Soil physicochemical properties and microbial biomass in agriculture and abandoned lands of Shivapuri-Nagarjun National Park, Kathmandu, Nepal

Evaluating soil physicochemical and biological properties helps to understand ecosystem functions and nutrient dynamics. This study assessed the physicochemical and microbial properties of soil in agricultural and abandoned lands within Shivpuri-Nagarjun National Park, Nepal. Soil samples were collected from both agriculture and abandoned lands at soil depths of 10 to30 cm during the pre-monsoon, monsoon, and post-monsoon seasons. Physicochemical parameters of the soil were analyzed by using standard protocols. The chloroform fumigation extraction method was applied for determination of microbial biomass carbon and nitrogen. Soil temperature decreased with increasing depth during both pre-monsoon and monsoon seasons, but it was increased on increasing the depth during post-monsoon. Agricultural land exhibited higher proportions of silt and clay, whereas abandoned land was characterized by a higher sand content. In agricultural soils, soil organic carbon was 2.63%, total nitrogen and available phosphorus were 0.21% and 72.16 kg ha^-1, respectively, while available potassium was 636.55 kg ha^-1. These nutrient levels peaked during the monsoon season and declined with increasing soil depth. Similarly, microbial biomass carbon (465.82 μg g^-1) and nitrogen (48.58  μg g^-1) were also higher in agricultural land, showing an increasing trend with depth. The microbial biomass carbon-to-nitrogen ratio ranged from 9.61 to 19.41. The first (PC1) and second (PC2) components of Principal Component Analysis (PCA) accounted for 35.5% and 12.4% of the total variance, respectively. Overall, agricultural land, upper soil layers, and the monsoon season were identified as the most influential factors contributing to improved soil characteristics.

1. INTRODUCTION

The pedosphere is the most active and vital layer of the Earth's surface (Herreño et al., 2023). Soil is an important natural resource, which provides numerous ecosystem services, supports plant growth and development, retains water, and nutrient cycling(Anikwe & Ife, 2023). The physicochemical properties of soil determine soil quality and provide information about the capacity of soil to supply mineral nutrients (Amgain et al., 2020;Nepal & Mandal, 2018). Similarly, soil microorganisms play a crucial role in maintaining soil biochemical properties and ecosystem functions(Dobrovol’skaya et al., 2015), soil organic matter processes(Nguyen-Sy et al., 2022)and formation of soil structure(Chandra et al., 2016). Soil microbial biomass, hence, is a dynamic labile pool of plant-available nutrients(Yokoyama et al., 2017). However, changes in practices on land use and management can affect soil physicochemical or biological properties (Abera & Belachew, 2011;Novara et al., 2017). Therefore, the results of soil mapping illustrate the distribution of soil fertility, which can help maintain soil fertility and improve land management(Suntoro et al., 2024).

Abandonment of agricultural lands is a significant phenomenon of land-use change with profound ecological and environmental consequences. It was estimated that the global area of farmland abandonment accounts for around 8-10% of the world’s total cultivated area(Campbell et al., 2008). Urbanization and industrialization have created a significant gap in income between the agricultural and non-agricultural sectors, leading to widespread abandonment of farmlands(Zhou et al., 2019). Agricultural activities in rural areas have declined significantly due to the outmigration of people to urban areas for employment, better education, and other amenities. Several studies have indicated that, on average, 37% of arable land in Nepal is abandoned(Dahal et al., 2020; Ojha et al., 2017).

Agricultural land abandonment is particularly pronounced in the buffer zones surrounding national parks, largely due to human–wildlife conflicts and restrictive park policies that limit resource use and traditional land management practices(Silwal et al., 2022). Soil infertility and degradation of soil quality may also contribute to land abandonment within park premises. Shivapuri-Nagarjun National Park (SNNP), situated near the capital city, Kathmandu, is a forested area, which comprises the majority of land cover (74.45%), followed by shrubland (20.45%), abandoned land (3.22%), and agricultural land (1.8%)(SNNP, 2017). This indicates that a significant area of land within the park has been abandoned, which was previously cultivated. However, studies regarding the causes and consequences of abandonment remain limited.

Previous studies in the national park have focused on vegetation(Dhakal et al., 2023), insects(Budhathoki et al., 2021), birds and wild mammals(Poudyal et al., 2023), and soil organic carbon(Shapkota & Kafle, 2021). These studies provide critical insights into vegetation, biodiversity, and ecosystem functioning, which could be helpful for sustainable forest management practices. In addition, analyzing the soil characteristics of abandoned and agricultural land is important for soil fertility assessment, understanding soil degradation processes, land restoration planning, and environmental impact assessment(Paudel et al., 2014). This study aims to assess the physicochemical properties and microbial biomass in the soils of agricultural and abandoned lands within the SNNP, taking into account seasonal variations and soil depth gradients.

2. MATERIALS AND METHODS

2.1. Study site

This study was carried out in Shivapuri-Nagarjun National Park (SNNP). The National Park is located in the mid-hill region towards the northern side of the Kathmandu Valley, Nepal (Fig. 1). There are two villages, where the study was confined within the National Park (i) Okhreni (27°79'53''N, 85°42'14''E; elevation 1950 m) and (ii) Chilauni (27°79'36''N, 85°42'18''E; elevation 1976 m). The area experiences a subtropical monsoon climate, with temperature range of 2°C to 17°C during the winter season and 19°C to 30°C during the summer, and the mean annual rainfall is 2,727 mm (SNNP, 2017).

The villages contain abundant agricultural lands, including both actively farmed areas and abandoned traces of past cultivation. Common crops like maize, wheat, millet, mustard, beans, etc. were grown in the agricultural fields. Seasonal vegetables are also grown alongside these staple crops. In contrast, the abandoned lands lack significant vegetation and are primarily used for grazing and forage collection.

Figure 1.Map of the study site (a) Nepal showing Kathmandu district, (b) Kathmandu district showing SNNP, and (c) SNNP showing the study site.

2.2. Soil sampling and soil analyses

Soil samples were collected from both agricultural and abandoned land sites during 2020 and 2021. Three transects were established in each land-use type. In each transect, three square plots of 10 × 10 m^²were established for soil sampling, with approximately 100 m spacing between plots. From each plot, soil cores were extracted from three different depths: (i) 0-10 cm, (ii) 10-20 cm, and (iv) 20-30 cm. The collected samples were thoroughly mixed from each plot to create a composite sample of approximately 1 kg. Soil samples were collected during pre-monsoon, monsoon, and post-monsoon seasons. Altogether 162 composite samples (2 land-use types × 3 transects × 3 plots × 3 depths × 3 seasons) were collected. The samples were dried, gently crushed, and sieved using a mesh with a pore size of ≤ 2 mm. Soil samples were transported in iceboxes and stored in the refrigerator (<4°C) until use.

A soil thermometer was used to measure the temperature on-site. The dry weight method

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