Effect of Different Land Use Types on Nutrient Distribution across Soil Depth in Busega Wetland, Uganda

Daniel Iku Abuni¹, Sylvia Nyamaizi^2*, Doreen Mbabazize³, Christine Atugonza⁴

¹Action Against Hunger, Arua, Uganda.
²Faculty of Land and Food Systems, The University of British Columbia, Main Hall, Vancouver, Canada.
³Department of Crops, Horticulture, and Soils, Egerton University, Njoro, Kenya.
⁴Water Resources Engineering, Faculty of Engineering and Technology, Busitema University, Tororo, Uganda.

DOI: 10.4236/ojss.2025.151005                

Abstract

Wetlands play a number of vital roles in the ecosystem, such as serving as nutrient sinks, preventing floods, storing carbon, and filtering water. Encroachment on wetlands has led to substantial economic and environmental losses, including water quality degradation, loss of biodiversity and natural habitats, reduced climate mitigation as well as social and health risks. This study evaluated the effect of different land use types on nutrient stock distribution across varying soil depths in Busega wetland. The soil samples were collected in three different land uses (annually cultivated areas, perennially cultivated areas, and the undisturbed wetland area) at three different depths (0 - 10 cm, 10 - 20 cm, and 20 - 30 cm) in 2021. The soil samples were analyzed for physicochemical soil properties including soil texture and nitrogen, phosphorus, calcium, and potassium concentrations. The interaction between land use type and soil depth did not have a significant effect on nutrient distribution. However, our results showed that the main effects of land use type and soil depth influenced nutrient stock distribution across the wetland. Higher nutrient concentrations were observed under perennial cropping system than in both annual cropping system and the undisturbed wetland area. Soils under perennial cropping systems had the highest soil organic matter (1.45%), calcium (2.06 Cmol/Kg) and potassium (0.091 Cmol/Kg) levels. Higher soil organic matter (1.40%), nitrogen (0.22%), calcium (1.74 Cmol/Kg), and potassium (0.07 Cmol/Kg) were found at the mid-soil depth of 10 - 20 cm. Our results show substantial nutrient changes due to agricultural activities in the Busega wetland, suggesting further research is urgently needed to determine if these changes have adverse effects on biodiversity and water quality of the wetland and nearby water resources.

Keywords

Annual Cropping System, Perennial Cropping System, Undisturbed Wetland Area, Wetland Encroachment, Wetland Degradation

1. Introduction

Wetlands are essential components of natural resources that offer ecosystem services and ensure environmental sustainability if well managed [1] [2]. Globally, wetlands occupy 5% to 8% of the total earth’s surface [3]. These are grouped as papyrus swamps, swamp forests, riverine wetlands, lake edges, flood plains, dambos and artificial wetlands [4]. In Uganda, wetlands cover about 13% of the total land area which is estimated at 30,105 km² land size [5] [6]. These wetlands are dominantly marshes, seasonal and permanently flooded swamps, and bogs [7]. Wetlands are a significant component of the ecosystem that promotes environmental quality through carbon sequestration, water purification, nutrient cycling, climate moderation, flood and erosion control, provide wildlife habitat and are a source of food, fuel and fresh water [3] [8].

Over the years globally, wetlands have been threatened by human encroachment and destruction resulting in sporadic wetland degradation. Almost half of the world’s wetland area has been destroyed completely and the left wetland area is facing degradation [2]. Although the Government of Uganda enacted strict laws to protect wetlands from encroachment, wetland cover continues to decline, causing flooding, soil degradation, and water pollution [5]. The encroachment is mainly driven by population increase, industrialization, urbanization, and the unclear land ownership in the wetland areas [9]-[11]. The increasing population has exerted a lot of pressure on the available land for farming, increasing farmers’ search for more farmland in the nearby wetland areas [12]. There is also limited knowledge by the locals of the importance of wetlands to the ecosystem and poor enforcement of stringent laws against wetland encroachment [5]. Wetland degradation has caused substantial land losses through soil erosion and nutrient loss, economic losses, and health risks [13] [14]. The degradation of wetlands through increased human agricultural activities is expected to increase the accumulation of nutrients mostly nitrogen (N) and phosphorus (P) in waterbodies such as Lake Victoria leading to eutrophication that is evidenced by growth of algal blooms [11].

Busega wetland (as it is locally known) in Uganda, a tropical perennial wetland covering 1200 m in surface area with around 450 km² of drainage catchment area, has been severely encroached upon and degraded. The encroachment has resulted in various introduced land uses in Busega wetland, including agriculture with different cropping systems, animal and fish rearing, bricklaying, sand mining, and industrialization [15]. Depending on the type of land use, soil physical, chemical, and biological properties are affected [16], which could interact to affect the wetland's soil quality and capacity to provide ecosystem services such as improved water quality, erosion control, and flood protection in Kampala city and surrounding areas. In addition, land use types such as cropping affect the distribution and availability of nutrients in the soil, which is influenced by several factors such as soil depth, tillage, fertilizer application and changes in nutrient cycling [17]-[19]. Although reports show that the shift of parts of Busega wetland from natural ecosystem to farmland owing to its fertile soils, water availability to crops throughout the year [20] has significantly lowered the wetland’s soil and water quality due to intensive fertilizer and pesticide use [21] [22], there is still no documentation on the impact of different land use types especially cultivation on the soil status across the soil profile of Busega wetland. One possible way of determining the effects of these introduced land use practices on wetland quality is to assess the soil physical and chemical properties [8] under different land use types currently undertaken in the wetland. The present study examined how different land use types affect nutrient stock distribution across varying soil depths in Busega wetland. We hypothesized that cropped land would have increased nutrient concentrations compared to the undisturbed wetland area across the varying soil depth in Busega wetland.

