Plant Species Effect on Soil Micronutrients and Aluminum in Secondary Forests at Masako Forest Reserve, Kisangani, Democratic Republic of Congo

Abstract

Plant species can significantly influence soil micronutrients. We assessed how soil micronutrients (B, Fe, Cu, Zn, Mn) and aluminum (Al) were affected by soil depth (SD) and plant species (PS) in a secondary forest at Masako Forest Reserve. Soil samples were collected in June 2022 and June 2023 along five PS (Entandrophragma utile, Hevea brasiliensis, Milettia laurentii, Musanga cecropoides, and Triculia africana). Four trees (replications) were selected per plant species. A completely randomized design was used with five PS and three SD (0–10 cm, 10–20, and 20–30 cm) and was replicated four times. To collect soil samples, a pit was dug at each sampling location (near a tree), and three soil samples were taken horizontally in the middle of each layer on one of the four faces of the pit, with a 5 cm height and 5 cm diameter cylinder. Soil samples were air-dried, mixed, and sieved to 2 mm, and a 20 g subsample was sent to Brookside Laboratories (OH, USA) for analyses of soil micronutrients. The results showed that most micronutrients were concentrated in the topsoil (0–10 cm). Plant species such as Treculia africana, Millettia laurentii, and Musanga cecropoides enhanced micronutrients in the soil in which they grew, especially iron (Fe) and zinc (Zn). The effect of the year of sampling on micronutrients was prevalent for many micronutrients, which remained significantly higher in 2022 than in 2023. These findings provide a foundational framework for developing nature-based biofortification strategies. By prioritizing key native plant species, local stakeholders can optimize soil health in the Congo Basin.

1. Introduction

In tropical Africa, deforestation is largely characterized by the clearing of primary forests, driven primarily by expanding subsistence agriculture, shifting cultivation, and commercial logging [1,2]. These activities often involve slash-and-burn techniques to create farmland, destroying mature and biodiverse ecosystems, leading to high rates of permanent, irreversible forest loss [3,4]. The Congo Basin, in particular, is experiencing significant primary forest loss due to smallholder agriculture, charcoal production, and, increasingly, commercial farming [5]. Deforestation significantly decreases soil micronutrients and overall fertility by disrupting nutrient cycling, increasing erosion, and reducing organic matter [6]. The removal of tree cover exposes soil to sun and rain, leading to topsoil loss, reduced microbial activity, and accelerated leaching of essential micronutrients. However, micronutrients like boron, iron, manganese, copper, and zinc are essential for plant growth and ecosystem health in tropical soils, where they are often limited by intense leaching from high rainfall and acidic conditions. Aluminum, while not a plant nutrient, plays a critical role in the chemistry of these soils, often acting as a major stressor due to its toxicity in acidic environments [7].

There are, however, secondary forests that can replace primary forests following deforestation, agricultural abandonment, or logging, comprising roughly half of all remaining tropical forests [8]. ITTO [9] suggested that secondary forests represent roughly 60% of the area now defined as tropical forests in the Congo Basin. These regenerating ecosystems often grow on abandoned cropland, functioning as vital carbon sinks and providing biodiversity buffers, although they differ in structure and species composition from primary forests [10]. It has been suggested that secondary forest regeneration can significantly contribute nutrients to the level of primary forests, primarily by recovering soil nutrient pools, increasing carbon sequestration, and improving ecosystem functionality over time [11]. As secondary forests age, they restore soil health and biomass, often reaching levels similar to those of undisturbed forests within 15 to 50 years [12].

While it is agreed that secondary tropical forests can recover similar levels of soil nutrients and carbon to primary forests, authors do not all agree on the specific time for this to occur [13]. Lamb [14] suggested that 15 to 40 years of regeneration is enough for a secondary forest to equate a primary forest. Holz et al. [15] studied the effects of history of use on secondary forest regeneration in the Upper Parana Atlantic Forest and were no longer able to differentiate the primary and secondary forests structurally or floristically after 20 years of succession. Brown and Lugo [16] reported that more organic matter is produced and transferred to the soil in younger secondary forests than is stored in above-ground vegetation. The impact of this on soil organic matter is significant and explains why the recovery of organic matter in the soil under secondary forests is relatively fast (50 yr or so). Jones et al. [17] compared their estimates to published data from younger and older secondary forests in the surrounding landscape and showed that soil carbon recovers within 40 years of forest regeneration, but above-ground biomass carbon stocks continue to increase over the past 100 years.

