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Current IssueEditorial BoardOnline First ArticlesArchive

Full Research Article

Impact of Agriculture and Road Construction on Soil Aggregates in Two Locations of Coastal Plain Sand, Southeastern Nigeria

U
U.C. Osakwe1,*
J
J.O. Ogunwole1
O
O. Omoju1
A
A.K. Oluleye1
K
K.A. Odetola1
1Department of Soil Science and Land Resources Management, Federal University Oye - Ekiti, Nigeria.

ABSTRACT

Background: Understanding how land use affects the stability of soil aggregates and soil organic carbon associated with water stable aggregates in soils within the coastal plain sands is crucial for sustainable management. This study examines how agriculture and road construction influence these factors in two locations of Abia State, Southeastern Nigeria.

Methods: The study was a 2 * 5 factorial experiment; denoting two locations (Umudike and Umuahia) and five land use practices which include Road Construction, Fallow, Grazing, Cultivation and Forest in a randomized complete block design. Soil sampling utilized the Transect method at a depth of 0 - 15 cm. Laboratory analysis on aggregate indices and pertinent soil characteristics were carried out.

Result: Soil texture differed between Umudike (sandy loam) and Umuahia (loamy sand). Umudike soil showed higher water-stable aggregates (WSA), lower CR (clay ratio: 7.19), DR (dispersion ratio: 0.36) clay dispersion index (CDI: 0.41), higher ASC (aggregated silt plus clay: 14.24 g/kg) and aggregated soil organic carbon (ASOC). Both locations are classified as erodible (DR > 15%). The main effect of land use revealed that forest, grazing and fallow enhanced aggregate stability and ASOC compared to cultivation and road construction.  The combined effect of location and land use on soil aggregates displayed variability of response, emphasizing the influence of particle size distribution, SOC status including management on aggregate indices. Correlation analysis supported this claim. Therefore, tailored conservation strategies are vital for enhancing structure and ASOC while asphalting with proper drainage is crucial to prevent land degradation.

INTRODUCTION

Stability of soil aggregate, reflecting particles’ adherence and resistance to disintegration, is crucial for carbon stabilization in aggregates (Ran et al., 2021). Soil organic carbon found in microaggregates enhances the formation of macroaggregates and plays a crucial role in the long-term retention of soil organic carbon (Sekaran et al., 2021). Stable aggregates prevent loss of associated carbon linked to soil erosion, maintain soil fertility, plant resilience to moisture fluctuations and mitigate climate change (Menon et al., 2020). Researchers have employed indices such as mean weight diameter (MWD) and colloidal dispersion indices to evaluate stability and predict soil erosion, particularly in tropical soils of Nigeria (Osakwe, 2018).
               
Coastal plain sand-derived soils as in Abia State are sandy; often exhibit low aggregate stability, clay and organic matter content, making them susceptible to erosion (Uluocha and Uwadiegwu 2015; Chukwu et al., 2023). They added that moderate soil erosion affects around 20% of the area (115,077.9 hectares). Additionally, 27% (60,271.4 hectares) is subject to low erosion and the remaining 33% of the State experiences very low levels of soil erosion. Therefore, regular assessment and evaluation for proper management is imperative. In addition, information on possible variability between soils in the study sites, crucial for site-specific management is lacking. Furthermore, Nigeria’s rapid urbanization, infrastructural development and agricultural expansion, while crucial for economic growth and food security, often overlook their impact on soil properties. Consequently, our study aimed to investigate how road construction and agriculture influence coastal plain sand soils in Abia State, southeastern Nigeria and the potential variability in response across locations.

MATERIALS AND METHODS

Description of study area
 
The research was conducted between 2018 and 2020 in Umudike and Umuahia between latitude 05o28′N and 05o32′N and longitude 007o28′E and 007o32′E, located in the rainforest agroecology of Abia State, southeastern Nigeria, characterized by a humid tropical climate (Igbozuruike, 1975). The area experiences average annual rainfall of 2176 mm  with minimum monthly air temperatures between 20oC and 24oC and maximum temperatures ranging from 28oC to 35oC. The humidity ranges from 51% to 87%. The primary parent material consists of Coastal Plain Sands, along with certain areas featuring alluvial sediments (Amanze et al., 2016). The soil type, classified as Ultisols, is sandy, predominantly kaolinite-based with low pH, base status and soil organic carbon (Lekwa and Whiteside 1986).
       