2. Materials and Methods

2.1. Site Description

The study was conducted in Busega wetland found in Wakiso district, Busiro county, Wakiso subcounty, Ssuumbwe parish, Buleenga A Kikaaya in central Uganda (00˚18’36” N 32˚31’12” E) at an elevation of 1158 m above main sea level. The study area has a drainage catchment area of 40 km² [23]. Busega wetland was naturally covered by swampy vegetation dominated by Echinochloa pyramidalis, Paspalum scrobiculatum, Cyperus papyrus, Typha capensis and Thelypteris acuminata species [23].

The wetland area receives a bimodal type of rainfall with total annual rainfall ranging from 1200 mm to 1700 mm in seasons of March to May and September to December. The average daily temperatures received range from 17˚C to 27˚C throughout the year [24]. Soils in the wetland are characterized by intermittent wetting and drying in the wet and dry seasons respectively, clayey in nature with greyish and dark soils and are classified as Gleysols [25]. Busega wetland, a reserved natural ecosystem is currently under threat by anthropogenic activities including agriculture. Farmers have encroached on the wetland growing mainly maize (Zea mays L.), sweet potatoes (Ipomea batatas L.), bananas (Musa spp), sugarcanes (Saccharum officinarum), and yams (Dioscoreaceae spp.) [26].

2.2. Field Selection and Soil Sampling

A preparatory reconnaissance transect survey was conducted across the wetland in December 2021 through which the main cropping systems in the wetland were identified. Both annual and perennial cropping systems were observed in the encroached area of the wetland. Annual cropping system in this study includes crops that complete their life cycle within one growing season, and the dominant annual crop grown in the studied area was maize (Zea mays). On the other hand, perennial cropping system includes crops that can live for more than two years without being replanted each year. Banana (Musa spp) was the dominant perennial crop in the studied area. Part of the land is undisturbed, which refers to the part of the wetland still in its natural state and has never been encroached with any human activities. In this study, the two systems of cultivation (annual and perennial) as well as the undisturbed area are referred to as the three types of land use (LU). There was no overlap among the three land use types.

For this study, purposive sampling was done separately to identify areas under annual and perennial cropping systems, and the undisturbed wetland area. For the two cropping systems, various fields were selected from the upper, middle, and lower areas of the wetland which represented three different blocks. Soil sampling was done during the dry season in December 2021 when the wetland was not very wet. Soil samples for analysis were collected using a 2.1-inch diameter auger following the zigzag method in the cropped land and the undisturbed area. Three soil samples at varying depths of 0 - 10 cm, 10 - 20 cm, and 20 - 30 cm from each cropping system and the undisturbed area per block were collected (Table 1). The study site and soil sampled area are shown in Figure 1. Soil clods, stones and objects were carefully removed using hands. The soil in the bucket was mixed thoroughly and a representative sample taken. All soil samples were packed in plastic bags and taken to Makerere Soil Science Laboratory, Kampala for soil laboratory analysis (during December 2021 to May 2022). All soil samples were placed on trays and hand crushed to avoid sticking together. The soil was air-dried at room temperature in an open-air space room for 14 days. The dried soil was crushed further using a mortar and pestle and sieved through a 2 mm sieve to get rid of gravels and plant roots.

Table 1. Experimental field layout.

Blocks(replicate)	Perennial (P) and depth (banana)	Annual (A) and depth (maize)	Undisturbed part (U)
1	P1 P1 P1	A1 A1 A1	U1 U1 U1
2	P2 P2 P2	A2 A2 A2	U2 U2 U2
3	P3 P3 P3	A3 A3 A3	U3 U3 U3

Where, P is perennial (banana), A is annual (maize) and U is undisturbed whereby P1 = (0 - 10) cm, P2 = (20 - 30) cm, P3 = (20 - 30) cm, A1 = (0 - 10) cm, A2 = (10 - 20) cm, A3 = (20 - 30) cm, U1 = (0 - 10) cm, U2 = (10 - 20) cm, U3 = (20 - 30) cm.

Figure 1. Study area map of Busega wetland. Red, yellow and black bold circles represent the fields sampled in this study.

2.3. Soil Analysis

Soil texture was determined by the hydrometer (Bouyoucos) method. Briefly, 50 g of air-dried sieved soil were saturated with distilled water and 10 ml of 10% sodium hexametaphosphate (Calgon solution) in a buffer cup [27]. The percentage of sand, silt and clay were used to determine the textural classes of the soil samples using the USDA-soil textural triangle [28]. Total N was determined using the Kjeldahl digestion and distillation method. Briefly, a mixture of 0.3 g of sieved soil and 2.5 ml of the digestion mixture was digested at 110˚C, followed by steam distillation using 40% sodium hydroxide [29]. The Bray 1 method was used to determine available P where 1 g of air-dried sieved soil was extracted using 10 ml of Bray1 extractant (0.025 M HCl and 0.03 M NH₄F) and shaken for 5 minutes. The concentrations of extracted P were measured colorimetrically at a wavelength of 880 nm [30]. Total organic carbon was determined using the Walkley-Black method. Briefly, 5 ml of potassium dichromate solution and 7.5 ml of concentrated sulphuric acid were added to 0.5 g of ground soil for carbon oxidation and digested for 30 minutes at a temperature of 155˚C. The excess volume of potassium dichromate was titrated with 0.5 M ferrous ammonium sulphate. The sample and blank titres were used in calculation of % carbon [31]. Soil pH was measured using the 1:2.5 (w/v) soil to water ratio method. This involved adding 50 ml of distilled water to 20 g of air-dried sieved soil. The mixture was vigorously shaken using a mechanical shaker. The pH values were read from the pH meter of model (AD1000, Adwa, Romania) [32]. Exchangeable cations of potassium (K) and calcium (Ca) were determined by adding 100 ml of 1M ammonium acetate to 5 g of air-dried sieved soil and shaken for 30 minutes. The extracts were filtered using No.42 Whatman paper and the concentration of K and Ca analyzed using flame photometry and atomic absorption spectrophotometry respectively [33].