In secondary tropical forests, plant species exert significant, often species-specific, effects on soil nutrients, including the availability of boron (B), iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), and aluminum (Al). These effects are primarily driven by differences in litter decomposition rates, root exudates, nutrient uptake strategies and, crucially, the ability to alter soil pH. Hobbie [18] reported that the rate of nutrient cycling, particularly of B, Fe, Cu, and Zn, is heavily influenced by the chemical composition of leaf litter. Ma et al. [19] suggested that differences in fine-root growth and associated microorganisms (e.g., mycorrhizal fungi) affect soil carbon, nutrient retention, and the transformation of P and metallic elements. Higher tree diversity in secondary forests is generally associated with increased soil fertility, often linked to higher nutrient uptake and return through litterfall [20]. Research also shows that soil Cu, Zn, Fe, and B concentrations often increase, while soil pH decreases (acidifies) with increasing forest age in secondary succession [21]. Past studies on the effect of plant species on soil nutrients, specifically micronutrients (B, Fe, Cu, Zn, Mn) and aluminum (Al) in tropical forests, often focused on how different functional groups (e.g., nitrogen-fixing vs. non-fixing trees) modify soil properties, particularly during secondary succession [22].

While we know that plant species alter local soil properties and nutrients through litter and root dynamics, much about secondary tropical forests remains a mystery. The unknowns primarily revolve around cause-and-effect mechanisms, successional timelines, and the broader role of plant diversity [13]. At Masako Forest Reserve, for example, research on the effect of plant species on soil micronutrients (boron, iron, copper, zinc, manganese) and aluminum (Al) is nonexistent [23]. Therefore, the overall goal of this study was to assess how soil depth and plant species affect soil micronutrients and aluminum in secondary forest at Masako Forest Reserve, Democratic Republic of Congo. Specific research objectives were:

(i)

To measure and compare the levels of specific micronutrients (B, Fe, Mn, Cu, Zn) and aluminum (Al) found in the soil beneath different plant species (Entandrophragma utile, Hevea brasiliensis, Milettia laurentii, Musanga cecropoides, and Triculia africana).

(ii)

To analyze how micronutrient concentrations fluctuate at different depths (0–10 cm, 10–20, and 20–30 cm) to determine if certain plant species promote higher nutrient retention in the topsoil compared to deeper layers.

(iii)

To establish which of the five plant species has the most beneficial effect on soil fertility (highest micronutrient level in soil beneath the plant species) for potential use in ecological restoration or agroforestry in the Congo Basin.

The following hypotheses were tested:

(i)

There is a significant difference in the concentration of soil micronutrients (B, Fe, Mn, Cu, Zn) and aluminum (Al) beneath the canopies of Entandrophragma utile, Hevea brasiliensis, Milettia laurentii, Musanga cecropoides, and Triculia africana.

(ii)

The concentrations of micronutrients and aluminum will vary significantly across different soil depths (0–10 cm, 10–20, and 20–30 cm), with the highest accumulation of available nutrients occurring in the topsoil due to litterfall decomposition.

(iii)

The soil beneath leguminous plant species (Millettia laurentii) will have more micronutrients than non-leguminous species because of litterfall content and decomposition.

2. Materials and Methods

2.1. Area of Study

The site of this study was Masako Forest Reserve, Kisangani, Democratic Republic of Congo (Figure 1). The geographic coordinates of Masako Forest Reserve are 0°36′ N and 25°13′ E. Masako Forest Reserve contains a tropical evergreen rainforest ecosystem defined by heavy thick vegetation [24]. The soils in the Masako forest, like all those in equatorial climates, are relatively poor [25]. They are mostly oxisols (ferrallisols according to FAO classification), with low humus content (average rate of carbon and organic nitrogen 1.5 and 0.1%), predominantly sandy (sand average rate: 70.1%, silt: 6.5%, clay: 23.4%), and controlled by kaolinite in the clayey fraction. They are acid soils (average water pH and KCl 1N, respectively, at 4.9 and 3.7) with low cation exchange capacity (CEC: <7 meq/100 g). The Masako forest within the Congo Basin stands out because this location maintains over 10,000 plant species among its endemic flora.