The land use history, methodology flow chart and location maps are presented in Table 1, Fig 1 and 2 respectively.

Table 1: Land use history.



Fig 1: Location maps.



Fig 2: Methodology flow chart.


 
Experimental design
 
The research was designed as a 2×5 factorial experiment in randomized complete block design (RCBD) with three replicates. The numbers are two locations (Umudike and Umuahia) and five land use practices (Fallow land, Arable land, Grazing land, Forest and Road construction). Each location’s designated land use areas were subdivided into three blocks and the transect method was used to collect soil samples from 0-15 cm depth. Samples from each block were combined to create 15 composite samples for each location. They were air-dried and passed through 4 mm and 2 mm sieves for laboratory analysis.
 
Laboratory analysis
 
The method of Gee and Bauder (1986) was used to assess the particle size distribution using sodium hexameta phosphate as a dispersing agent. Colloidal stability was determined by substituting water for the dispersing agent and computed as follows:




 
ASC =  % clay + % silt (calgon) - % clay + % silt (H2O)
 
CR = % Sand+ % silt/ % clay
                                                               (Bouyoucus, 1951),
 
DR, Dispersion Ratio, CDI, Clay Dispersion Index and ASC, Aggregated Silt plus Clay.
       
The method of Kemper and Rosenau (1986) was employed to determine percent water stable aggregates. This process included placing 25 grams of air-dried aggregates larger than 2 mm on the top sieve of a nest containing sieves measuring 2.0 mm, 0.25 mm and 0.053 mm. After pre-soaking in water for 10 minutes, aggregates underwent vertical oscillation 40 times with a 4 cm amplitude. Remaining aggregates on each sieve were oven-dried at 60°C for 24 hours and weighed. The weight of aggregates < 0.053 mm was calculated by subtracting masses collected on specific sieves from original sample weight. The retained aggregates in each sieve size class represented water-stable aggregates (WSA). The MWD was calculated as follows:


Where,
xi = Represents the average diameter of aggregates within a specific size range, while wi denotes the weight of aggregates in that range as a fraction of the total dry weight of the sample analyzed.
       
The determination of SOC in bulk soil and water stable aggregates was conducted using the Walkey and Black wet oxidation method, which was modified by Nelson and Sommer in 1982.
 
Statistical analysis
 
Data obtained were subjected to statistical analysis using Minitab software, with the Tukey test applied for mean separation at a 5% probability level. Linear correlation analysis between PSD, SOCb and aggregate indices were conducted with SPSS software.

RESULTS AND DISCUSSION

Effect of location and land use on aggregate indices
 
Soil texture and particle size distribution (PSD)
       
Table 2 shows the impact of location, land use and their interaction on soil texture and PSD. Soils in Umudike exhibited a sandy loam texture, while Umuahia soils were loamy sand, reflecting a coarser texture in Umuahia. Umudike had higher clay content (138 g/kg) than Umuahia (108 g/kg), while sand content followed the opposite trend. Though texture is mostly inherent, localized leaching, illuviation and pedogenic processes often influence variability (Osakwe et al., 2022; Oguike et al., 2023).

Table 2: Effect of location and land use on particle size distribution and texture.


       
Forest and grazing lands exhibited lower sand and higher clay contents than other land uses, attributed to organic matter inputs and canopy protection against erosion. Cultivated lands showed the highest sand content (854 g/kg) and the lowest clay (93 g/kg) and silt (46 g/kg), likely due to tillage and erosion-consistent with Onwuka (2020). Dinh and Shima (2024) also found bare soils had more sand than forest soils. These interactions underscore cultivation’s negative effect on PSD, modulated by location.
 
Soil organic carbon in aggregates (ASOC) and bulk soil (SOCb)
 
Fig 3 and 4 reveal that Umudike had higher ASOC in both the 4-2 mm (25.27 g/kg) and 2-0.25 mm (28.95 g/kg) fractions, along with greater bulk SOC (20.67 g/kg) than Umuahia. This supports Yang et al. (2022), who emphasized  macroag- gregates’ role in SOC storage. In both locations, the <0.25 mm ASOC was similar, indicating diminishing location effects with decreasing aggregate size, as also noted by Osakwe et al. (2024) and Mao et al. (2020).

