Plants, like all other living things, require a variety of nutrients for their growth and development (Silva et al., 2000). Nutrients along with carbon dioxide (CO2), water (H2O) and sunlight collectively enable plants to convert light energy into chemical energy by the photo-synthesis, facilitating their growth and the providing oxygen to the environment. Plants require 17 essential elements where Carbon, Hydrogen and Oxygen are derived from the atmosphere, the soil and water; the remaining 13 vital components (Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sulphur, Iron, Zinc, Manganese, Copper, Boron, Molybdenum, Nickel, and Chlorine) are sourced from either soil minerals and organic matter or through the application of organic or the inorganic fertilizers (Sridhara et al., 2022). Soil nutrients are divided into two categories: macronutri-ents such as nitrogen, potassium and phosphorus etc. are required by plants in large quantities and micro-nutrients such as iron, zinc and copper etc. are needed in smaller amounts but are still essential for plant growth (Food, 2022). Every nutrient has its distinct function on plants. Among all nutrients, Nitro-gen (N), potassium (K), phosphorus (P), and sulfur (S) are four of the most crucial nutrients required by plants in large quantities (Silva et al., 2000; IPNI, 1999). Nitrogen is largely responsible for the growth of plant leaves; it is a component of chlorophyll, which is essential for photosynthesis, and is also a building block of amino acids, proteins, and nucleic acids (Gebreslassie, 2016; Plunkett et al., 2022). Phosphorus plays a pivotal role in root growth, flower and fruit development, seed production, as well as energy transfer and storage within plants (IPNI, 1999). Potassium aids in water balance regulation, the stress tolerance improvement, disease resistance enhancement, enzyme activation, and protein synthesis (Silva et al., 2000; Ekhlas et al., 2014; Asmare et al., 2023).  

Sulfur is important for the synthesis of amino acids, proteins, and enzymes which also involved in chloro-phyll formation and helps improve the efficiency of nitrogen utilization (Plunkett et al., 2022). The levels of soil nutrients may fluctuate based on the various factors, including soil composition, depth, prevailing climate conditions, and land management practices (Jiaying et al., 2022 and Leghari et al., 2016). For example, land use like intensive agriculture can deplete soil nutrients over time, while urban development can result in soil compaction and reduced water infiltration (Yousaf et al., 2021; Islam et al., 2021). Soil nutrient status in hilly agricultural land can vary depending on factors such as slope and elevation of land, soil texture, management techniques, and weather conditions (Leul et al., 2023). Steep slopes can make the land vulner-able to erosion, leading to the loss of topsoil, organic matter, and nutrients and leaching of nutrients can also occur during periods of high rainfall (Siswanto and Sule, 2019). Also, Soil depth has a substantial effect on organic matter, nutrient availability and plant growth. Nutrients tend to accumulate near the surface due to the decomposition of organic matters, root activity and, mineralization of nutrients (Eldin and Ibrahim, 2006). The upper layer of soil, known as the topsoil, usually contains the highest concentration of organic matters and nutrients. Soil pH can change with soil depth also. Changes in pH with depth can affect organic matter and the nutrient availability patterns (Grandgirard et al., 2002). Hilly soils have distinctive qualities because of terrain, climate, and geological formations among other things and these soils have particular physical, chemical, and biological charac-teristics that affect the availability and cycling of nutrients (Grandgirard et al., 2002). For efficient land management and sustainable forestry practices, it is critical to understand the status of the soil nutrients in hilly forest land. The variations in soil nutrient content (fertility) and the variables affecting their distribution in Chittagong Hill Tracts are highlighted in numerous researches (Rasul and Thapa, 2005; Hassan et al., 2017; Chowdhury et al., 2007; Uddin et al., 2017; and Al-Mamun et al., 2021).

Therefore, the objectives of this study were; (i) to evaluate the dynamic changes of soil organic matter (OM) and nutrients with the land use changes; and (ii) to investigate the dynamic changes of soil OM and nutrients (TN, P, K and S) in different soil depths in Bandarban Sadar Upazila. 

Table 1 shows the summary statistics for the soil vari-ables between two soil depths of different land uses (barren, agriculture and forest land) in the present study area. Fig. 4, 5, 6, and 7 show the graphical representation of soil pH, OM, TN, P, K & S pattern. To compare the significance differences among the land uses (barren vs agriculture vs forest), soil depth, plot numbers in terms of soil variables, three-way ANOVA was conducted and represented in Table 2, 3, 4, and 5 where significant differences are marked in bold. Residuals of the model were normally distributed according to the Shapiro-Wilk test and variances among the groups were homogeneous according to the Bartletts test. Fig. 3 also represent the correlation matrix among the soil variables. 