Masako Forest Reserve started under the Ministry of Environment administration until the University of Kisangani officially transformed it into its field research station. The change in management has established new opportunities to undertake ecology studies and monitor the environment, along with protecting biodiversity and various other essential areas of work. The reserve receives worldwide recognition as a biodiversity hotspot of the DRC since it contains various endemic species found within its flora and fauna [26]. The area is experiencing climate-related issues, such as increased temperatures and erratic rainfall, which have negatively affected agricultural productivity and reduced the availability of water. In June 2022 and 2023, Kisangani experienced its typical early dry season conditions, characterized by warm, humid weather, relatively high daily temperatures often reaching around 30 °C, and a decrease in, but not total absence of, rainfall, with roughly 114 mm, often described as part of the region’s drier, more comfortable period. Rainfall amounts significantly decreased compared to previous months, averaging around 114 mm over the month.

2.2. Experimental Design, Soil Sampling, and Analysis of Soil Nutrients

The study was conducted in June 2022 and in June 2023 in primary and secondary forests, fallows, and agricultural fields, the four land use types found at Masako Forest Reserve. This paper focuses on the remnant secondary forest. In each of the four land uses, a completely randomized design was used with 5 plant species (PS) and 3 soil sampling depths (SD), all replicated 4 times. Five of the most dominant plant species were chosen in each land use, and 4 trees (replications) were studied for each selected plant species. Trees studied in 2022 were not the same as in 2023; therefore, tree locations also differed within each land use and each year.

In the secondary forest, plant species of interest were Entandrophragma utile, Hevea brasiliensis, Milettia laurentii, Musanga cecropoides, and Triculia africana. Depending on the size of the tree, soil samples were collected at about 1 m distance from the trunk. Trees were randomly selected, and the distance between them was not measured. To collect soil samples, a pit of about 1 m long, 1 m wide, and about 50 cm deep was dug at each sampling location (near the tree). Soil samples were taken horizontally at one of the four faces of the pit in the middle of each of the 0–10 cm, 10–20, and 20–30 cm layers (Figure 2).

For soil physical properties (manuscript in preparation), a 5 cm diameter and 5 cm height soil core was inserted horizontally in the middle of each layer to collect the soil samples, which were later sent to the Center for Biodiversity laboratory, Faculty of Science at the University of Kisangani for further analyses.

For soil chemical properties and nutrients, three soil samples were also taken with the same cylinder in the middle of each layer as for soil physical properties and put into bags. These samples were brought to the Department of Soil Science Laboratory at the “Institut Facultaire des Sciences Agronomiques” (IFA-Yangambi) where they were air-dried, thoroughly mixed, and sieved 2 mm, and then a 20 g subsample was taken and sent to Brookside Laboratories (New Bremen, OH, USA) for the analysis of soil micronutrients: copper (Cu), iron (Fe), boron (B), manganese (Mn), zinc (Zn), and aluminum (Al). Other soil nutrients as well as soil chemical properties were also analyzed to assess the fertility and general nutrient status of the soil, but they are not included in this report. For each sampling year, 240 soil samples [(4 land uses × 5 plant species × 3 soil depths) × 4 replicates)] were collected and shipped to Brookside Laboratories.

2.3. Statistical Analysis

After laboratory analysis, data on soil micronutrients was recorded into an Excel sheet and then transferred into Statistix statistical software, 10th version, and checked for normality before further analysis. For the tables of summaries of simple statistics (n = 12) for B, Fe, Mn, Cu, Zn, and Al, normality was assumed as the means were closer to their medians. For the yearly (n = 60) and combined 2 years (n = 120) analyses of the effect of soil depth (SD) and plant species (PS) on soil micronutrients, normality was again assumed since the means were closer to their medians, except for a few instances for Mn and zinc.

After computing the summaries of statistics, a two-factor factorial analysis of variance was conducted on the data with soil sampling depth (SD) as the first factor at three levels (0–10 cm, 10–20, and 20–30 cm) and plant species (PS) as the second factor at five levels (plant species 1, plant species 2, 3, and 4, and plant species 5) for the yearly analyses. A third factor (year of sampling at two levels: 2022 and 2023) was added for the combined analysis. Where a significant effect was found, means were separated using the LSD test at a 0.05 probability level. The results of the summaries of statistics for 2022 are presented in

Table 1. Descriptive statistics for soil micronutrients and aluminum in the secondary forest in 2022: 1. Entandrophragma utile.