Fig 3: The effect of location on aggregate associated soil organic carbon and soil organic carbon in bulk soil.



Fig 4: Effect of land use on aggregate associated soil organic carbon and soil organic carbon in bulk soil.


       
Land use effects showed that road construction eliminated ASOC in the 4-2 mm fraction (0 g/kg), demonstrating the adverse impact of soil disturbance, consistent with Sezgin et al. (2019). Grazing areas had the highest ASOC across all sizes: 25.27 g/kg (4-2 mm), 38.95 g/kg (2-0.25 mm) and 23.56 g/kg (<0.053 mm). Forest soils showed the highest SOCb. These trends suggest that organic matter quality and type-not just quantity-drive ASOC levels. Prabakaran et al. (2023) found that vegetation and litter-fall significantly influenced carbon pools across land uses.

Table 3: The interaction of location and land use on ASOC and SOCb.


       
SOCb followed the order: forest > grazing > fallow > cultivated > road, confirming findings by Martens et al. (2004), who reported that forests and pastures stored 46% and 25% more SOC than cropped soils, respectively, with forest increasing SOC by 29% over pasture. SOC declines in cultivated land stem from crop residue removal, intensive tillage, oxidation and erosion (Mengstie et al., 2020; Osakwe, 2018). Variability in SOC across land uses is also shaped by vegetation, microbial activity, substrate quality and root turnover.
       
Land use significantly influenced ASOC, aligning with Yi et al. (2018). Interaction effects (Table 3) indicated road construction caused significant ASOC loss in macroaggregates at both sites, while grazing, fallow and forest enhanced ASOC. ASOC contributes to soil structural stability, so its decline under road construction likely degrades soil physical properties and ecosystem functions (Ayoubi et al., 2020).
       
Grazing and forest lands in Umudike showed superior ASOC and SOCb levels, likely due to better management practices (Table 1; Fig 4). These results emphasize the importance of sustainable land use for enhancing SOC and promoting climate resilience.
 
Aggregate size distribution (ASD) and mean weight diameter (MWD)
 
Table 4 shows that Umudike soils had a higher proportion of water-stable aggregates (WSA) in all size fractions. Despite this, no significant location effect was observed on MWD. Greater WSA in Umudike is likely due to its higher clay and SOC content (Tables 2 and 3), which promote aggregation. Li et al. (2023) reported that fine particles and SOC support aggregate formation. Umudike also had a higher proportion of microaggregates (33%) than Umuahia, linked to its finer texture. Similar findings were reported by Gao et al. (2024) and Wu et al. (2024) for Mollisols and silty loam soils.
Land use effects on ASD were as follows:
•   4.00–2.00 mm: Grazing = Forest > Fallow > Road > Cultivated
•   2.00–0.25 mm: Grazing > Fallow > Forest > Road = Cultivated
•   0.25–0.053 mm: Road = Cultivated > Fallow > Forest > Grazing
•   <0.053 mm: Road = Cultivated > Fallow = Forest = Grazing

Table 4: Effect of location and land use on aggregate size distribution (ASD).


       
Grazing, forest and fallow lands retained larger aggregates better than road and cultivated lands. Higher WSA in the smallest fractions under road and cultivation implies greater breakdown and weaker structure. MWD values were higher in forest, grazing and fallow, supporting Nwite (2015), who observed improved aggregation in these systems.
       
Interaction effects revealed that macroaggregation was lowest at Umuahia’s road construction sites (4.53%) and in Umudike’s cultivated lands (6.41%), showing severe degradation. Grazing and forest lands recorded the highest macroaggregation (42.08-44.31%). Road construction and cultivation caused up to 88% decline in macroaggregates, consistent with Osakwe et al. (2024) in southeastern Nigerian Ultisols.
       
Enhanced macroaggregation implies  not only reduced compaction (Obalum and Obi 2014) but also improves root penetration, erosion control and nutrient cycling (Zhang et al., 2012). These findings underline the need for land management practices that improve soil aggregation.
 