Summary statistics for the soil variables 

Table 1 provides a comprehensive overview of key soil properties across three different land cover types: agricultural (A), forested (F), and barren (B) land. For pH, agricultural land (A) has the highest mean value (6.8), while barren land (B) has the lowest (6.4), with forested land (F) falling in between (6.6). The mean values of pH in each land type indicate that it is relatively acidic. Organic matter (OM) percentage is highest in forested land (9.9%) and lowest in barren land (4.3%). Total nitrogen (TN) percentage shows similar trends, with the highest mean value in forested land (0.4%) and the lowest in barren land (0.1%). Phosphorus (P) content is highest in agricultural land (9.5µg/g) and lowest in barren land (6.2µg/g). Pota-ssium (K) content is relatively consistent across all land types, with small variations. Sulfur (S) content is highest in agricultural land (5.80µg/g) and lowest in barren land (4.1µg/g).

Correlation among soil variables

Overall, a significant positive correlation is observed between TN and OM, suggesting that when the TN content increases, the OM content also tends to rise. This association underscores the mutual influence of nitrogen (N) and OM dynamics in the soil. Moreover, a strong positive correlation is evident between TN and K, indicating that an increase in TN levels is typically accompanied by an increase in K content. This positive correlation highlights the interconnect-tedness of N and K in soil nutrient availability. In contrast, a weak negative correlation is found between P and S, implying that when P levels increase, there is a slight decrease in S content. Though relatively weak, this negative correlation suggests that P and S might influence each other to some extent.

Soil pH

In Table 2, factors such as Land use (A) and depth (C) showed significant effects, as indicated by the high  F-

values and very low P-values. Plot (B) also had a significant effect, although with a slightly higher P-value. The interactions among these factors (A × B, A × C, B × C, and A × B × C) also showed very signi-ficant effects on soil pH. These findings suggested that age, plot, and depth individually influence the soil pH, as well as their interactions have a substantial impact.

According to this bar graph represented in Fig. 4, the highest pH value is observed in the barren land for a 15-30 cm soil depth, while the lowest pH value is observed in the forest land for a 15-30 cm soil depth. Forest and agricultural land appeared to have a higher pH level in the upper soil layer (0-15 cm) than the lower soil layer (15-30 cm), but barren land showed a high pH in the lower soil level. Additionally, the pH values indicated that all land use types in our study area contain slightly acidic soils.

Soil Organic Matter (OM)

All the factors, as well as the interaction among these factors, showed highly significant and substantial impacts in determining soil organic matter as indicated by high F-values and very low P-values (Table 3).

According to Fig. 5, the upper soil layer (0-15 cm) appears to be more enriched with organic matter than the lower soil layer (15-30 cm), and the upper soil layer (0-15 cm) of forested areas is enriched with more organic matter than both agricultural and barren land. In contrast, barren land showed the lowest value of organic matter in both soil layers. The upper layer of agricultural land showed a moderate value, while the lower layer showed the highest value among the three land use systems.

Soil Total nitrogen (TN) and Phosphorus (P)

Table 4 shows all the factors, as well as their inter-actions showed highly significant & substantial impacts in determining soil TN and P, as indicated by high F-values and very low P-values. But the only inter-actions between land use and soil depth do not have a substantial impact because of high P-value (0.18113 and 0.0409) for soil TN and P respectively (Table 4).

Fig. 6 shows the upper soil layer (0-15 cm) is enriched with a higher TN & P than the lower layer (15-30 cm). Additionally, forest land showed a higher percentage of total TN & P compared to agricultural land and barren land in both soil layers.

Soil Potassium (K) & Sulfur (S)

Table 5 shows all the factors showed highly signi-ficant and substantial impacts in determining soil K & S, as indicated by high F-values and very low P-values. The interactions of these factors (A×B, A×C, B×C, and A×B×C) also had a significant effect, although with a slightly higher P-value respectively (Table 5).

Table 5: Three-way analysis of variance of soil K & S at different soil depths (0-15cm and 15-30cm) under different land uses (Agriculture, forest and barren land) in different plots (1, 2, 3, 4 and 5) of the study area.

Fig. 7: Bar graph showing soil K (a) & S (b) differences in 2 different (0-15 cm and 15-30 cm) soil depths at each land use type: agricultural, forest, and barren land; different letters indicate the significant differences in the same layer of different stand ages at the 0.05 level.

Fig. 7 shows the upper soil layer (0-15 cm) is enriched with higher soil K (a) & S (b) than the lower layer (15-30 cm). Additionally, forest land showed a higher percentage of soil K & S than agricultural land and barren land in both soil layers.