2.4. Data Analysis

Statistical analyses were performed using R version 4.3.1 (R Core Team, 2023). Prior to data analysis, data normality was tested using the Shapiro Wilk test. Data transformation was done on data that was not normally distributed to meet assumptions of ANOVA. Two-factor Analysis of Variance (ANOVA) was conducted (Land Use Type (LU), Depth (D) and Land Use * Depth interaction) and blocks as a random effect. Treatment means that were significantly different were separated using Tukey’s Honestly Significant Difference (HSD) test at p ≤ 0.05.

3. Results

3.1. Effect of Land Use Type and Depth on Soil Texture

Table 2. Effect of land use type and depth on soil texture in Busega wetland.

Land use type	Annual			Perennial			Undisturbed			LSD0.05	CV%
Depth (cm)	0 - 10	10 - 20	20 - 30	0 - 10	10 - 20	20 - 30	0 - 10	10 - 20	20 - 30
Sand %	75.0	76.3	78.3	65.0	61.7	57.7	69.0	62.3	73.0	10.8	3.5
Silt %	15.7	14.3	11.7	21.7	23.0	25.7	23.7	29.7	15.7	8.0	7.2
Clay %	12.7	12.7	10.0	18.7	19.3	22.7	15.3	22.0	16.0	7.0	28.8

There was no interactive effect of land use type * depth on the sand (p = 0.277) and clay (p = 0.877) soil proportions (Table 2). However, the interaction was slightly significant for silt (p = 0.054) showing inconsistent changes in silt content in the different land use types at the various soil depths. Increase in soil depth (from 0 - 10 cm to 10 - 20 cm to 20 - 30 cm) resulted in a decrease in silt content under annual cropping system (15.7% to 14.3% to 11.7%) respectively. In contrast, an increase in soil depth (from 0 - 10 cm to 10 - 20 cm to 20 - 30 cm) resulted in an increase in silt content (21.7% to 23% to 25.7%) respectively under perennial cropping systems. For the undisturbed wetland area, silt content increased from 23.7% to 29.7% and decreased to 15.7% as soil depth increased down the soil profile. Land use types significantly affected sand (p = 0.0004), clay (p = 0.001) and silt (p = 0.0005) content. Sand proportions were significantly higher than silt and clay content under all land use types. The annual cropping system had the highest sand proportion = 76.5% compared to both the perennial cropping system (61.4%) and the undisturbed wetland area (68.1%) (Table 2) although the trends under different cropping systems were inconsistent. Perennial cropping system had the highest silt content (23.46%), followed by the undisturbed wetland area (23.03%) while the lowest silt content (13.9%) was found under the annual cropping system. Clay content significantly varied across the different land use types with more clay (15.1%) under perennial cropping system and the least clay content (10%) under annual cropping system (Table 2). The main effect of soil depth alone had no significant effect on sand (p = 0.543), clay (p = 0.231) and silt (p = 0.129) proportions.

3.2. Effect of Land Use Type and Depth on Soil Organic Matter

The interactive effect of land use type * depth on soil organic matter (SOM) was not significant while the main effect of land use type significantly (p = 0.005) influenced SOM. Soils under perennial cropping system had the highest SOM content (1.45%) compared to the annual cropping system (0.56%) and the undisturbed wetland areas (1.25%) (Table 3). The main effect of depth highly (p = 0.012) influenced SOM. At 0 - 10 cm soil depth, SOM (1.60%) under perennial cropping system was more than twice as that found in annual cropping system (0.68%). Soil organic matter at 10 - 20 cm soil depth for perennial cropping system and annual cropping systems area almost doubled that obtained under the undisturbed wetland area at the same depth. At 20 - 30 cm soil depth, SOM under perennial cropping system was significantly higher than SOM under annual cropping system and the undisturbed wetland area. The amount of SOM was in order of Undisturbed < Annual < Perennial land use type when averaged across the three soil depths (Table 3).

Table 3. Effect of land use type and depth on soil organic matter (%) and soil pH.

Land use type	Annual			Perennial			Undisturbed			LSD0.5	CV%
Depth (cm)	0 - 10	10 - 20	20 - 30	0 - 10	10 - 20	20 - 30	0 - 10	10 - 20	20 - 30
SOM (%)	0.68	1.58	1.44	1.60	1.88	1.71	0.41	0.87	0.59	0.88	4.60
Soil pH	6.57	6.77	6.17	5.89	6.05	6.38	5.60	5.50	5.76	8.00	0.48

3.3. Effect of Land Use Type and Depth on Soil pH

The interactive effect of land use type * depth significantly (p = 0.043) affected soil pH (Table 3). The highest (6.77) soil pH was observed under annual cropping system at 10 - 20 cm soil depth. Similarly, the lowest (5.50) soil pH was observed at 10 - 20 cm soil depth but in the undisturbed wetland area. There were inconsistent changes in soil pH with increment in soil depth across the different land use types. For example, as soil depth increased from 0 - 10 cm to 10 - 20 cm, there was an increase in soil pH i.e. (6.57 to 6.77), and (5.89 to 6.05) in annual and perennial cropping systems respectively. Contrastingly, this trend was not observed under the undisturbed wetland area. Soil pH decreased with increase in depth with higher soil pH (5.60) obtained at 0 - 10 cm and lower pH (5.50) recorded at 10 - 20 cm. Soil pH increased with depth for perennial cropping system (from 6.05 to 6.38 for 10 - 20 cm to 20 - 30 cm) and under the undisturbed wetland area (5.50 to 5.76 from 10 - 20 cm to 20 - 30 cm soil depth). In contrast, soil pH decreased (from 6.77 to 6.17) under annual cropping system from soil depths at 10 - 20 cm to 20 - 30 cm (Table 3).

3.4. Effect of Land Use Type and Depth on N and P Concentrations

The interaction of land use type * depth had no significant effect on N stock (p = 0.067). We observed significant differences of N among different land use types (p = 0.011). The annual cropping systems recorded significantly lower N (0.073%) followed by perennial (0.170%) and the undisturbed wetland area (0.244%). The main effect of depth alone significantly influenced soil N levels (p = 0.025) with 10 - 20 cm depth recording the highest N (0.220%) and the lowest N (0.078%) at 20 - 30 cm soil depth (Figure 2).

