Effects of Land-Use Dynamics on Soil Organic Carbon and Total Nitrogen Stock, Western Ethiopia

Leul, Yitayh; Assen, Mohammed; Damene, Shimeles; Legass, Asmamaw

doi:https://doi.org/10.1155/2023/5080313

Applied and Environmental Soil Science

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 5080313 | https://doi.org/10.1155/2023/5080313

Effects of Land-Use Dynamics on Soil Organic Carbon and Total Nitrogen Stock, Western Ethiopia

Yitayh Leul,^1,2Mohammed Assen,¹Shimeles Damene,³and Asmamaw Legass¹

Academic Editor: Evgeny Abakumov

Received21 Jun 2022

Revised08 Feb 2023

Accepted11 Feb 2023

Published18 Feb 2023

Abstract

Soil organic carbon (SOC) and total nitrogen (TN) stock are key indicators of soil quality in tropical regions; however, their status is often degraded, especially due to massive deforestation in natural forest areas associated with extensive agricultural land use. The aim of this study was to investigate the dynamics of SOC and TN stock in different land-use systems in the Abobo woreda, Western Ethiopia. To analyze their status, 80 disturbed (composite) and 45 undisturbed soil samples were collected from the top 20 cm of soil in five major land-use types: natural forestlands, grasslands, recently developed commercial farmlands, old commercial farmlands, and small-scale cultivated lands. The results showed that SOC stock varied significantly across the different land-use types, with mean stock ranging from 32.23 Mg·ha⁻¹ in recently developed commercial farmlands to 54.54 Mg·ha⁻¹ in natural forestlands. The mean TN stock ranged from 2.54 Mg·ha⁻¹ in recently developed commercial farmlands to 4.63 Mg·ha⁻¹ in natural forestlands. With natural forestlands as a baseline and the duration ranging in age from 15 to 45 years since land-use conversion, the mean annual absolute rates of change in SOC and TN stock loss were 0.49, 1.49, 0.39, and 0.45 Mg·ha⁻¹·yr⁻¹ and 0.05, 0.14, 0.03, and 0.04 Mg·ha⁻¹·yr⁻¹ for grasslands, recently developed commercial farmlands, old commercial farmlands, and small-scale cultivated lands, respectively. The results of this study revealed that soil disturbance during forestland conversion to tillage enhanced the decomposition rate of organic matter in recently developed commercial farmlands. Nevertheless, after agricultural abandonment and vegetation restoration, the SOC and TN stock capacities were enriched in the old commercial farmlands. It is, therefore, important to effectively restore vegetation and implement sustainable land-use management practices.

1. Introduction

Soil plays a crucial role in the global carbon cycle as it functions as both a source and a sink of atmospheric carbon dioxide [1]. It contains at least three times more carbon than the atmosphere [2, 3]. The large-scale exploitation of forest resources is a pressing environmental concern in many developing countries [4]. This often results from converting forests into agricultural land for subsistence farming and large-scale agricultural investments (LAIs) [5]. Such deforestation and land-use change significantly impact the global carbon cycle by increasing carbon emissions into the atmosphere at both local and global levels [6, 7].

Land-use change and management can impact soil organic carbon and total nitrogen stock due to increased carbon degradation and erosion from soil disturbance [8, 9]. This leads to alterations in ecosystem carbon and results in biodiversity loss, poor soil quality, low land productivity, and desertification [10]. Over the last 150 years, the rate of soil organic carbon and total nitrogen loss from land-use change has steadily increased, reaching approximately 2 Pg·C per year [11]. Land-use change has also resulted in net carbon dioxide emissions of approximately 156 Pg·C over the same period [12], which is similar to the CO₂ emissions estimates of 1.7 Pg/year from 1980 to 1989 [13], 1.6 Pg/year from 1989 to 1998 [13], and 1.1 Pg/year from 2000 to 2009 [14].