According to the results, it is clear that the value of soil pH is higher in agricultural land compared to forest land; as shown in Figure 4. Agricultural land typically has a higher soil pH due to the use of lime and other soil amendments to neutralize acidity and improve crop growth (Al-Mamun et al., 2021). In contrast, the forest land tends to have a lower soil pH due to the accumulation of organic matter and the presence of microorganisms that produce acidic compounds and this observation (Hossain et al., 2014; Wang et al., 2022; Yuan et al., 2023). Overall, these findings align with previous researches (Zhou et al., 2019; Filippi et al., 2019) and provide valuable insights into the rela-tionship between land use patterns, soil depths, and soil pH. In terms of soil organic matter (OM), forest land exhibits significantly higher levels of OM com-pared to agricultural land and barren land in hilly regions. A notable study conducted by Hossain et al. (2014), in hilly landscapes found that forested areas exhibited substantial accumulations of organic matter due to the continuous input of plant residues, leaf litter, and decaying organic material (Fang et al., 2015; Cardinael et al., 2020). This organic matter enrichment is a result of the diverse and abundant vegetation in forest ecosystems, promoting the accumulation of biomass and organic debris on the forest floor. Agri-cultural land practices such as tillage, harvesting, and removal of crop residues lead to reduced organic mat-ter input and accelerated decomposition, limiting the buildup of organic matter in the soil. In contrast, Barren land has low organic matter because it lacks vegetation and organic material, which are essential sources for the accumulation of organic matter in both lair of soil. These findings highlight the vital role of forests in carbon sequestration and the preservation of soil organic matter, emphasizing the importance of sustainable land management practices to maintain soil fertility and ecosystem health in hilly regions. Our findings indicate that the values of soil total nitrogen (TN), phosphorous (P) and potassium (K) is higher in forest land compared to agricultural land and barren land. Lowest TN and K values showed by barren land because it lacks vegetation and moisture content; and lowest P values showed by agricultural land. The dense and diverse vegetation in forests promotes nitro-gen fixation and assimilation by plants and micro-organisms, leading to increased nitrogen content in the soil (Sardar et al., 2023). Also, Forest vegetation efficiently capture and recycle phosphorus and potas-sium, contributing to its enrichment in the forest soil. Agricultural practices, such as intensive fertilization and nitrogen-demanding crops, often result in higher nitrogen losses through leaching and runoff, leading to reduced nitrogen retention in agricultural soils in Chittagong Hill Tract. Surprisingly, among all land use types, barren land and forest land showed maximum and minimum S content respectively. Limited nutrient cycling in barren land may cause maximum S content in barren land. Without thriving ecosystems to effi-ciently cycle nutrients, sulfur tends to remain more concentrated in the soil of barren lands, making it appear to have higher sulfur content compared to more fertile areas.  On the other hand, the value of soil TN, P, K and S are comparatively higher in the topsoil than the deep soil because of active nutrient cycling and accumulation of plant residue. However, the present study investigated the dynamics of soil pH, organic matter and nutrient (TN, P, K and S) in order to the understate their trend in 3 different land use types under 2 soil depths (0-15 cm, and 15-30 cm). And finally, the insights of our findings and observations are essential for optimizing agricultural production, preserving soil health, reducing environmental impact, and promoting sustainable land management practices in Bandarban Sadar Upazila.

Using Analysis of Variance and Post Hoc Analysis, this research examined how the soil parameters vary across 3 different land use patterns (agriculture, forest and barren) and 2 soil depths (0-15 cm and 15-30 cm) at Bandarban Sadar, Chittagong Hill Tracts of Bangla-desh. The findings of this research revealed significant differences in the soil nutrient levels among forest, agricultural, & barren lands, with specific implications for each land use type. Moreover, the study highlighted the vertical distribution of soil nutrients, with topsoil generally exhibiting higher nutrient concentrations compared to deeper soil layers. The results appeared that the agricultural land generally had higher soil S content and pH levels compared to forest land, while organic matter, total nitrogen, phosphorus and pota-ssium levels were relatively higher in forest land because of more litters and forest residue. The study also revealed that the topsoil tended to have higher nutrient concentrations than the deep soil layer due to biological activity and nutrients cycling. The correla-tion analyses provided valuable insights into the inter-play of soil variables that can aid in understanding nutrient interactions. Although the study has some limitations including relatively small sample size, the focusing on a specific region, not considering seasonal variations, the significance of the study concluded the successfully. For future research, it is suggested that longitudinal studies be conducted to observe soil nutri-ent dynamics over time, encompassing multiple regi-ons with varying soil types and environmental condi-tions for improved generalization. However, the over-all research underscores the significance of land use practices and soil depth as crucial determinants in shaping soil nutrient dynamics. The findings empha-size the importance of adopting sustainable land man-agement strategies to maintain soil health, preserve soil fertility, & promote long-term agricultural productivity at the Bandarban Sadar, Chittagong Hill Tracts of the Bangladesh.

The authors feel great pleasure to express his gratitude and thanks to S. M. Zobair Al Arman, Laboratory Assistant Agricultural Scientific Officer, SRDI for his cordial help and provide of important information, helpful discussion and explanation, laboratory facilities and valuable suggestion during lab work in SRDI, Noakhali.  

The authors declare no conflict of interest