Means followed by the same letter are not significantly different according to Tukey’s honestly significantly different test (p ≤ 0.05).

Figure 2. Mean interactive effect of land use type and depth on N levels in Busega wetland as of December 2021.

Means followed by the same letter are not significantly different according to Tukey’s honestly significantly different test (p ≤ 0.05).

Figure 3. Mean interactive effect of land use type and depth on available P in Busega wetland as of December 2021.

The interaction of land use type * depth had no significant effect on P concentrations (Figure 3). However, P concentrations significantly varied (p < 0.001) within different land use types. The P concentrations were significantly higher under the annual cropping system but decreased by about 40% to 20% within perennial cropping system and the undisturbed wetland area respectively. The P concentrations did not vary across soil depths (p = 0.492). Overall, the P levels were low in the soils of the study area.

3.5. Effect of Land Use Type and Depth on Ca and K Concentrations

The interaction between land use type * depth on Ca concentrations was not significant (p = 0.256) (Figure 4). In contrast, the main effect of land use type was strongly significant on soil Ca concentrations (p < 0.001). High Ca levels were obtained under the perennial cropping system while annual cropping system and the undisturbed wetland area had relatively lower Ca levels. Soil Ca concentrations significantly differed across the various soil depths (p = 0.003). Higher Ca concentration was observed in the topsoil of 0 - 10 cm, 10 - 20 cm than in deeper soils of 20 - 30 cm. Averaging all land use types, the highest Ca levels were obtained at 10 - 20 cm soil depth (1.741 Cmol/Kg) while the lowest Ca (0.963 Cmol/Kg) was found at the 20 - 30 cm soil depth (Figure 4).

Means followed by the same letter are not significantly different according to Tukey’s honestly significantly different test (p ≤ 0.05).

Figure 4. Mean interactive effect of land use type and depth on Ca levels in Busega wetland as of December 2021.

Means followed by the same letter are not significantly different according to Tukey’s honestly significantly different test (p ≤ 0.05).

Figure 5. Mean interactive effect of land use type and depth on K levels in Busega wetland as of December 2021.

Similarly to Ca, the interactive effect of land use type * depth was not significant on K concentrations (p = 0.424). (Figure 5). In contrast, the main effect of land use type had a great significant effect on K concetrations (p < 0.001). Perennial cropping system had significantly higher K concetrations, followed by the annual cropping system and the undisturbed wetland area. The main effect of soil depth alone was not significant on K concentrations in the present study (p = 0.110).

4. Discussion

4.1. Effects of Land Use Types and Soil Depth on Soil Texture

The lack of significant interactive effect of depth and land use type on sand and clay proportions justifies that soil texture is an inherent stable physical property. Soil texture is also one of the physicochemical soil properties that does not change easily in a short time and is characterized by its stability to withstand mechanical pressures [34] [35]. The inconsistent trends in silt content under the different land use types at the varying soil depths was due to the susceptibility of silt particles to erosive forces and differences in vegetative cover under the various land use types. Literature has reported that the small nature of silt particles compared to sand makes silt more prone to soil erosion [36]. The differences in clay, silt and sand contents in the different land use types is due to the difference in crops grown, agronomic practices such as manure additions and tillage. The higher clay content in perennial cropping system and the undisturbed wetland area could be due to clay deposition caused by limited cultivation and soil erosion. The higher sand content under annual cropping system is attributed to seasonal land preparation that promotes loss of clay and silt proportions through soil erosion. Results from many studies showed that soil texture varied across different land use types owing to differences in weather condition, agronomic practices, the cropping system duration, and topographical variations [36]-[38]. The absence of soil depth effects on soil texture could be explained by the same parent material from which soils of the wide region of the wetland developed. Busega wetland soils are Gleysols characterized by intermittent wetting and drying, that developed from clayey parent material with a higher percentage of sand soil proportions [39]. In contrast to our results, other studies found significant changes in clay, silt, and sand proportions across various depths within soil profiles [40] [41].

4.2. Effects of Land Use Types and Soil Depth on Soil Organic Matter

The higher amount of SOM under perennial cropping system is due to limited cultivation and the accumulation of organic materials that later decompose to form SOM [42] [43]. Under annual cropping systems, low SOM values result from repeated cultivation, which increases topsoil loss through soil erosion and facilitates microbial organic matter breakdown [44] [45]. The unexpected low SOM in the undisturbed wetland area could have been a result of limited plant litter. Another reason for low SOM in the undisturbed wetland area is the anaerobic soil conditions (since this part is still covered by water) that are known to lower the decomposition of plant litter which builds SOM [46] [47]. The lower SOM at 0 - 10 cm soil depth could be due to increased mineralization of the SOM into energy and nutrients for plant utilization. On the other hand, the higher SOM at 10 - 20 cm is due to accumulation of organic materials such as dried leaves, dead leaves, grasses, and other decomposed organic matter common at this depth leading to SOM accumulation. Furthermore, the higher SOM at 10 - 20 cm soil depth could be related to the much root density growing in this range of soil depth. A reduction in SOM down the soil profile can be attributed to a reduced deposition of plant materials that form SOM and reduced microbial activity in deeper soils. This lower SOM at a depth of 20 - 30 cm indicates that the soil in the studied area (Figure 1) has a well-developed soil profile that separates the upper and subsequent soil layers.

4.3. Effects of Land Use Types and Soil Depth on Soil pH, N and P

The significant interactive effect of land use type and depth on soil pH indicates the importance of these two studied factors in determining acidity and alkalinity of the soil solution. Higher soil pH values under annual cropping systems could be attributed to a range of factors including the high sand content, agronomic practices such as fertilizer application for example calcium phosphate and lime that increases soil pH [36] [48]. The lowest soil pH under the undisturbed wetland area could have been due to increased clay content compared to other land use types. High clay content reduces hydrogen ions on the clay colloidal surfaces resulting in low soil pH. This occurs when base cations replace hydrogen ions responsible for acidity on the clay surfaces leading to an increase of hydrogen ions in the soil solution [49]. Overall, the abrupt changes in soil pH in different land uses at different depths can be attributed to the soil’s buffering capacity. Soil pH is a sensitive parameter, and soils tend to resist changes in their pH caused by external forces [50] [51].