Microaggregate stability indices
 
Table 5 shows that Umudike had better microaggregate stability, with lower clay ratio (CR: 7.1), clay dispersion index (CDI: 0.41), dispersion ratio (DR: 0.35) and higher aggregated silt + clay (ASC: 14.24 g/kg) than Umuahia. These values suggest Umuahia soils are more erosion-prone. Umudike’s higher clay and lower sand content explain this stability. However, both sites are highly erodible (DR > 15%, Middleton, 1930).

Table 5: Effect of location and land use on microaggregate stability indices.


       
Land use impacted microaggregate stability. Forest soils showed the lowest CR (5.6), CDI (0.30) and DR (0.26), whereas cultivated and road-graded lands had the highest CR (10.24 and 9.1 respectively). Cultivated soils also had the highest CDI and DR (0.65 and 0.54 respectively). This agrees with Osakwe (2021), who reported reduced microaggregate stability due to tillage. Elevated dispersion indices indicate susceptibility to soil structure loss and increased erosion.
       
Interaction effects revealed that road construction increased CR in Umudike, while CDI rose under grazing (Umuahia) and cultivation (Umudike). Grazing at Umuahia (community-managed) showed higher dispersion than Umudike (better-managed pasture), demonstrating management’s role. ASC was lowest under road construction in Umudike but highest in Umuahia due to clay increase with depth, showing location influence.
       
Despite grazing and forest soils’ superior microaggregate stability, coastal plain sands remain inherently erodible, as indicated by CR values (5.6-10.54; Bouyoucos, 1935) and DR >15% (Middleton, 1930).
 
Correlation analysis
 
Table 6 presents significant correlations among PSD, SOCb, aggregate indices and ASOC, consistent with Yi et al. (2018) and Hoang (2024). Sand content correlated positively with dispersion (r = 0.88** - 0.94**) and negatively with aggregation (-0.58 to -0.91). Sand also negatively correlated with ASOC in all size classes, indicating reduced carbon storage. Clay content, in contrast, promoted macroaggregation and reduced dispersion.

Table 6: Correlation of aggregate stability indices, aggregate soil organic carbon (ASOC) with particle size distribution and SOC in bulk soil (SOCb).


       
Silt content decreased DR and increased ASC and WSA, especially in Umuahia, due to its coarser texture. Li et al. (2023) also found positive clay and negative sand correlations with aggregate stability.
       
SOCb was strongly correlated with ASC, MWD, WSA1 and ASOC1 in Umudike (r = 0.71 - 0.96) and with MWD, WSA1, WSA2 and ASOC1 in Umuahia (r = 0.56 - 0.85), suggesting SOC’s key role in macroaggregation. SOCb negatively correlated with dispersion indices only in Umudike (r = -0.83 to -0.89), indicating site-specific dynamics. Negative correlations between SOCb and WSA3 and ASOC3 suggest macroaggregates are the main carbon storage zones, corroborating Yudina et al. (2022).

CONCLUSION

Agricultural and road construction activities reduced aggregate stability and ASOC, while grazing, forest and fallow lands improved them. Pedogenic and management factors influenced location-specific outcomes. Macroaggregates were primary SOC storage zones, not microaggregates. Road construction in Umuahia enhanced microaggregate stability due to deeper clay accumulation, contrasting with increased dispersion in Umudike. Well-managed pastures in Umudike improved ASOC levels.
       
Correlation analysis highlighted the importance of particle size and SOC in stabilizing aggregates and enhancing ASOC. Conservation measures should be tailored to specific soil characteristics and road construction should consider soil properties to mitigate degradation, especially in the erosion-prone coastal plain sands of southeastern Nigeria.
       
Correlation analysis highlighted the importance of particle size distribution and bulk SOC in promoting aggregate stability and ASOC. It is recommended that conservation measures be tailored to each location’s soil characteristics. Additionally, customizing road construction projects according to specific soil properties will help mitigate soil degradation in the highly erodible coastal plain sands of southeastern Nigeria.

ACKNOWLEDGEMENT

The study received support from Federal University Oye-Ekiti TETFUND Research Grant, led by Prof. Joshua O. Ogunwole. The donation of the Alexander Von Humboldt to the second author for equipment is appreciated.

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.