Following the conversion of native forests to extensive agricultural lands, such as those in tropical Mexico [7], the Amazon forest in Brazil [15], the Caribbean region [16], and sub-Saharan Africa (SSA) [9], a 20–50% loss in soil organic carbon and total nitrogen stock has been observed [17–19]. A review by Guo and Gifford [20] indicated that soil organic carbon and total nitrogen stock in grasslands are more impacted by land-use changes than in forests and grasslands, resulting in significant losses of these stocks. Conversion of forests and agroforestry to cropland has been found to cause a decline in soil organic carbon and total nitrogen stock, with reduction ranging from 3.3 to 8.0 Mg·ha⁻¹ in the southwestern highlands of sub-Saharan Africa [21–23] to 50–87% in the northern highlands of East Africa [24, 25]. On the other hand, the opposite has been shown to increase the soil organic carbon stock [26, 27].

Ethiopia is known for its diverse vegetation and climatic zones [28, 29]. However, the increasing demand for agricultural land leads to deforestation, caused by a permanent change in land use for commodity production [30]. Since 2000, large-scale agricultural investments have leased extensive land areas to local and foreign investors, causing an upsurge in deforestation and land-use changes [31]. The promotion of government-led large-scale commercial farming has also led to elimination of considerable natural vegetation in the western lowlands of Ethiopia [32, 33]. Deforestation has had a detrimental impact on soil organic carbon and total nitrogen stock, the quality and quantity of organic inputs, land productivity, and soil disturbance [8, 23, 26, 30, 32–35]. Land-use alterations from natural forests to agroecosystems in sub-humid and humid tropical regions have negatively affected soil quality by emitting more carbon into the atmosphere, reducing soil organic carbon and total nitrogen levels. The sub-humid tropical lowland agroecosystems in Western Ethiopia, for instance, have been widely replaced by large-scale commercial farms, resulting in the clearing of most natural vegetation and, subsequently, a decline in soil organic carbon and total nitrogen stock. Examining their dynamics in different soil pools under varying land-use systems is crucial.

The present study aimed to (1) assesses the differences in soil organic carbon and total nitrogen storage capacity between cultivated and natural forestlands, (2) evaluate the impact of land conversion on soil organic carbon and total nitrogen losses, and (3) determine the rate of change in soil organic carbon and total nitrogen as a function of cultivation time. It was hypothesized that natural forests have greater carbon and nitrogen storage than cultivated soils and that there would be a significant correlation between the net change in soil organic carbon and total nitrogen accumulation and the duration of cultivation. The study took advantage of fields established at different times and under diverse land-use systems to assess the changes in soil organic carbon and total nitrogen over a long period after converting natural forestland to multiple agricultural land-use systems.

2. Materials and Methods

2.1. Description of the Study Area

The study was carried out in the Abobo district of Gambella National Regional State, Western Ethiopia. It is situated between 07°46′N and 08°4′N latitude and 34°0′E and 34°45′E longitude and covers 200,225 hectares (Figure 1). The geology of the area primarily comprises Quaternary undifferentiated alluvial and lacustrine deposits [36]. The dominant landform is plain, characterized by relatively low elevations ranging from 400 to 631 meters above sea level (masl) with flat (0.2–0.5%) to gently undulating (2–5%) slopes. The geological and geomorphological characteristics of the area influence the soil formation, properties, and distribution in the Abobo district. There are various soil types found in the region, including Vertisols, Cambisols, Luvisols, Gleysols, Nitisols, Leptosols, Fluvisols, Ferralsols, and Planosols [33]. These soils are found in specific locations, with Cambisols on the upper slope of old commercial farmland plots. Luvisols and Plinthosols are found in the midslope, largely occupying smallholder cultivated farmland plots and grasslands. Vertisols are found on the lower slopes, particularly in recently developed commercial farmland plots and natural forestlands [37]. The main rivers and streams in the region are the Baro, Akobo, Gilo, Jikow, Gnandera, and Koikoye, which originate from the highlands and are all tributaries of the Nile River.

The study area has been described as having an “Aw” (tropical savanna) climate, with average monthly temperatures ranging from 17°C to 19°C in November and January and maximum monthly temperatures reaching 40°C in February and March. As confirmed by available meteorological data (1983 to 2020) [38], the average total annual precipitation recorded by the meteorological station located in the study area is 137 mm (Figure 2). Approximately 26% of the rain occurs in spring between February and May, whereas most (44%) of the rain occurs in summer from June to August.

