Jahnavi et al., 2020a) [6] . In contrast, maize soils exhibited the lowest mean SOC ( [20,10,1] . Paddy (4.90±0.82 g kg⁻¹) and barren soils (5.47±0.73 g kg⁻¹) showed moderate levels. These findings emphasize the critical role of vegetation cover and management in maintaining soil carbon stocks.

Cation Exchange Capacity (CEC)

Cation Exchange Capacity ranged from 6.18 to 27.82 cmol (p⁺) kg⁻¹ across land use covers. The highest mean CEC was recorded in natural forest soils (21.59±4.21 cmol (p⁺) kg⁻¹), closely followed by arecanut plantations (20.39±4.54 cmol (p⁺) kg⁻¹) (fig. 4). This trend reflects the positive influence of organic matter inputs and stable clay humus complexes that enhance nutrient retention. Paddy (17.46±3.99 cmol (p⁺) kg⁻¹) and maize soils (13.77±3.17 cmol (p⁺) kg⁻¹) showed moderate CEC, while barren soils exhibited the lowest mean (8.93±2.21 cmol (p⁺) kg⁻¹), which might be due to the absence of vegetation, poor organic input and often coarser soil texture that limits cation retention. These results are in line with findings by (Seema et al., 2022; Ravikumar and Govindaraju, 2019) [18,16] .

Soil Microbial Biomass Carbon (SMBC)

SMBC values varied from 183.92 to 575.11 mg kg⁻¹, indicating substantial variation among land use covers. The highest microbial biomass was recorded in natural forest soils (469.80±65.68 mg kg⁻¹), followed by arecanut plantations (373.49±28.73 mg kg⁻¹). Paddy (349.22±51.61 mg kg⁻¹) and barren soils (285.96±48.78 mg kg⁻¹) exhibited moderate levels, while maize soils showed the lowest (278.35±57.93 mg kg⁻¹) (fig. 5), which might be due the differences reflect variations in SOC availability, organic residue input and land management practices. Forest and perennial plantations maintain higher microbial biomass due to continuous organic additions, whereas intensive annual cropping covers reduce microbial activity (Jahnavi et al., 2020b) [7] .

Available Nitrogen (N)

Among the different land use cover available nitrogen content ranged from 149.63 to 432.41 kg ha⁻¹. The highest mean nitrogen was recorded in arecanut soils (325.14±62.15 kg ha⁻¹), followed by natural forest soils (305.77±63.84 kg ha⁻¹). Maize soils showed the lowest mean nitrogen content (216.30±42.47 kg ha⁻¹) (fig. 6). The higher nitrogen availability in arecanut and forest soils can be attributed to greater organic matter accumulation and minimal soil disturbance, whereas intensive cultivation and crop uptake deplete nitrogen in maize covers (Jahnavi et al., 2020a; Anand et al., 2024) [6,2] . [12,11,21] .

Available Phosphorus (P₂O₅)

Available potassium (K₂O)

Available potassium content varied from 85.93 to 421.38 kg ha⁻¹ significantly among the land use covers. The highest mean potassium content was recorded in the arecanut land use cover (327.20 ± 56.04 kg ha⁻¹), followed by the natural forest cover (262.37 ± 56.26 kg ha⁻¹). In contrast, the lowest mean potassium level was observed in the barren land use cover, with a value of (127.61 ± 28.73 kg ha⁻¹) (fig. 8). The higher available potassium status in arecanut soils may be attributed to organic carbon enrichment, permanent ground cover that reduces leaching and regular application of potassic fertilizers, along with the upward movement of K from subsoil layers through capillary rise. On the contrary, the lower available potassium status in barren soils reflects the combined effects of nutrient depletion, lack of organic matter and parent material limitations (Madivalappa et al., 2025b; Syed et al., 2023) [11,21] .

DTPA-extractable micro nutrients status of soils under different land use cover

The data presented in Table 4 indicate the status of various micronutrients under different land use covers in Tarikere taluk of Chikkamagaluru district.

Iron (Fe)

The DTPA-extractable iron (Fe) content across land use covers ranged from 4.98 to 22.42 mg kg⁻¹. Among the land use covers, the arecanut land use cover recorded the highest mean Fe content (15.73±4.06 mg kg⁻¹), followed closely by paddy (15.51±5.02 mg kg⁻¹) and natural forest (13.35±2.31 mg kg⁻¹). The lowest mean Fe content was observed under maize land use cover (8.84±2.52 mg kg⁻¹) (fig. 9). Overall, Fe availability was sufficient across all land use covers, which can be attributed to the granite gneiss parent material of the study area, inherently rich in iron (Ravikumar et al., 2019) [16] .