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    Full Research Article

    Impact of Agriculture and Road Construction on Soil Aggregates in Two Locations of Coastal Plain Sand, Southeastern Nigeria

    U
    U.C. Osakwe1,*
    J
    J.O. Ogunwole1
    O
    O. Omoju1
    A
    A.K. Oluleye1
    K
    K.A. Odetola1
    1Department of Soil Science and Land Resources Management, Federal University Oye - Ekiti, Nigeria.

    ABSTRACT

    Background: Understanding how land use affects the stability of soil aggregates and soil organic carbon associated with water stable aggregates in soils within the coastal plain sands is crucial for sustainable management. This study examines how agriculture and road construction influence these factors in two locations of Abia State, Southeastern Nigeria.

    Methods: The study was a 2 * 5 factorial experiment; denoting two locations (Umudike and Umuahia) and five land use practices which include Road Construction, Fallow, Grazing, Cultivation and Forest in a randomized complete block design. Soil sampling utilized the Transect method at a depth of 0 - 15 cm. Laboratory analysis on aggregate indices and pertinent soil characteristics were carried out.

    Result: Soil texture differed between Umudike (sandy loam) and Umuahia (loamy sand). Umudike soil showed higher water-stable aggregates (WSA), lower CR (clay ratio: 7.19), DR (dispersion ratio: 0.36) clay dispersion index (CDI: 0.41), higher ASC (aggregated silt plus clay: 14.24 g/kg) and aggregated soil organic carbon (ASOC). Both locations are classified as erodible (DR > 15%). The main effect of land use revealed that forest, grazing and fallow enhanced aggregate stability and ASOC compared to cultivation and road construction.  The combined effect of location and land use on soil aggregates displayed variability of response, emphasizing the influence of particle size distribution, SOC status including management on aggregate indices. Correlation analysis supported this claim. Therefore, tailored conservation strategies are vital for enhancing structure and ASOC while asphalting with proper drainage is crucial to prevent land degradation.

    INTRODUCTION

    Stability of soil aggregate, reflecting particles’ adherence and resistance to disintegration, is crucial for carbon stabilization in aggregates (Ran et al., 2021). Soil organic carbon found in microaggregates enhances the formation of macroaggregates and plays a crucial role in the long-term retention of soil organic carbon (Sekaran et al., 2021). Stable aggregates prevent loss of associated carbon linked to soil erosion, maintain soil fertility, plant resilience to moisture fluctuations and mitigate climate change (Menon et al., 2020). Researchers have employed indices such as mean weight diameter (MWD) and colloidal dispersion indices to evaluate stability and predict soil erosion, particularly in tropical soils of Nigeria (Osakwe, 2018).
                   
    Coastal plain sand-derived soils as in Abia State are sandy; often exhibit low aggregate stability, clay and organic matter content, making them susceptible to erosion (Uluocha and Uwadiegwu 2015; Chukwu et al., 2023). They added that moderate soil erosion affects around 20% of the area (115,077.9 hectares). Additionally, 27% (60,271.4 hectares) is subject to low erosion and the remaining 33% of the State experiences very low levels of soil erosion. Therefore, regular assessment and evaluation for proper management is imperative. In addition, information on possible variability between soils in the study sites, crucial for site-specific management is lacking. Furthermore, Nigeria’s rapid urbanization, infrastructural development and agricultural expansion, while crucial for economic growth and food security, often overlook their impact on soil properties. Consequently, our study aimed to investigate how road construction and agriculture influence coastal plain sand soils in Abia State, southeastern Nigeria and the potential variability in response across locations.

    MATERIALS AND METHODS

    Description of study area
     
    The research was conducted between 2018 and 2020 in Umudike and Umuahia between latitude 05o28′N and 05o32′N and longitude 007o28′E and 007o32′E, located in the rainforest agroecology of Abia State, southeastern Nigeria, characterized by a humid tropical climate (Igbozuruike, 1975). The area experiences average annual rainfall of 2176 mm  with minimum monthly air temperatures between 20oC and 24oC and maximum temperatures ranging from 28oC to 35oC. The humidity ranges from 51% to 87%. The primary parent material consists of Coastal Plain Sands, along with certain areas featuring alluvial sediments (Amanze et al., 2016). The soil type, classified as Ultisols, is sandy, predominantly kaolinite-based with low pH, base status and soil organic carbon (Lekwa and Whiteside 1986).
           