The low quantities of N in the annual cropping system can be attributed to N being one of the most limiting nutrients, readily utilized by crops such as maize that is widely grown in Busega wetland. In addition, over cultivation under annual cropping systems may facilitate N loss through water runoff, leaching and volatilization as the land is tilled repeatedly every season. These results are consistent with previous studies which reported that N is very mobile in the soil and is easily lost through different pathways with an increase in land clearing and tillage [52] [53]. It is possible that the higher N content in perennially cropped areas can be attributed to the higher SOM accrued from decomposition of plant matter and limited land cultivation in perennial crops. Several studies have reported an increase in N levels with an increase in SOM [42] [43]. As expected, N input and outputs influenced N concentrations in different soil layers across the soil profile with upper soil layers recording higher N stock due to SOM accumulation. Our results are in agreement with those of [54] who reported that N accumulated in upper soil layers where high SOM was found. The lower N stock in the lower soil layers could be due to plant nitrate utilization by the crop in the upper layers and low SOM leading to limited nitrate leaching.

The higher P concentrations under annual cropping system could be attributed to fertilizer application, given that soil P levels were low in the studied area. Another reason for these high P levels could be because of higher sand: clay ratio found under annual cropping system. In literature, soils with high clay content provide large surface area to strongly adsorb phosphate [55]-[57]. Contrarily, some studies found that the concentrations of available P were lower under annual cropping system than perennial cropping system as a result of water runoff, and crop removal [36] [58] [59]. The lower available P in perennial cropping system and the undisturbed wetland area is caused by the higher clay content that provides a large surface for phosphate sorption. Soil phosphate sorbed to clay surfaces makes it unavailable to be detected in the soil solutions [60]. Contrary to our study, [54] obtained higher levels of P under perennial cropping system which was attributed to reduced soil erosion and increased SOM. Available P was generally lower in the presently studied soils than the critical value of 30 kg∙P∙ha⁻¹ which is a characteristic of most Ugandan soils. This is because of phosphate sorption from high Fe and Al hydroxides dominant in these soils which causes low soil phosphate concentrations in the soil solution [54]. The low P levels are beneficial for the natural wetland soils to avoid increased risks of P loss to nearby water bodies in which Busega wetland discharges its water.

4.4. Effects of Land Use Types and Soil Depth on Base Cations, Ca and K

Unlike N and P, we found higher Ca and K levels under perennial cropping system than under annual cropping system and the undisturbed wetland area. The accumulation of Ca and K in the upper soil layers is explained by weathering of soil minerals which is a primary source of base cations and atmospheric deposition in topsoil [61]-[63]. Furthermore, higher amount of Ca and K in the surface layers may be due to management practices such as mulching, limited cultivation that promote soil and water conservation which reduce leaching of the base cations. The significant amount of Ca and K under perennial cropping systems as compared to annual and the undisturbed wetland area is explained by the functions of deeper root systems on recovering cations like Ca and K to upper soil layers through plant nutrient cycling [61] [62]. The results of this study show significant alterations in nutrient distribution across the varying soil depths under different land use types in the wetland calling for sustainable practices to maintain this natural ecosystem. However, soil samples used for this study were collected from the wetland during the dry season. To minimize the effects of seasonal variability on the results, the authors recommend that future studies collect soil samples during both the wet and dry seasons of the year.

5. Conclusion

The different land use types affected nutrient distribution across the varying soil depth in Busega wetland. The land use type and soil depth interaction had no significant effect on soil texture, SOM, N, P, Ca, and K except for soil pH. The different land use types had more significant influence on the soil nutrient distribution than soil depth. Overall, higher nutrient concentrations were observed under perennial cropping system than in both annual cropping system and the undisturbed wetland area. Soils under perennial cropping systems had higher SOM (1.45%) and N (0.170%) content than SOM (0.68%) and N (0.073%) under annual cropping systems. Our results show alteration in SOM and original nutrient stocks when cultivation is introduced in the wetland which could in turn affect the original capacity of this wetland to perform its ecosystem functions. Based on the findings of this preliminary study in Busega wetland, it becomes clear that wetland conservation is necessary due to their ecosystem-critical importance. Unlike perennial and annual crops currently grown in encroached areas of the Busega wetland, natural marshes and papyrus help conserve soil and water by purifying water, cycling nutrients, controlling floods, and minimizing erosion. We suggest further research to better understand the effects of cultivation introduced to this natural wetland on water quality.

Acknowledgements

Abuni Daniel would like to thank Dr. Sylvia Nyamaizi for supervising him during this undergraduate research project that led to the production of this research paper. The authors would like to acknowledge the support of Dr. Sylvia Nyamaizi from The University of British Columbia for funding this research through the Land Use Effects on Soil and Water Quality (LUESAW) project in Uganda, Africa.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References