Manganese (Mn)

The concentration of DTPA-extractable manganese (Mn) varied from 2.06 to 17.41 mg kg⁻¹ across the different land use covers.

The arecanut land use cover recorded the highest mean Mn content (11.50±4.02 mg kg⁻¹), followed by paddy (9.87±3.10 mg kg⁻¹) and natural forest (9.51±2.62 mg kg⁻¹), while the lowest was in maize land use cover (4.10±1.40 mg kg⁻¹) (fig. 10). Higher Mn availability in arecanut soils may be due to organic matter recycling through frond and husk additions, coupled with moist soil conditions that favor Mn mobilization and solubility. Plantation soils are also moderately acidic, which enhances Mn availability (Seema et al., 2022) [18] .

Zinc (Zn)

The DTPA-extractable zinc (Zn) content across land use covers ranged from 0.18 to 1.70 mg kg⁻¹. The highest mean Zn content was observed in paddy soils (1.00±0.45 mg kg⁻¹), whereas the lowest was recorded in maize soils (0.66±0.19 mg kg⁻¹) (fig. 11). Although Zn availability was marginal in a few samples, the majority of soils fell within the sufficient range. Variability in Zn content may be explained by differences in soil pH and organic matter content. Soils with higher organic matter facilitate Zn mineralization, thereby increasing its availability (Ravikumar et al., 2019) [16] .

Copper (Cu)

The availability of DTPA-extractable copper (Cu) ranged from 0.44 to 4.83 mg kg⁻¹, with sufficient levels observed across all land use covers. The arecanut land use cover recorded the highest mean Cu content (2.74±1.29 mg kg⁻¹), followed by natural forest (2.21±1.05 mg kg⁻¹), while the lowest was in maize soils (0.97±0.34 mg kg⁻¹) (fig. 12). The higher Cu levels in arecanut plantations may be attributed to the regular application of copper sulphate for disease management, along with the contribution from organic manures (Seema et al., 2022) [18] . Soil pH and organic matter are key factors regulating the solubility and availability of Cu and other micronutrients (Lindsay and Norvell, 1978; Ravikumar et al., 2019) [9,16] . [6,7] . On the other hand, the comparatively lower nutrient status in maize and barren lands suggests depletion of nutrient reserves due to intensive cultivation, residue removal, and lack of organic inputs (Subramanyan et al., 2023; Madivalappa et al., 2025a) [20,10] . These results highlight the strong interrelationship between land use pattern and soil nutrient cycling, where vegetation cover and management practices play a decisive role in sustaining nutrient pools (Bhattacharyya et al., 2015; Ravikumar and Govindaraju, 2019) [3, 16] .

Soil Properties

The overall trend indicates that land use conversion from perennial systems to annual cropping or barren conditions reduces soil organic matter and impairs nutrient retention and microbial functions (Lal, 2020; Seema et al., 2022) [8,18] . This emphasizes the need for adopting sustainable nutrient management strategies tailored to specific land use types.

Practices such as integrated nutrient management (INM), incorporation of crop residues, use of organic manures, green manuring, biofertilizers, and reduced tillage can enhance soil organic carbon, improve nutrient-use efficiency, and promote long-term soil fertility (Menasinakai et al., 2024; Anand et al., 2024) [12,2] . In addition, adoption of agroforestry-based land use systems and site-specific nutrient management (SSNM) approaches would help in recycling nutrients, conserving soil moisture, and ensuring sustainable productivity in the southern transition zone (Ravikumar et al., 2019; Reddy et al., 2015) [16,17] . The findings of the present study suggest that maintaining continuous vegetation cover and promoting balanced nutrient management are key to sustaining soil fertility and ecological stability under varying land use systems of Tarikere taluk.

Conclusion

This study revealed that soil texture across the studied land use covers ranged from sandy loam to clay loam. Soil pH varied from slightly acidic to slightly alkaline, with electrical conductivity remaining within the normal range across all land use covers. Soil organic carbon (SOC) and cation exchange capacity (CEC) were highest under natural forest compared to other land use covers. Available nitrogen (N) and phosphorus (P₂O₅) were found to be in the low to medium range, whereas available potassium (K₂O) varied from medium to high. Among the DTPA extractable micronutrients Fe, Mn, Zn and Cu were generally sufficient, although a few soil samples exhibited zinc deficiency. Overall, the findings emphasize the need for land usespecific soil management practices to maintain and enhance soil health in the region.

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