    The land use history, methodology flow chart and location maps are presented in Table 1, Fig 1 and 2 respectively.

    Table 1: Land use history.



    Fig 1: Location maps.



    Fig 2: Methodology flow chart.


     
    Experimental design
     
    The research was designed as a 2×5 factorial experiment in randomized complete block design (RCBD) with three replicates. The numbers are two locations (Umudike and Umuahia) and five land use practices (Fallow land, Arable land, Grazing land, Forest and Road construction). Each location’s designated land use areas were subdivided into three blocks and the transect method was used to collect soil samples from 0-15 cm depth. Samples from each block were combined to create 15 composite samples for each location. They were air-dried and passed through 4 mm and 2 mm sieves for laboratory analysis.
     
    Laboratory analysis
     
    The method of Gee and Bauder (1986) was used to assess the particle size distribution using sodium hexameta phosphate as a dispersing agent. Colloidal stability was determined by substituting water for the dispersing agent and computed as follows:




     
    ASC =  % clay + % silt (calgon) - % clay + % silt (H2O)
     
    CR = % Sand+ % silt/ % clay
                                                                   (Bouyoucus, 1951),
     
    DR, Dispersion Ratio, CDI, Clay Dispersion Index and ASC, Aggregated Silt plus Clay.
           
    The method of Kemper and Rosenau (1986) was employed to determine percent water stable aggregates. This process included placing 25 grams of air-dried aggregates larger than 2 mm on the top sieve of a nest containing sieves measuring 2.0 mm, 0.25 mm and 0.053 mm. After pre-soaking in water for 10 minutes, aggregates underwent vertical oscillation 40 times with a 4 cm amplitude. Remaining aggregates on each sieve were oven-dried at 60°C for 24 hours and weighed. The weight of aggregates < 0.053 mm was calculated by subtracting masses collected on specific sieves from original sample weight. The retained aggregates in each sieve size class represented water-stable aggregates (WSA). The MWD was calculated as follows:


    Where,
    xi = Represents the average diameter of aggregates within a specific size range, while wi denotes the weight of aggregates in that range as a fraction of the total dry weight of the sample analyzed.
           
    The determination of SOC in bulk soil and water stable aggregates was conducted using the Walkey and Black wet oxidation method, which was modified by Nelson and Sommer in 1982.
     
    Statistical analysis
     
    Data obtained were subjected to statistical analysis using Minitab software, with the Tukey test applied for mean separation at a 5% probability level. Linear correlation analysis between PSD, SOCb and aggregate indices were conducted with SPSS software.

    RESULTS AND DISCUSSION

    Effect of location and land use on aggregate indices
     
    Soil texture and particle size distribution (PSD)
           
    Table 2 shows the impact of location, land use and their interaction on soil texture and PSD. Soils in Umudike exhibited a sandy loam texture, while Umuahia soils were loamy sand, reflecting a coarser texture in Umuahia. Umudike had higher clay content (138 g/kg) than Umuahia (108 g/kg), while sand content followed the opposite trend. Though texture is mostly inherent, localized leaching, illuviation and pedogenic processes often influence variability (Osakwe et al., 2022; Oguike et al., 2023).

    Table 2: Effect of location and land use on particle size distribution and texture.


           
    Forest and grazing lands exhibited lower sand and higher clay contents than other land uses, attributed to organic matter inputs and canopy protection against erosion. Cultivated lands showed the highest sand content (854 g/kg) and the lowest clay (93 g/kg) and silt (46 g/kg), likely due to tillage and erosion-consistent with Onwuka (2020). Dinh and Shima (2024) also found bare soils had more sand than forest soils. These interactions underscore cultivation’s negative effect on PSD, modulated by location.
     
    Soil organic carbon in aggregates (ASOC) and bulk soil (SOCb)
     
    Fig 3 and 4 reveal that Umudike had higher ASOC in both the 4-2 mm (25.27 g/kg) and 2-0.25 mm (28.95 g/kg) fractions, along with greater bulk SOC (20.67 g/kg) than Umuahia. This supports Yang et al. (2022), who emphasized  macroag- gregates’ role in SOC storage. In both locations, the <0.25 mm ASOC was similar, indicating diminishing location effects with decreasing aggregate size, as also noted by Osakwe et al. (2024) and Mao et al. (2020).