The natural vegetation types include Combretum Terminalia woodland, wooded grassland, and lowland (semi) evergreen forest (lowland dry peripherals of the semideciduous Guineo-Congolian type). Combretum Terminalia is distinguished by small- to medium-sized trees and grass strata with relatively large deciduous leaves frequently burned in the dry season. Commonly observed species include yetan zaf (Boswellia papyrifera), Anogeissus leiocarpa, Stereospermum kunthianum, and Boswellia papyrifera, with other species generally comprising Terminalia and Combretum [39]. Lowland and evergreen forests are mainly found at elevations between 450 and 650 masl. This forested area is prone to be burned for subsistence agricultural purposes [28]. The characteristic species of this forest type is Baphia abyssinica, mixed with less common species, including Celtis toka, Diospyros abyssinica, Malacantha alnifolia, Zanha golungensis, Lecaniodiscus, Trichilia, and Zanthoxylum [29].

In terms of population, the area is inhabited by a mix of native and resettled groups. Most of the total population lives in rural areas, where households primarily depend on natural resources for their livelihoods. Some ethnic groups primarily practice shifting cultivation to produce maize and sorghum, supplemented by fishing, hunting, and collecting wild fruits and roots as sources of their livelihood [40], thus having differential effects on the local ecosystem. In the 1970s and 1980s, government-sponsored resettlement was implemented to relocate drought-affected and landless farmers from other parts of the country, especially Northern Ethiopia and Southern Ethiopia, to sites in the study area following the impacts of the 1973/74 and 1984/5 droughts. These populations have transformed the natural forest and vegetation cover into sedentary mixed agriculture for subsistence, cultivating maize, sorghum, and groundnuts.

The Ethiopian government introduced the first large-scale mechanized farming in the 1980s in the study area. Between 1984 and 1994, almost 140,000 hectares of natural forestland in the Gambella Region were cleared, and the population relocated to develop large-scale state-owned mechanized farms [33]. More recent governments, especially those in 2000, have opened up large-scale agricultural investment opportunities in the study area. With the policy shift from state-owned to private investment in large-scale mechanized farms, a large share of the land with natural forest cover was leased out to commercial agricultural enterprises, some of which have been transformed into large-scale commercial plantations, with natural vegetation being cleared by bulldozing and/or burning. As a result, since 1980, the study area has largely undergone the development of mechanized government- and privately owned farms and government-sponsored resettlement projects [41], causing substantial natural forest destruction. For instance, in 2004, 10,000 hectares of state-owned large-scale mechanized farms in the Abobo area were leased to private companies, while between 2006 and 2011, an estimated 1.4 million hectares of land were transferred to foreign and domestic investors, with private investors having put nearly 3,569 hectares into cultivation [42].

Moreover, Rahmato [43] showed that the government has continued to promote agricultural and agro-industrial products for export and local markets. As a result, major crops such as maize (Zea mays L.), sorghum (Sorghum bicolor L.), groundnut (Arachis hypogaea), sesame (Sesamum aestivum), cotton (Gossypium sp.), haricot bean (Phaseolus vulgaris L.), soybean (Glycine max), and rice (Oryza sativa) have been cultivated in the large-scale commercial farms of the study area.

2.2. Field Survey and Soil Sample Collection

The study area’s previous and current land-use histories were collected from local elders, and a preliminary field visit was conducted using a 1 : 50,000 topographic map augmented with aerial images from 1967. Additional information about the current land use was obtained from the 2019 (OLI) Landsat image. Five main land-use types of the study area were identified: natural forestlands, grasslands, recently developed large-scale commercial farmland (i.e., with less than 15 years of cultivation period), relatively old commercial farmland (i.e., with over 30 years of cultivation period), and small-scale cultivated lands that have experienced over 45 years of cultivation period (smallholder, cultivated farmland soils) (Table 1).