	Soil Micronutrients and Aluminum
Descriptive Statistics	B	Fe	Mn	Cu	Zn	Al
	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Mean	0.39	182.75	0.76	0.46	0.07	400.75
Standard dev (SD)	0.12	35.06	1.21	0.16	0.22	76.86
Variance	0.01	1229.30	1.47	0.03	0.05	5906.80
C.V.	30.82	19.19	160.29	35.83	298.63	19.18
Minimum	0.24	112.00	0.01	0.27	0.01	297.00
Median	0.37	182.00	0.01	0.42	0.01	393.50
Maximum	0.66	253.00	4.00	0.72	0.76	522.00
Skewness	0.88	−0.02	1.77	0.52	3.02	0.18
Kurtosis	0.34	0.36	2.31	−1.14	7.09	−1.09

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum.

,

Table 2. Descriptive statistics for soil micronutrients and aluminum in the secondary forest in 2022: 2. Hevea brasiliensis.

	Soil Micronutrients and Aluminum
Descriptive Statistics	B	Fe	Mn	Cu	Zn	Al
	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Mean	0.21	133.75	0.59	0.45	0.01	411.25
Standard dev (SD)	0.13	19.06	0.99	0.23	0.00	67.56
Variance	0.02	363.11	0.98	0.06	0.00	4564.80
C.V.	61.19	14.25	168.13	51.72	0.00	16.43
Minimum	0.01	102.00	0.01	0.01	0.01	268.00
Median	0.25	133.50	0.01	0.43	0.01	415.50
Maximum	0.35	163.00	3.00	0.95	0.01	504.00
Skewness	−0.74	−0.06	1.50	0.26	0.01	−0.57
Kurtosis	−0.89	−0.81	0.96	0.41	0.01	−0.23

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum.

,

Table 3. Descriptive statistics for soil micronutrients and aluminum in the secondary forest in 2022: 3. Milettia laurentii.

	Soil Micronutrients and Aluminum
Descriptive Statistics	B	Fe	Mn	Cu	Zn	Al
	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Mean	0.41	210.00	1.50	0.53	0.09	462.83
Standard dev (SD)	0.12	44.72	1.73	0.31	0.29	135.88
Variance	0.01	1999.60	2.99	0.10	0.08	18,464.00
C.V.	29.48	21.29	114.89	58.42	308.96	29.36
Minimum	0.26	156.00	0.01	0.22	0.01	332.00
Median	0.41	188.50	1.00	0.40	0.01	417.00
Maximum	0.63	285.00	5.00	1.21	1.00	732.00
Skewness	0.51	0.42	0.83	1.21	3.02	0.80
Kurtosis	−0.62	−1.32	−0.57	0.13	7.09	−0.81

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum.

,

Table 4. Descriptive statistics for soil micronutrients and aluminum in the secondary forest in 2022: 4. Musanga cecropoides.

	Soil Micronutrients and Aluminum
Descriptive Statistics	B	Fe	Mn	Cu	Zn	Al
	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Mean	0.12	114.50	40.08	0.66	4.99	294.33
Standard dev (SD)	0.14	21.80	39.47	0.40	8.21	112.16
Variance	0.02	475.36	1557.90	0.16	67.41	12,581.00
C.V.	117.86	19.04	98.47	61.06	164.56	38.11
Minimum	0.01	79.00	2.00	0.01	0.01	100.00
Median	0.10	124.50	31.00	0.55	0.92	277.00
Maximum	0.35	142.00	115.00	1.42	24.31	495.00
Skewness	0.57	−0.41	0.45	0.33	1.57	0.29
Kurtosis	−1.41	−1.24	−1.15	−0.67	0.93	−0.57

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum.

and

Table 5. Descriptive statistics for soil micronutrients and aluminum in the secondary forest in 2022: 5. Treculia africana.

	Soil Micronutrients and Aluminum
Descriptive Statistics	B	Fe	Mn	Cu	Zn	Al
	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Mean	0.50	215.67	2.34	0.44	0.59	433.50
Standard dev (SD)	0.25	72.21	4.07	0.22	0.90	157.97
Variance	0.06	5214.10	16.59	0.05	0.81	24,956.00
C.V.	49.18	33.48	174.22	49.45	153.22	36.44
Minimum	0.01	115.00	0.01	0.01	0.01	205.00
Median	0.51	233.50	1.00	0.42	0.01	375.50
Maximum	0.84	341.00	14.00	0.83	2.39	714.00
Skewness	−0.29	0.02	2.20	−0.05	1.36	0.67
Kurtosis	−0.57	−1.16	3.79	−0.16	0.32	−0.53

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum.

. The summaries of statistics for 2023 are not presented to avoid data redundancy. The analyses of variance for 2022, 2023, and 2022 + 2023 are presented in

Table 6. Analysis of variance for the effect of soil sampling depth and plant species on soil micronutrients and aluminum in the secondary forest in June 2022.