[1]	Alikhani, S., Nummi, P. and Ojala, A. (2021) Urban Wetlands: A Review on Ecological and Cultural Values. Water, 13, Article 3301. https://doi.org/10.3390/w13223301
[2]	Delle Grazie, F.M. and Gill, L.W. (2022) Review of the Ecosystem Services of Temperate Wetlands and Their Valuation Tools. Water, 14, Article 1345. https://doi.org/10.3390/w14091345
[3]	Ballut-Dajud, G.A., Sandoval Herazo, L.C., Fernández-Lambert, G., Marín-Muñiz, J.L., López Méndez, M.C. and Betanzo-Torres, E.A. (2022) Factors Affecting Wetland Loss: A Review. Land, 11, Article 434. https://doi.org/10.3390/land11030434
[4]	Fynn, R.W.S., Murray-Hudson, M., Dhliwayo, M. and Scholte, P. (2015) African Wetlands and Their Seasonal Use by Wild and Domestic Herbivores. Wetlands Ecology and Management, 23, 559-581. https://doi.org/10.1007/s11273-015-9430-6
[5]	Matovu, B., Sarfo, I., Bbira, Y., Yeboah, E., Muhoozi, Y. and Lukambagire, I. (2024) Navigating through Complexity by Profiling the Main Threats to Sustainable Tropical Wetlands Management and Governance: A Case Study of Mityana District, Uganda. Discover Environment, 2, Article No. 18. https://doi.org/10.1007/s44274-024-00041-5
[6]	Kalanzi, W. (2015) Local Governments and Wetland Conservation in Uganda: Contributions and Challenges. Journal of Public Administration, 50, 157-171.
[7]	James Gideon, O. and Bernard, B. (2018) Effects of Human Wetland Encroachment on the Degradation of Lubigi Wetland System, Kampala City Uganda. Environment and Ecology Research, 6, 562-570. https://doi.org/10.13189/eer.2018.060606
[8]	Wang, Q., Liu, J., Wang, Y., Guan, J., Liu, Q. and Lv, D. (2012) Land Use Effects on Soil Quality along a Native Wetland to Cropland Chronosequence. European Journal of Soil Biology, 53, 114-120. https://doi.org/10.1016/j.ejsobi.2012.09.008
[9]	Ntambirweki, J. (1998) The Evolution of Policy and Legislation on Wetlands in Uganda. Case Study Prep Tech Consult des Methodol to Rev Laws Institutions Relev to Wetl Switz. http://scholar.google.com/scholar?hl=en&q=%0A+%0A+Ntambirweki+%2C+John+.+1998.+The+evolution+of+policy+and+legislation+on+wetlands+in+Uganda+.+In+Designing+methodologies+to+review+laws+and+institutions+relevant+to+wetlands.+Gland%2C+Switzerland+%3A+Ramsar+.+Ntambirweki%2C+John.+1998.+The+evolution+of+policy+and+legislation+on+wetlands+in+Uganda.+In+Designing+methodolo-gies+to+review+laws+and+institutions+relevant+to+wetlands.+Gland%2C+Switzerland%3A+Ramsar.+
[10]	Mafabi, P. (2000) The Role of Wetland Policies in the Conservation of Waterbirds: The Case of Uganda. Ostrich, 71, 96-98. https://doi.org/10.1080/00306525.2000.9639880
[11]	Kansiime, F., Kateyo, E., Oryem-Origa, H. and Mucunguzi, P. (2007) Nutrient Status and Retention in Pristine and Disturbed Wetlands in Uganda: Management Implications. Wetlands Ecology and Management, 15, 453-467. https://doi.org/10.1007/s11273-007-9054-6
[12]	Toor, M.D. (2021) Nutrients and Their Importance in Agriculture Crop Production: A Review. Indian Journal of Pure & Applied Biosciences, 9, 1-6. https://doi.org/10.18782/2582-2845.8527
[13]	Kundu, S., Kundu, B., Rana, N.K. and Mahato, S. (2024) Wetland Degradation and Its Impacts on Livelihoods and Sustainable Development Goals: An Overview. Sustainable Production and Consumption, 48, 419-434. https://doi.org/10.1016/j.spc.2024.05.024
[14]	Lal, R. (2015) Restoring Soil Quality to Mitigate Soil Degradation. Sustainability, 7, 5875-5895. https://doi.org/10.3390/su7055875
[15]	Kayima, J.K., Mayo, A.W. and Nobert, J. (2018) Ecological Characteristics and Morphological Features of the Lubigi Wetland in Uganda. Environment and Ecology Research, 6, 218-228. https://doi.org/10.13189/eer.2018.060402
[16]	Zhang, Y., Wang, L., Jiang, J., Zhang, J., Zhang, Z. and Zhang, M. (2022) Application of Soil Quality Index to Determine the Effects of Different Vegetation Types on Soil Quality in the Yellow River Delta Wetland. Ecological Indicators, 141, Article ID: 109116. https://doi.org/10.1016/j.ecolind.2022.109116
[17]	Li, Y., Gong, J., Liu, J., Hou, W., Moroenyane, I., Liu, Y., et al. (2022) Effects of Different Land Use Types and Soil Depth on Soil Nutrients and Soil Bacterial Communities in a Karst Area, Southwest China. Soil Systems, 6, Article 20. https://doi.org/10.3390/soilsystems6010020
[18]	Fetene, E.M. and Amera, M.Y. (2018) The Effects of Land Use Types and Soil Depth on Soil Properties of Agedit Watershed, Northwest Ethiopia. Ethiopian Journal of Science and Technology, 11, 39-56. https://doi.org/10.4314/ejst.v11i1.4
[19]	Gong, J., Hou, W., Liu, J., Malik, K., Kong, X., Wang, L., et al. (2022) Effects of Different Land Use Types and Soil Depths on Soil Mineral Elements, Soil Enzyme Activity, and Fungal Community in Karst Area of Southwest China. International Journal of Environmental Research and Public Health, 19, Article 3120. https://doi.org/10.3390/ijerph19053120
[20]	Turyahabwe, N., Kakuru, W., Tweheyo, M. and Tumusiime, D.M. (2013) Contribution of Wetland Resources to Household Food Security in Uganda. Agriculture & Food Security, 2, Article No. 5. https://doi.org/10.1186/2048-7010-2-5