The soil survey teams conducted transect walks using a field vehicle to document various soil types, slope gradients, and vegetation in different land-use types. The objective of the transect research was to determine suitable sampling intervals that effectively recorded ecological variations. From each identified land-use type, both disturbed (composite) and undisturbed soil samples were collected using an auger from the four corners and the center of a 10 m × 10 m square plot of the surface soil (0–20 cm).

According to Figure 1, the land-use/cover types in the study area include forestlands (16%), woodlands (7%), wetlands (17%), shrublands (10%), cultivated lands (17%), grasslands (14%), commercial farms (both recently developed and old) (18%), and water bodies (1%). Soil sampling was mainly based on the objectively selected land-cover types such as natural forestlands, grasslands, recently developed commercial farms, old commercial farms, and smallholder cultivated farmland soils. The soil samples were taken based on the proportion of the area for each selected land-use type, management differences between land-use types, and available resources. 80 disturbed (composite) and 45 undisturbed soil samples were collected (Table 1).

We decided to use a consistent soil sample depth of 0–20 cm for all land-use types after considering several important factors. Firstly, the top 20 cm of soil is widely recognized as an important layer for soil studies as it contains high concentrations of soil nutrients and is highly vulnerable to the impact of land-use change. Over 70–90% of the total soil organic carbon and total nitrogen stock is found in the upper 20 cm of soil [8, 21]. Secondly, the centers of the other sampling plots were developed at a 200-meter distance from the center point of each land use or cover type. All sampling points were georeferenced using the Global Positioning System (GPS). As the area has more or less uniform topographic and climatic features, sampling was based primarily on land-use cover and management differences.

Approximately 2 kg of the composite soil sample was collected from each sampling plot. The collected soil samples were taken to the laboratory for further sample preparation and analysis. The disturbed soil samples were air-dried and crushed to pass through a 2 mm sieve before analysis. In addition, undisturbed core samples were taken from the center of each plot for each land-use/cover type with a cylindrical metal core sampler (volume = 100 cm³) to measure the soil bulk density () of different land-use/cover types in the study area.

2.3. Laboratory Analysis

We analyzed the composite soil samples at the National Soil Laboratory of the Ministry of Agriculture in Addis Ababa, Ethiopia, for soil organic carbon, total nitrogen, and particle size class. The soil particle size distribution was determined using the Bouyoucos hydrometer and 1 : 2.5 soil-water suspension method [44]. The amount of soil organic carbon was determined using the Walkley and Black oxidation method [45]. Total nitrogen was determined using the Kjeldahl method [46]. Soil bulk density () was determined using the undisturbed (core) sampling method after soil samples had been dried in an oven at 105°C to constant weight for 24 hours [47], as the study area was free of rocks, no adjustment was made for rock volume. These undisturbed (core) soil samples were analyzed for bulk density at the Ethiopian Construction Design and Supervision Enterprise (ECDSE) soil testing laboratory in Addis Ababa, Ethiopia.

2.4. Statistical Analysis

All statistical analyses were conducted using Statistical Package for Social Sciences (SPSS) for Windows, Version 25 [48]. One-way ANOVA was performed to test whether the mean soil organic carbon and total nitrogen stock varied significantly among land-use/cover types. Differences between means of treatments were considered significant at 0.05, 0.01, and 0.001 levels using Tukey’s studentized (HSD) test. The soil value for each land-use type was used to calculate the SOC and TN stock (Mg·ha⁻¹) using the Ellert and Bettany [49] and Gelaw et al. [34] models, as shown in equation (1), using the soil value for each land-use/cover type.where = soil organic carbon or total nitrogen stock (Mg·ha⁻¹). = soil organic carbon or total nitrogen contents (kg·Mg⁻¹). = dry bulk density (Mg·m⁻³). = thickness of the soil layer (m).