    Fig 3: The effect of location on aggregate associated soil organic carbon and soil organic carbon in bulk soil.



    Fig 4: Effect of land use on aggregate associated soil organic carbon and soil organic carbon in bulk soil.


           
    Land use effects showed that road construction eliminated ASOC in the 4-2 mm fraction (0 g/kg), demonstrating the adverse impact of soil disturbance, consistent with Sezgin et al. (2019). Grazing areas had the highest ASOC across all sizes: 25.27 g/kg (4-2 mm), 38.95 g/kg (2-0.25 mm) and 23.56 g/kg (<0.053 mm). Forest soils showed the highest SOCb. These trends suggest that organic matter quality and type-not just quantity-drive ASOC levels. Prabakaran et al. (2023) found that vegetation and litter-fall significantly influenced carbon pools across land uses.

    Table 3: The interaction of location and land use on ASOC and SOCb.


           
    SOCb followed the order: forest > grazing > fallow > cultivated > road, confirming findings by Martens et al. (2004), who reported that forests and pastures stored 46% and 25% more SOC than cropped soils, respectively, with forest increasing SOC by 29% over pasture. SOC declines in cultivated land stem from crop residue removal, intensive tillage, oxidation and erosion (Mengstie et al., 2020; Osakwe, 2018). Variability in SOC across land uses is also shaped by vegetation, microbial activity, substrate quality and root turnover.
           
    Land use significantly influenced ASOC, aligning with Yi et al. (2018). Interaction effects (Table 3) indicated road construction caused significant ASOC loss in macroaggregates at both sites, while grazing, fallow and forest enhanced ASOC. ASOC contributes to soil structural stability, so its decline under road construction likely degrades soil physical properties and ecosystem functions (Ayoubi et al., 2020).
           
    Grazing and forest lands in Umudike showed superior ASOC and SOCb levels, likely due to better management practices (Table 1; Fig 4). These results emphasize the importance of sustainable land use for enhancing SOC and promoting climate resilience.
     
    Aggregate size distribution (ASD) and mean weight diameter (MWD)
     
    Table 4 shows that Umudike soils had a higher proportion of water-stable aggregates (WSA) in all size fractions. Despite this, no significant location effect was observed on MWD. Greater WSA in Umudike is likely due to its higher clay and SOC content (Tables 2 and 3), which promote aggregation. Li et al. (2023) reported that fine particles and SOC support aggregate formation. Umudike also had a higher proportion of microaggregates (33%) than Umuahia, linked to its finer texture. Similar findings were reported by Gao et al. (2024) and Wu et al. (2024) for Mollisols and silty loam soils.
    Land use effects on ASD were as follows:
    •   4.00–2.00 mm: Grazing = Forest > Fallow > Road > Cultivated
    •   2.00–0.25 mm: Grazing > Fallow > Forest > Road = Cultivated
    •   0.25–0.053 mm: Road = Cultivated > Fallow > Forest > Grazing
    •   <0.053 mm: Road = Cultivated > Fallow = Forest = Grazing

    Table 4: Effect of location and land use on aggregate size distribution (ASD).


           
    Grazing, forest and fallow lands retained larger aggregates better than road and cultivated lands. Higher WSA in the smallest fractions under road and cultivation implies greater breakdown and weaker structure. MWD values were higher in forest, grazing and fallow, supporting Nwite (2015), who observed improved aggregation in these systems.
           
    Interaction effects revealed that macroaggregation was lowest at Umuahia’s road construction sites (4.53%) and in Umudike’s cultivated lands (6.41%), showing severe degradation. Grazing and forest lands recorded the highest macroaggregation (42.08-44.31%). Road construction and cultivation caused up to 88% decline in macroaggregates, consistent with Osakwe et al. (2024) in southeastern Nigerian Ultisols.
           
    Enhanced macroaggregation implies  not only reduced compaction (Obalum and Obi 2014) but also improves root penetration, erosion control and nutrient cycling (Zhang et al., 2012). These findings underline the need for land management practices that improve soil aggregation.
     