Treatment		B	Fe	Mn	Cu	Zn	Al
		(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Soil sampling depth (SD)
0–10 cm		0.32	177.55	18.30 a	0.58	1.07	344.80 b
10–20 cm		0.34	172.75	4.06 b	0.42	1.06	413.00 ab
20–30 cm		0.32	163.70	4.81 b	0.52	1.32	443.80 a
Plant species
Entandrophragma utile		0.39 a	182.75 a	0.76 b	0.46	0.07 b	400.75 a
Hevea brasiliensis		0.21 b	133.75 b	0.59 b	0.45	0.01 b	411.25 a
Milettia laurentii		0.41 a	210.00 a	1.50 b	0.53	0.09 b	462.83 a
Musanga cecropoides		0.12 b	114.50 b	40.08 a	0.66	4.99 a	294.33 b
Treculia africana		0.50 a	215.67 a	2.34 b	0.44	0.59 b	433.50 a
Analysis of variance
Sources de variation	dl
Rep	3
SD	2	0.8923	0.5797	0.0014	0.2392	0.9687	0.0216
PS	4	0.0001	0.0001	0.0001	0.3064	0.0087	0.0075
Interaction
SD × PS	8	0.9726	0.4444	0.0001	0.8846	0.9938	0.9036
Error	42
Total	59

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum. Means with the same letter are not significant among them.

,

Table 7. Analysis of variance for the effect of soil sampling depth and plant species on soil micronutrients and aluminum in the secondary forest in June 2023.

Treatment		B	Fe	Mn	Cu	Zn	Al
		(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Soil sampling depth (SD)
0–10 cm		0.22	190.25	5.45 a	0.58	1.56	629.45
10–20 cm		0.19	165.10	1.70 b	0.64	0.96	586.25
20–30 cm		0.18	161.35	1.35 b	0.70	0.76	609.65
Plant species
Entandrophragma utile		0.18	145.42 b	3.25	0.70	0.76 b	594.67
Hevea brasiliensis		0.20	135.50 b	1.50	0.70	0.40 b	561.83
Milettia laurentii		0.18	199.25 a	1.50	0.47	0.46 b	625.50
Musanga cecropoides		0.19	188.33 a	2.33	0.55	1.10 b	717.25
Treculia africana		0.24	192.67 a	5.58	0.79	2.76 a	543.00
Analysis of variance
Sources de variation	dl
Rep	3
SD	2	0.3228	0.1188	0.0166	0.4916	0.2609	0.7729
PS	4	0.431	0.003	0.2145	0.1573	0.0037	0.2024
Interaction
SD × PS	8	0.4872	0.4803	0.1708	0.5127	0.6117	0.1224
Error	42
Total	59

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum. Means with the same letter are not significant among them.

and

Table 8. Analysis of variance for the effect of year of sampling, soil sampling depth, and plant species on soil micronutrients and aluminum in the secondary forest in June 2022 and 2023.

Treatment		B	Fe	Mn	Cu	Zn	Al
		(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)	(mg/kg)
Year of sampling (YS)
June 2022		0.33 a	171.33 a	9.05 a	0.51 b	1.15 a	400.53 b
June 2023		0.20 b	172.23 a	2.83 b	0.65 a	1.09 a	608.45 a
Soil sampling depth (SD)
0–10 cm		0.27 a	183.90 a	11.88 a	0.58 a	1.31 a	487.13 a
10–20 cm		0.27 a	168.93 a	2.88 b	0.54 a	1.01 a	499.63 a
20–30 cm		0.25 a	162.52 a	3.08 b	0.61 a	1.04 a	526.72 a
Plant species (PS)
Milettia laurentii		0.29 ab	204.63 a	1.50 b	0.51 a	0.28 b	544.17 a
Musanga cecropoides		0.15 c	151.42 bc	21.21 a	0.61 a	3.04 a	505.79 a
Hevea brasiliensis		0.21 c	134.62 c	1.05 b	0.58 a	0.21 b	486.54 a
Entandrophragma utile		0.29 b	164.08 b	2.00 b	0.58 a	0.42 b	497.71 a
Treculia africana		0.37 a	204.17 a	3.96 b	0.61 a	1.67 ab	488.25 a
Analysis of variance
Sources de variation	df
Rep	3
YS	1	0.0001	0.9167	0.0008	0.0196	0.9195	0.0001
SD	2	0.8025	0.1191	0.0001	0.5566	0.8858	0.5134
PS	4	0.0001	0.0001	0.0001	0.7888	0.0056	0.7046
Interactions
YS × SD	2	0.687	0.6053	0.0349	0.2481	0.7426	0.1689
YS × PS	4	0.0001	0.001	0.0001	0.0538	0.0133	0.007
PS × SD	8	0.8122	0.2882	0.0001	0.8956	0.9693	0.1862
YS × SD × PS	8	0.9878	0.8237	0.0001	0.5238	0.9917	0.2561
Error	87
Total	119