The SOC (or TN) stock rate is estimated depending on the changes in soil organic carbon (or total nitrogen) stock in different time sequences (15, 30, and 45 years) converted to recently developed commercial farms, old commercial farms, smallholders, cultivated farmland soils, and grassland land-use types. The study set the soil organic carbon (or total nitrogen) stock of prior land-use types and the control (natural forestlands) land use as the baseline for calculation. The percentage and rate of change in soil organic carbon (or total nitrogen) after land-use conversion are estimated depending on the soil organic carbon stock changes between the previous and present land-use types. We computed them using Deng et al. [50] equation (2) model. We first calculated the SOC (or total nitrogen) stock (Mg·ha⁻¹) at every site after land-use conversion.where is the percentage of change in SOC (or TN) stock; represents the SOC (or TN) stock of prior land-use types (natural forestland) before land-use conversion (Mg·ha⁻¹); and represents the SOC (or TN) stock after land-use conversions, such as grasslands, recently developed commercial farms, old commercial farms, and smallholder cultivated farmland soil land use. We used the mean annual absolute rate of change in SOC (or TN) stock (Mg·ha⁻¹·yr⁻¹). The calculated equation is as follows.

The percentage and rate of change in value bulk density after land-use conversion are estimated as a function of value bulk density changes between the previous and current land-use types. We calculated the bulk density contained in the value using equation (4) model.where is the percentage of change in bulk density (Mg·m⁻³), represents the value bulk density of prior land-use types (natural forestland) before land-use conversion (Mg·m⁻³), and represents the value of bulk density after land-use conversions, such as grasslands, recently developed commercial farms, old commercial farms, and smallholder cultivated farmland soil land use.

3. Results and Discussion

3.1. Soil Particle Size Classes and Bulk Density Value

A one-way ANOVA analysis indicated that the sand fraction showed a statistically significant difference among the different land-use/cover categories (Table 2). The change from a forest to agricultural land resulted in a change in the sand content due to the selective removal of the clay and silt fractions. The bulk density of the studied soils ranged from 1.17 Mg·m⁻³ to 1.42 Mg·m⁻³ (Table 2). Our analysis showed a statistically significant difference in between the different land uses/covers and an inverse correlation with soil organic matter (r = −0.499). These sub-humid tropical highland agroecosystems have experienced substantial depletion of soil organic carbon and total nitrogen of soils due to ongoing cultivation and removal of natural vegetation. The increase in bulk density following deforestation from natural forestland to recently developed commercial farm soils and smallholder, cultivated farmland soils was 20.98% and 17.09%, respectively, as shown in Table 3.

The soil bulk density across different land-use and cover patterns affects the soil organic matter (SOM) in a negative manner [26]. An increase in soil compaction due to frequent agricultural practices and farm tractors is directly correlated with the decline in soil organic matter, total nitrogen content, and biomass [51]. The soil layer in forests, with a depth of 0–20 cm, has higher clay and silt proportions due to the protection provided by the forest canopy, debris, and roots against leaching and erosion [34]. This results in higher soil organic carbon levels, which are often linked to soils with higher clay content [30]. The presence of clay in soils increases SOM, as it helps protect organic matter molecules from degradation by microbes. Clay particles bond with organic matter, retarding the decomposition process, and also increase the potential for aggregate formation, which further protects organic matter from mineralization [52].

3.2. Soil Organic Carbon and Total Nitrogen Contents in Five Different Land-Use/Cover Categories

The study revealed significant differences in the amount of organic carbon and total nitrogen in upper soil (0–20 cm depth) based on land-use/cover type. The results indicated that the highest levels of organic carbon were observed in forestland soils (23.45 g·kg⁻¹), followed by old commercial farmland soils (17.24 g·kg⁻¹), grassland soils (16.76 g·kg⁻¹), and small-scale cultivated land soils (13.05 g·kg⁻¹). Notably, organic carbon levels in recently developed commercial farmland soils (11.52 g·kg⁻¹) were significantly lower than those in other land-use types. These findings emphasize the significant impact of land use on soil carbon levels and the necessity for responsible land-use practices to preserve soil health and sustain soil carbon levels, as presented in Table 4.