    Microaggregate stability indices
     
    Table 5 shows that Umudike had better microaggregate stability, with lower clay ratio (CR: 7.1), clay dispersion index (CDI: 0.41), dispersion ratio (DR: 0.35) and higher aggregated silt + clay (ASC: 14.24 g/kg) than Umuahia. These values suggest Umuahia soils are more erosion-prone. Umudike’s higher clay and lower sand content explain this stability. However, both sites are highly erodible (DR > 15%, Middleton, 1930).

    Table 5: Effect of location and land use on microaggregate stability indices.


           
    Land use impacted microaggregate stability. Forest soils showed the lowest CR (5.6), CDI (0.30) and DR (0.26), whereas cultivated and road-graded lands had the highest CR (10.24 and 9.1 respectively). Cultivated soils also had the highest CDI and DR (0.65 and 0.54 respectively). This agrees with Osakwe (2021), who reported reduced microaggregate stability due to tillage. Elevated dispersion indices indicate susceptibility to soil structure loss and increased erosion.
           
    Interaction effects revealed that road construction increased CR in Umudike, while CDI rose under grazing (Umuahia) and cultivation (Umudike). Grazing at Umuahia (community-managed) showed higher dispersion than Umudike (better-managed pasture), demonstrating management’s role. ASC was lowest under road construction in Umudike but highest in Umuahia due to clay increase with depth, showing location influence.
           
    Despite grazing and forest soils’ superior microaggregate stability, coastal plain sands remain inherently erodible, as indicated by CR values (5.6-10.54; Bouyoucos, 1935) and DR >15% (Middleton, 1930).
     
    Correlation analysis
     
    Table 6 presents significant correlations among PSD, SOCb, aggregate indices and ASOC, consistent with Yi et al. (2018) and Hoang (2024). Sand content correlated positively with dispersion (r = 0.88** - 0.94**) and negatively with aggregation (-0.58 to -0.91). Sand also negatively correlated with ASOC in all size classes, indicating reduced carbon storage. Clay content, in contrast, promoted macroaggregation and reduced dispersion.

    Table 6: Correlation of aggregate stability indices, aggregate soil organic carbon (ASOC) with particle size distribution and SOC in bulk soil (SOCb).


           
    Silt content decreased DR and increased ASC and WSA, especially in Umuahia, due to its coarser texture. Li et al. (2023) also found positive clay and negative sand correlations with aggregate stability.
           
    SOCb was strongly correlated with ASC, MWD, WSA1 and ASOC1 in Umudike (r = 0.71 - 0.96) and with MWD, WSA1, WSA2 and ASOC1 in Umuahia (r = 0.56 - 0.85), suggesting SOC’s key role in macroaggregation. SOCb negatively correlated with dispersion indices only in Umudike (r = -0.83 to -0.89), indicating site-specific dynamics. Negative correlations between SOCb and WSA3 and ASOC3 suggest macroaggregates are the main carbon storage zones, corroborating Yudina et al. (2022).

    CONCLUSION

    Agricultural and road construction activities reduced aggregate stability and ASOC, while grazing, forest and fallow lands improved them. Pedogenic and management factors influenced location-specific outcomes. Macroaggregates were primary SOC storage zones, not microaggregates. Road construction in Umuahia enhanced microaggregate stability due to deeper clay accumulation, contrasting with increased dispersion in Umudike. Well-managed pastures in Umudike improved ASOC levels.
           
    Correlation analysis highlighted the importance of particle size and SOC in stabilizing aggregates and enhancing ASOC. Conservation measures should be tailored to specific soil characteristics and road construction should consider soil properties to mitigate degradation, especially in the erosion-prone coastal plain sands of southeastern Nigeria.
           
    Correlation analysis highlighted the importance of particle size distribution and bulk SOC in promoting aggregate stability and ASOC. It is recommended that conservation measures be tailored to each location’s soil characteristics. Additionally, customizing road construction projects according to specific soil properties will help mitigate soil degradation in the highly erodible coastal plain sands of southeastern Nigeria.

    ACKNOWLEDGEMENT

    The study received support from Federal University Oye-Ekiti TETFUND Research Grant, led by Prof. Joshua O. Ogunwole. The donation of the Alexander Von Humboldt to the second author for equipment is appreciated.

    CONFLICT OF INTEREST

    The authors declare that they have no conflict of interest.

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