[21]	Kumar, N., Kumar, A., Marwein, B.M., Verma, D.K., Jayabalan, I., Kumar, A., et al. (2021) Agricultural Activities Causing Water Pollution and Its Mitigation—A Review. International Journal of Modern Agriculture, 10, 590-609.
[22]	Zahoor, I. and Mushtaq, A. (2023) Water Pollution from Agricultural Activities: A Critical Global Review. International Journal of Chemical and Biochemical Sciences, 23, 164-176.
[23]	Kayima, J.K. and Mayo, A.W. (2020) Nitrogen Removal Buffer Capacity of the Lubigi Wetland in Uganda. Physics and Chemistry of the Earth, Parts A/B/C, 117, Article ID: 102883. https://doi.org/10.1016/j.pce.2020.102883
[24]	Opio, A., Jones, M.B., Kansiime, F. and Otiti, T. (2014) Growth and Development of Cyperus papyrus in a Tropical Wetland. Open Journal of Ecology, 4, 113-123.
[25]	FAO (2014) World Reference Base for Soil Resources 2014, Update 2015 (World Soil Resources Reports 106).
[26]	James Gideon, O. and Bernard, B. (2018) Effects of Human Wetland Encroachment on the Degradation of Lubigi Wetland System, Kampala City Uganda. Environment and Ecology Research, 6, 562-570. https://doi.org/10.13189/eer.2018.060606
[27]	Gee, G.W. and Bauder, J.W. (1986) Particle-Size Analysis. In: Klute, A., Ed., Methods of Soil Analysis: Part 1, Physical and Mineralogical Methods, ASA, SSSA, 383-411. https://doi.org/10.1007/978-3-540-31211-6_2
[28]	Whitney, M. (1911) The Use of Soils East of the Great Plains Region. Forgotten Books.
[29]	Jackson, M. (1973) Soil Chemical Analysis. Prentice Hall, 498.
[30]	Bray, R.H. and Kurtz, L.T. (1945) Determination of Total, Organic, and Available Forms of Phosphorus in Soils. Soil Science, 59, 39-46. https://doi.org/10.1097/00010694-194501000-00006
[31]	Nelson, D.W. and Sommers, L.E. (1982) Total Carbon, Organic Carbon, and Organic Matter. In: Page, A.L., Ed., Methods of Soil Analysis, Part 2—Chemical and Microbiological Properties, American Society of Agronomy, 539-579.
[32]	Hendershot, W., Lalande, H. and Duquette, M. (2007) Soil Reaction and Exchangeable Acidity. In: Carter, M.R. and Gregorich, E., Eds., Soil Sampling and Methods of Analysis, Second Edition, CRC Press, 173-178. https://doi.org/10.1201/9781420005271.ch16
[33]	Hendershot, W., Lalande, H. and Duquette, M. (2007) Ion Exchange and Exchangeable Cations. In: Carter, M.R. and Gregorich, E., Eds., Soil Sampling and Methods of Analysis, Second Edition, CRC Press, 196-206. https://doi.org/10.1201/9781420005271.ch18
[34]	Yan, F., Fu, Y., Paradelo, M., Zhang, F. and Arthur, E. (2023) Long-term Manure and Cropping Systems Effect on Soil Water Vapour Sorption Characteristics Is Controlled by Soil Texture. Geoderma, 436, Article ID: 116533. https://doi.org/10.1016/j.geoderma.2023.116533
[35]	Amsili, J.P., van Es, H.M. and Schindelbeck, R.R. (2021) Cropping System and Soil Texture Shape Soil Health Outcomes and Scoring Functions. Soil Security, 4, Article ID: 100012. https://doi.org/10.1016/j.soisec.2021.100012
[36]	Tesfahunegn, G.B. and Gebru, T.A. (2020) Variation in Soil Properties under Different Cropping and Other Land-Use Systems in Dura Catchment, Northern Ethiopia. PLOS ONE, 15, e0222476. https://doi.org/10.1371/journal.pone.0222476
[37]	Voundi Nkana, J.C. and Tonye, J. (2002) Assessment of Certain Soil Properties Related to Different Land-Use Systems in the Kaya Watershed of the Humid Forest Zone of Cameroon. Land Degradation & Development, 14, 57-67. https://doi.org/10.1002/ldr.519
[38]	Agoumé, V. and Birang, A.M. (2009) Impact of Land-Use Systems on Some Physical and Chemical Soil Properties of an Oxisol in the Humid Forest Zone of Southern Cameroon. Tropicultura, 27, 15-20.
[39]	Kyebogola, S., Burras, L.C., Miller, B.A., Semalulu, O., Yost, R.S., Tenywa, M.M., et al. (2020) Comparing Uganda's Indigenous Soil Classification System with World Reference Base and USDA Soil Taxonomy to Predict Soil Productivity. Geoderma Regional, 22, e00296. https://doi.org/10.1016/j.geodrs.2020.e00296
[40]	Coblinski, J.A., Giasson, É., Demattê, J.A.M., Dotto, A.C., Costa, J.J.F. and Vašát, R. (2020) Prediction of Soil Texture Classes through Different Wavelength Regions of Reflectance Spectroscopy at Various Soil Depths. CATENA, 189, Article ID: 104485. https://doi.org/10.1016/j.catena.2020.104485
[41]	Li, X., Chang, S.X. and Salifu, K.F. (2014) Soil Texture and Layering Effects on Water and Salt Dynamics in the Presence of a Water Table: A Review. Environmental Reviews, 22, 41-50. https://doi.org/10.1139/er-2013-0035
[42]	Ledo, A., Smith, P., Zerihun, A., Whitaker, J., Vicente‐Vicente, J.L., Qin, Z., et al. (2020) Changes in Soil Organic Carbon under Perennial Crops. Global Change Biology, 26, 4158-4168. https://doi.org/10.1111/gcb.15120
[43]	Bai, Z., Caspari, T., Gonzalez, M.R., Batjes, N.H., Mäder, P., Bünemann, E.K., et al. (2018) Effects of Agricultural Management Practices on Soil Quality: A Review of Long-Term Experiments for Europe and China. Agriculture, Ecosystems & Environment, 265, 1-7. https://doi.org/10.1016/j.agee.2018.05.028