B = boron; Fe = iron; Mn = manganese; Cu = copper; Zn = zinc; Al = aluminum. Means with the same letters are not significant among them.

.

3. Results and Discussion

3.1. Descriptive Statistics for Soil Micronutrients Under Entandrophragma utile in June 2022

Entandrophragma utile is a non-nitrogen-fixing species of the Meliaceae family. It is called the sipo or utile, a species of large tree in the genus Entandrophragma, native to nearly all of tropical Africa facing the Atlantic, from Guinea to Angola, and as far east as Uganda [27]. It produces large, complex, and relatively nutrient-rich leaves. However, the litter typically features a moderate to high lignin and polyphenol content, slowing the rate of decomposition but releasing stabilized organic matter over time. It has a robust, deep taproot system paired with large lateral roots that often form large buttresses. This enables access to deeper soil layers for nutrient and water scavenging [28]. The steady decomposition of its dense canopy litter creates localized organic acids and humus, which act as chelating agents. This lowers soil pH slightly, which can increase the solubility and plant availability of micronutrients like iron (Fe), zinc (Zn), and manganese (Mn) in the topsoil. Its deep taproots can also pump up leached deep-soil nutrients to the surface via litterfall [29].

Table 1 presents the descriptive statistics for soil micronutrients and aluminum concentrations in soils occupied by Entandrophragma utile in the secondary forest plots during the 2022 sampling season. Boron (B) had a mean of 0.39 mg/kg, which is relatively moderate compared to other forest plots in the study. In fact, B is extracted by hot water, and our mean value is close to the critical sufficiency level of 0.5 mg/kg, as reported by Havlin et al. [30]. The coefficient of variation (24%) indicates that boron distribution under Entandrophragma utile was fairly uniform across the sampled points. Iron (Fe) had a mean value of 182.75 mg/kg and a notably high coefficient of variation (112%). This value is close to the toxicity limit. In fact, the toxicity limit for DTPA-extracted Fe is 200 mg/kg, according to Schafer [31]. The risk of Fe toxicity is not rare in tropical soils. It is more available under acidity and waterlogging conditions [27].

Manganese (Mn) had a mean of 0.76 mg/kg, which is below the relative sufficiency threshold of 1 mg/kg, according to Havlin et al. [32]. The coefficient of variation (0,01%) was also low, suggesting low variability and uniformity across the sampling points. Copper had a mean concentration of 0.46 mg/kg, twice below the critical sufficiency level of 0.6 mg/kg (Havlin et al. [30]), with a moderate coefficient of variation (27%). Given copper’s strong binding affinity to organic matter [33], these results may reflect the active organic turnover from Entandrophragma utile, although this plant is rich in lignin and has a relatively high C/N ratio. With a mean of just 0.07 mg/kg, zinc levels were strikingly below the deficiency threshold of 0.5 mg/kg [32,34].

In addition, the low coefficient of variation (0.01%) pointed to an even distribution across the sampling points. The low availability of zinc could be attributed to strong adsorption onto soil mineral surfaces or uptake by plant roots, and both phenomena are common in tropical secondary forests [35]. Aluminum exhibited a mean of 400.75 mg/kg, which was reflective of the natural acid soil conditions of Masako Forest Reserve. The coefficient of variation (297%) suggests high variability, which could stem from the influence of soil and plant processes. Overall, Entandrophragma utile reflected a distinct nutrient signature shaped by the plant’s ecological traits and its impact on soil chemical cycling, underlining the importance of species-specific interactions in a secondary forest ecosystem [33]. Entandrophragma utile promoted slow and conservative nutrient cycling.