The study found that total nitrogen levels in topsoil (0–20 cm depth) varied based on land use/cover. The highest total nitrogen levels were found in forestland soils (2.01 g·kg⁻¹), followed by old commercial farmland soils (1.51 g·kg⁻¹), grassland soils (1.35 g·kg⁻¹), small-scale cultivated land (1.05 g·kg⁻¹), and recently developed commercial farmland soils (0.91 g·kg⁻¹). The total nitrogen levels in recently developed commercial farmland soils were significantly lower than those in natural forest and grassland soils . The total nitrogen levels in small-scale cultivated land were not significantly different from those in old commercial farmland soils. The substantial reduction in organic carbon content in recently developed commercial farmland soils is a concern and may be attributed to poor land management practices. The indiscriminate burning of biomass for land preparation can accelerate soil organic carbon decomposition and decrease total nitrogen levels due to decreased microfaunal activity and increased soil bulk density following tree removal [1].

The soil’s ability to store or release carbon is impacted by several environmental factors, such as vegetation cover, climatic conditions, and management practices [53]. High temperatures and favorable moisture conditions lead to accelerated decomposition of organic matter, releasing nutrients and reducing the soil’s capacity to store soil organic carbon and total nitrogen. Table 3 shows that soil organic carbon and total nitrogen contents in old commercial and recently developed commercial farmland soils declined by 26.45% and 50.87% and by 24.73% and 54.72%, respectively, compared to natural forestland soils. Organic matter is a crucial source of total nitrogen [21], and commercial farmlands play a substantial role in soil organic carbon and total nitrogen losses during land-use conversions. However, factors such as crop absorption, leaching, surface erosion, poor management practices, removal of crop residue, and burning after harvest contribute to low levels of soil organic carbon and total nitrogen in small-scale cultivated lands, especially in recently developed and old commercial farmland soils. Compared to recently developed commercial farmlands, old commercial farmland soils that have undergone relative abandonment and exclosure had higher soil organic carbon and total nitrogen contents due to increased vegetation cover and reduced soil erosion, resulting from planting more tree species. These findings align with Damene et al. [27] and Chen et al. [54] who found that restoring farmland positively impacted soil organic carbon and total nitrogen contents.

3.3. Effects of Land-Use Changes on Soil Organic Carbon and Total Nitrogen Stock Accumulation

The soil’s ability to store or release carbon is influenced by environmental factors such as vegetation cover, climatic conditions, and management practices [53]. A decrease in organic matter in the soil can occur due to high temperatures and optimal moisture conditions, leading to the rapid release of nutrients and a decrease in the soil’s ability to store organic carbon and total nitrogen.

Table 4 shows that there was a significant difference in the mean soil organic carbon and total nitrogen stock of the topsoil (0–20 cm) across the various land-use types studied. The highest soil organic carbon stock in the 0–20 cm depth was observed in the natural forestland (54.54 Mg·ha⁻¹), followed by old commercial farmland (42.75 Mg·ha⁻¹), while the lowest soil organic carbon stock (32.23 Mg·ha⁻¹) was found in recently developed commercial farmland. The highest total nitrogen stock in the topsoil was measured in the natural forestland (4.63 Mg·ha⁻¹), followed by old commercial farmland (3.75 Mg·ha⁻¹) and grassland soils (3.21 Mg·ha⁻¹). In comparison, the lowest total nitrogen stock was observed in small-scale cultivated land (2.73 Mg·ha⁻¹) and recently developed commercial farmland (2.54 Mg·ha⁻¹) (Table 4).

The results demonstrate that forestland has a higher stock of soil organic carbon and total nitrogen due to the accumulation of above-ground leaves, deadwood litter, ground litter, and underground root litter [55]. In comparison, the soil organic carbon stock in old and recently developed commercial farmland soils decreased by 21.62% and 40.91%, respectively. Similarly, the total N stock in old and recently developed commercial farmland soils decreased by 19.02% and 45%, respectively (Table 3). This decrease is attributed to removing above-ground biomass for livestock feed in grassland and cultivated land and frequent fires in recently developed commercial farmlands [8]. Additionally, soil disturbance during site preparation and tillage exposes organic matter to decomposition and leads to rapid losses in soil organic carbon and total nitrogen stock in recently developed croplands [56].