[44]	Burle, M.L., Mielniczuk, J. and Focchi, S. (1997) Effect of Cropping Systems on Soil Chemical Characteristics, with Emphasis on Soil Acidification. Plant and Soil, 190, 309-316. https://doi.org/10.1023/a:1004266831343
[45]	Li, Q., Gu, F., Zhou, Y., Xu, T., Wang, L., Zuo, Q., et al. (2021) Changes in the Impacts of Topographic Factors, Soil Texture, and Cropping Systems on Topsoil Chemical Properties in the Mountainous Areas of the Subtropical Monsoon Region from 2007 to 2017: A Case Study in Hefeng, China. International Journal of Environmental Research and Public Health, 18, Article 832. https://doi.org/10.3390/ijerph18020832
[46]	Steinmuller, H.E. and Chambers, L.G. (2019) Characterization of Coastal Wetland Soil Organic Matter: Implications for Wetland Submergence. Science of the Total Environment, 677, 648-659. https://doi.org/10.1016/j.scitotenv.2019.04.405
[47]	McLatchey, G.P. and Reddy, K.R. (1998) Regulation of Organic Matter Decomposition and Nutrient Release in a Wetland Soil. Journal of Environmental Quality, 27, 1268-1274. https://doi.org/10.2134/jeq1998.00472425002700050036x
[48]	Daba, N.A., Li, D., Huang, J., Han, T., Zhang, L., Ali, S., et al. (2021) Long-Term Fertilization and Lime-Induced Soil pH Changes Affect Nitrogen Use Efficiency and Grain Yields in Acidic Soil under Wheat-Maize Rotation. Agronomy, 11, Article 2069. https://doi.org/10.3390/agronomy11102069
[49]	Emiru, N. and Gebrekidan, H. (2013) Effect of Land Use Changes and Soil Depth on Soil Organic Matter, Total Nitrogen and Available Phosphorus Contents of Soils in Senbat Water-Shed, Western Ethiopia. ARPN Journal of Agricultural and Biological Science, 8, 206-212.
[50]	Lu, H., Li, K., Nkoh, J.N., Shi, Y., He, X., Hong, Z., et al. (2022) Effects of the Increases in Soil pH and pH Buffering Capacity Induced by Crop Residue Biochars on Available Cd Contents in Acidic Paddy Soils. Chemosphere, 301, Article ID: 134674. https://doi.org/10.1016/j.chemosphere.2022.134674
[51]	Ng, J.F., Ahmed, O.H., Jalloh, M.B., Omar, L., Kwan, Y.M., Musah, A.A., et al. (2022) Soil Nutrient Retention and pH Buffering Capacity Are Enhanced by Calciprill and Sodium Silicate. Agronomy, 12, Article 219. https://doi.org/10.3390/agronomy12010219
[52]	Hoang, H.G., Thuy, B.T.P., Lin, C., Vo, D.N., Tran, H.T., Bahari, M.B., et al. (2022) The Nitrogen Cycle and Mitigation Strategies for Nitrogen Loss during Organic Waste Composting: A Review. Chemosphere, 300, Article ID: 134514. https://doi.org/10.1016/j.chemosphere.2022.134514
[53]	Mahmud, K., Panday, D., Mergoum, A. and Missaoui, A. (2021) Nitrogen Losses and Potential Mitigation Strategies for a Sustainable Agroecosystem. Sustainability, 13, Article 2400. https://doi.org/10.3390/su13042400
[54]	Nyamaizi, S., Olupot, G., Tumuhe, C.L., Basamba, T.A. and Musinguzi, J. (2018) Impact of Toposequence and Type of Cropping System on Soil Properties in Mid-Western Uganda. International Journal of Advanced Technology and Innovative Research, 10, 808-815. https://www.ijatir.org
[55]	Johan, P.D., Ahmed, O.H., Omar, L. and Hasbullah, N.A. (2021) Phosphorus Transformation in Soils Following Co-Application of Charcoal and Wood Ash. Agronomy, 11, Article 2010. https://doi.org/10.3390/agronomy11102010
[56]	Gérard, F. (2016) Clay Minerals, Iron/Aluminum Oxides, and Their Contribution to Phosphate Sorption in Soils—A Myth Revisited. Geoderma, 262, 213-226. https://doi.org/10.1016/j.geoderma.2015.08.036
[57]	Atalay, A. (2001) Variation in Phosphorus Sorption with Soil Particle Size. Soil and Sediment Contamination: An International Journal, 10, 317-335. https://doi.org/10.1080/20015891109284
[58]	Nweke, I.A. and Nnabude, P.C. (2014) Organic Carbon, Total Nitrogen and Available Phosphorous Concentration in Aggregate Fractions of Four Soils under Two Land Use Systems. International Journal of Applied Research, 2, 273-288.
[59]	O’ Flynn, C.J., Fenton, O., Wall, D., Brennan, R.B., McLaughlin, M.J. and Healy, M.G. (2017) Influence of Soil Phosphorus Status, Texture, pH and Metal Content on the Efficacy of Amendments to Pig Slurry in Reducing Phosphorus Losses. Soil Use and Management, 34, 1-8. https://doi.org/10.1111/sum.12391
[60]	Nyamaizi, S., Messiga, A.J., Cade-Menun, B., Cornelis, J.-T. and Smukler, S.M. (2025) Environmental Phosphorus Risk Classes for Silage Corn in the Fraser Valley, Canada. Agriculture, Ecosystems & Environment, 381, Article ID: 109423. https://doi.org/10.1016/j.agee.2024.109423
[61]	Bedel, L., Poszwa, A., van der Heijden, G., Legout, A., Aquilina, L. and Ranger, J. (2016) Unexpected Calcium Sources in Deep Soil Layers in Low-Fertility Forest Soils Identified by Strontium Isotopes (Lorraine Plateau, Eastern France). Geoderma, 264, 103-116. https://doi.org/10.1016/j.geoderma.2015.09.020
[62]	Dijkstra, F.A. and Smits, M.M. (2002) Tree Species Effects on Calcium Cycling: The Role of Calcium Uptake in Deep Soils. Ecosystems, 5, 385-398. https://doi.org/10.1007/s10021-001-0082-4
[63]	Crews, T.E. (2005) Perennial Crops and Endogenous Nutrient Supplies. Renewable Agriculture and Food Systems, 20, 25-37. https://doi.org/10.1079/raf200497