3.1.1. Descriptive Statistics for Soil Micronutrients Under Hevea brasiliensis in June 2022

Hevea brasiliensis is a non-nitrogen-fixing species of the Euphorbiaceae family. Known as the Pará rubber tree, sharinga tree, seringueira, or, most commonly, rubber tree or rubber plant, it is a flowering plant belonging to the spurge family, Euphorbiaceae. It is originally native to the Amazon basin but is now pantropical in its distribution [36]. However, it forms arbuscular mycorrhizal (AM) associations. It produces dense, tough, and fibrous leaf litter, with a higher C:N ratio and higher tannin levels, causing it to decompose very slowly. It has a powerful, deep taproot along with an extensive network of shallow, aggressive lateral roots that spread widely to anchor the tall canopy [28]. The slow decomposition of rubber tree litter forms a thick, persistent mulch layer that shields the soil from erosion and moderates soil temperature and moisture. This stable environment fosters rich microbial activity that enhances the slow release of bound trace minerals. The wide lateral root networks extract nutrients from the upper soil strata, while the AM fungi physically expand the rhizosphere, secreting glomalin and organic acids to dissolve and absorb micronutrients [29].

Table 2 summarizes the distribution of soil micronutrients under the canopy of Hevea brasiliensis within the secondary forest plots, offering insight into this plant species’ possible influence on soil micronutrients in the 2022 growing season. The soil under this tree had a mean boron content of 0.21 mg/kg, suggesting that the soil under Hevea brasiliensis maintained boron below the sufficient level [30]. The coefficient of variation of 61.19% reflects noticeable spatial variability commonly seen in soil studies. Iron (Fe) concentration averaged 133.75 mg/kg, a value above the sufficient threshold of 4.5 mg/kg [30], but it did not attain the toxicity threshold of 200 mg/kg [31]. In addition, Fe also had a relatively low CV of 14.25%, highlighting a more uniform distribution across the sampling locations.

Manganese exhibited a low mean concentration of 0.59 mg/kg, well below the sufficiency level of 1.0 mg/kg [30], but it showed significant variability (CV = 168.13%), as commonly seen in soil studies for parameters like soil nutrients. This might suggest that while Mn was generally low in soils under Hevea brasiliensis, isolated pockets with elevated levels existed [37]. The skewness (1.50) confirms the presence of outliers. Copper (Cu) had a mean concentration of 0.45 mg/kg, close to the critical sufficiency level of 0.6 mg/kg [30], with moderate variability (CV = 51.72%). Zinc had a low and consistent concentration of 0.01 mg/kg across all observations, as demonstrated by the zero coefficient of variation (CV = 0.00%). This value was also below the deficiency threshold of 0.5 mg/kg suggested by Lindsay and Norvell [32] and Havlin et al. [30]. This result suggests that Zn availability in soils under Hevea brasiliensis is uniformly limited, possibly constrained by strong soil adsorption mechanisms, plant uptake efficiency, or leaching losses [38].

Aluminum had a mean concentration of 411.25 mg/kg, with relatively low variability (CV = 16.43%). This uniformity may reflect the strongly acidic nature of the soils in Masako Forest Reserve, as aluminum becomes more soluble and mobile in such environments, particularly in the absence of significant pH-buffering organic matter inputs [22].

Overall, the data indicates that Hevea brasiliensis, an introduced species often used in agroforestry systems, seems to maintain relatively stable micronutrient dynamics in terms of iron, copper, and aluminum, while zinc availability remains particularly constrained in its understory soil. This observation underscores the complex interplay between plant species, soil chemistry, and nutrient cycling in secondary forests and highlights the importance of considering species-specific litter inputs and root interactions when assessing soil fertility and forest restoration potential [31].

3.1.2. Descriptive Statistics for Soil Micronutrients Under Milettia laurentii in June 2022

Milletia laurentii is a nitrogen-fixing species of the Fabaceae family. It is a legume tree from Africa and is native to the Republic of Congo, the Democratic Republic of Congo, Cameroon, Gabon, and Equatorial Guinea. The species is listed as “endangered” in the IUCN Red List, principally due to the destruction of its habitat and over-exploitation for timber [39]. It is associated with arbuscular mycorrhizal fungi (AMF), which are highly effective at scavenging immobile soil phosphorus. Because it drops N-rich, moderately decomposable litter, it actively releases organic acids during decomposition, which slightly lowers soil pH [28]. This localized acidification and the addition of root exudates increase the solubility and plant availability of micronutrients like iron, manganese, and zinc. Furthermore, its nitrogen fixation boosts the overall nutrient economy, enhancing microbial activity that further mobilizes trace elements [40].