Clearing forests and the subsequent tillage practices negatively impact soil organic carbon and total nitrogen stock by reducing litter input and hindering soil organic carbon accumulation [57]. On the other hand, the increase in soil organic carbon and total nitrogen stock in old commercial farmlands suggests the potential for restoration through vegetation, which can improve soil organic carbon and total nitrogen accumulation and protect against the loss of cations through leaching and biological processes [26, 27]. These findings align with the research of Li et al. [58] on changes in soil organic carbon and total nitrogen stock in agricultural soils.

Small-scale cultivated lands typically have lower soil organic carbon stock due to the low input of organic matter from harvested farm residues. This removal of nutrients results in reduced soil quality for carbon sequestration. A small portion of crop residues is returned to the soil, which may not be sufficient to balance the supply of litter to increase the soil’s organic carbon potential [6]. The low soil organic carbon stock can be attributed to decomposition, leaching, and soil erosion losses. Poor land management and postharvest grazing may also contribute to the low soil organic carbon storage level in the topsoil [21]. The lack of vegetative cover after harvest can also result in significant topsoil and organic matter loss, especially in areas with high precipitation levels and high erosion rates.

3.4. Rates of Changes in the Soil Organic Carbon and Total Nitrogen Stock in Different Land-Use Types

Compared to the baseline of natural forest land, the other land uses showed the most incredible mean annual absolute rate of change in soil organic carbon stock. Recently developed commercial farmland showed the largest mean average net yearly loss of soil organic carbon stock (1.49 Mg·ha⁻¹·yr⁻¹), followed by small-scale cultivated land (0.45 ha⁻¹·yr⁻¹) and old commercial farmland (0.39 ha⁻¹·yr⁻¹) (Table 5). The trend of accumulation of total nitrogen stock was similarly impacted by the conversion of forests to commercial farming, resulting in a total nitrogen stock loss of 0.14 Mg·ha⁻¹·yr⁻¹ in recently developed commercial farmlands, 0.05 Mg·ha⁻¹·yr⁻¹ in the grasslands, 0.04 Mg·ha⁻¹·yr⁻¹ in the small-scale cultivated lands, and 0.03 Mg·ha⁻¹·yr⁻¹ in old commercial farmland soils (Table 5). The low total nitrogen stock accumulation rate under old commercial farmland may be related to nitrogen loss through leaching or low total nitrogen content in crop residues. This is consistent with Solomon et al. [23] and Lemenih et al. [59] who observed a decreasing trend in soil organic carbon and total nitrogen stock across diverse land-use patterns in Southern Ethiopia. The correlation between soil organic carbon accumulation and total nitrogen stock is because most N is part of soil organic matter.

The difference in total nitrogen stock between forest and cultivated land indicates that the conversion of forestland to smallholder, cultivated farmland led to a loss of 0.03 Mg·ha⁻¹·yr⁻¹ of the total nitrogen stock over 45 years (Table 3). Previous studies showed that cultivated land had a 58–70% reduction in soil organic carbon compared with forestland after a 30-year cultivation period [21, 60, 61]. However, long-term farmland restoration has improved soil organic carbon and total nitrogen stock rates of change in old commercial farmland. Thus, a significant fraction of soil organic carbon and total nitrogen can be captured and stored in plant biomass and soil. Hence, the typical soil organic carbon and total nitrogen stock in intensively used smallholder, cultivated farmland soils with poor vegetation restoration lead to a high rate of soil organic carbon and total nitrogen stock degradation because of the poor total nitrogen content in vegetation litter. Gelaw et al. [34] reported that crop residue use did not specifically improve total nitrogen stock at the farm level in a semiarid watershed in Tigray, Northern Ethiopia, because of poor total nitrogen content in the crop residue.

3.5. Carbon-to-Nitrogen Ratio and Correlation of Carbon-Nitrogen Stock

Carbon-to-nitrogen ratios (C : N) are taken as an indicator of available nitrogen to be taken up by plants. The current study shows that soils with a C : N ratio of 11.39% in old commercial farmlands, 14.53% in smallholder, cultivated farmland soils, and 13.12% in recently developed commercial farmland soils require slow organic matter decay (Table 4). However, this range generally indicates the presence of an acceptable extent of mineralization, where the C : N ratios of the study area show an easy breakdown of soil organic matter. The C : N ratio is an essential indicator of carbon storage, greenhouse gas emissions, soil organic matter quality, and the microorganism community structure that changes the soil organic carbon and total nitrogen stock [62]. This was attributed to an increase in the mineralization rate and a decrease in the soil accumulation of soil organic carbon and total nitrogen stock; even though the results of a one-way ANOVA revealed an insignificant difference in the carbon-to-nitrogen ratio, it was estimated to decline by 3.32% with natural forestland use types. The higher correlation between soil organic carbon and total nitrogen stock showed that the main source of total nitrogen is the decomposition of organic matter [55, 63]. In general, the soil C : N ratio is an essential indicator of the soil nutrient storage and recycling capacity, carbon storage, greenhouse gas emissions, and the microorganism community structure that changes the soil quality of ecosystems.

3.6. The Implication of Changes in Soil Organic Carbon and Total Nitrogen Stock following the Conversion of Forests to Farmlands

This study highlights the impact of forestland and agricultural land-use systems on the rate of change in soil organic carbon and total nitrogen contents and stocks. Results indicate that recently developed commercial farmlands have lower soil organic carbon and total nitrogen contents and stocks than natural forestlands, grasslands, and abandoned or restored old commercial farmlands. This is due to the increased carbon breakdown caused by soil disturbance in the recently developed commercial farmlands.

The amount of soil organic carbon can vary based on land use and management practices, and any carbon stock loss can negatively impact soil function and the environment [19, 64]. Deforestation, which involves the conversion of forests into farmlands, releases a significant amount of carbon into the atmosphere [10, 57, 65]. This is particularly severe in the study area due to the conversion of natural forestlands into large-scale commercial farmlands through land-use intensification and clearing of natural vegetation.

Forest fires also play a crucial role in influencing the soil organic carbon and total nitrogen stock cycles, as they frequently occur alongside land-use conversion and escalate land degradation [66]. On the other hand, restoring croplands with vegetation types and grasslands can increase soil organic carbon, total nitrogen content, and storage due to increased plant biomass and soil organic matter inputs [67]. Maintaining organic matter degradation can improve soil quality by increasing the amount of organic carbon in agricultural soils [68], which can significantly impact the distribution of soil organic carbon and nutrients. Given these findings, further research is needed to evaluate the factors affecting soil organic carbon and nutrients and to develop strategies for maintaining and improving soil quality in agricultural soils.

4. Conclusion

Our findings indicated that clearing natural vegetation for large-scale agricultural purposes resulted in significant soil organic carbon losses. This can be attributed to the disturbance of natural forests, which decreases soil organic carbon and total nitrogen supplies. The long-term impact on diverse agricultural land-use systems’ soil carbon sequestration capacity is significant. This capacity is influenced by the land-use alternatives chosen and the management techniques implemented during the postconversion phase. Soil carbon sequestration can take a century or more to compensate for the initial losses. Our findings showed that soil organic carbon and total nitrogen stock increased after abandoning former commercial farmland. This suggests that abandonment can enhance soil organic carbon and total nitrogen accumulation.

On the other hand, converting natural vegetation cover to recently developed commercial farmland resulted in significant soil organic carbon and total nitrogen loss. Burning vegetation for large-scale agricultural investment in recently developed commercial farmland plots could cause a considerable loss of soil organic carbon and total nitrogen in the topsoil. The impact of these changes on the soil organic carbon and total nitrogen sequestration capacity of different agricultural land-use systems must be considered. The low soil organic carbon and total nitrogen stock in small-scale farmland may be due to inadequate land management. Therefore, when making land-use and land-cover management plans, the impact of these changes on soil organic carbon and total nitrogen should be considered. Land management techniques should be enhanced to ensure the long-term sustainability of tropical forestland soils and their ability to provide ecosystem services.

Data Availability

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors thank Addis Ababa University for financing this study, local experts for providing the required information, and Saudi Star large-scale agricultural investment for lodging us during our field study. Addis Ababa University Research and Technology Transfer Vice President Office funded this work.

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