Table 3 presents the descriptive statistics for soil micronutrient concentrations in soils under Milettia laurentii in the secondary forest. The data shows that boron levels recorded a mean of 0.41 mg/kg, a value close to the sufficiency level. The coefficient of variation was 29.48%, suggesting moderate variability in boron distribution across the sampling plots. Iron concentration averaged 210.00 mg/kg, a value above the sufficient threshold of 4.5 mg/kg [30], and it slightly exceeded the toxicity threshold of 200 mg/kg [31]. This may indicate robust presence and potentially favorable conditions for root respiration and enzymatic activities [37]. The coefficient of variability (CV = 21.29%) was relatively low, suggesting consistent iron distribution.

Manganese (Mn), on the other hand, exhibited a mean of 1.50 mg/kg, close to the sufficiency level, but it showed a high variability (CV = 114.89%), indicating considerable spatial heterogeneity. Copper (Cu) had a mean concentration of 0.53 mg/kg, close to the critical sufficiency level of 0.6 mg/kg according to Havlin et al. [30], with a relatively high CV (58.42%), highlighting variability in copper distribution. Copper dynamics are closely linked to soil organic matter content and microbial activity. Under this species, microbial activity is high. Arbuscular mycorrhizae (AM) improve Zn and Cu availability, both of which can vary significantly across even small spatial scales, particularly under heterogeneous canopy structures like those of Milettia laurentii [38].

Zinc was low on average (0.09 mg/kg) and displayed the highest variability (CV = 308.96%) among all micronutrients studied. This suggests that zinc availability was highly uneven [29]. The high skewness (3.02) and kurtosis (7.09) further confirm the presence of extreme outliers, where a few locations had significantly elevated Zn levels while the rest remained deficient. Aluminum was present at a mean concentration of 462.83 mg/kg, indicating acidic soil conditions typical of humid tropical forests [40]. The moderate variation (CV = 29.36%) and skewness (0.80) suggest that although aluminum levels were generally consistent, certain locations exhibited higher-than-average concentrations. Elevated aluminum levels in the rooting zone could potentially limit nutrient uptake due to toxicity, though Milettia laurentii may exert some resistance through root exudates or association with tolerant mycorrhizal fungi [38]. In summary, the data from Milettia laurentii plots reflected a nutrient environment shaped by dynamic biological inputs and moderate-to-high spatial heterogeneity, particularly in trace metals like Mn and Zn.

The leguminous nature of Milettia laurentii and its litter quality may have contributed to improved micronutrient cycling, although localized limitations in Zn and Mn warrant further investigations in relation to plant productivity and ecosystem functioning.

3.1.3. Descriptive Statistics for Soil Micronutrients Under Musanga cecropioides in June 2022

Musanga cercopoides is a non-nitrogen-fixing species of the Urticaceae (formerly Cecropiaceae) family. Musanga cecropioides, the African corkwood tree or umbrella tree, is found in tropical Africa from Sierra Leone to Angola and east to Uganda. This tree is also known as parasolier, n’govoge, govwi, doe, kombo-kombo, musanga, and musanda [41]. It is a rapidly growing pioneer species essential for secondary forest regeneration in tropical Africa. It produces high-quality, “fast” litter characterized by low C:N ratios and high concentrations of base cations (Ca, Mg, K). This leads to rapid decomposition and a quick release of nutrients back into the topsoil [28]. The tree features a shallow, expansive root system with prominent stilt roots (adventitious roots) that provide stability in moist soils and maximize nutrient capture from the decomposing litter layer. It typically enhances micronutrient availability by mobilizing elements like Fe, Mn, and Zn through fast organic matter turnover and specialized fungal symbiosis [42].

Table 4 presents the summary of statistics of micronutrients in soils under Musanga cecropoides. The data illustrates significant variability in nutrient dynamics, reflecting the influence of Musanga cecropoides ecological characteristics on soil chemistry. Boron had a mean concentration of 0.12 mg/kg, a value below the critical sufficiency level (0.5 mg/kg) as reported by Havlin et al. [30]. Boron is quickly released, but there is a high risk of leaching. The coefficient of variation (CV = 117.86%) was very high, indicating considerable spatial heterogeneity. The minimum value (0.01 mg/kg) and maximum (0.35 mg/kg), along with high skewness (0.57), might suggest that while most areas recorded low B levels, a few hotspots contained elevated concentrations.

Iron (Fe) averaged 114.50 mg/kg, a value above the sufficient threshold of 4.5 mg/kg [30], but it did not attain the toxicity threshold of 200 mg/kg [31]. Fe also showed moderate variation across the plots (CV = 19.04%). The narrow spread in iron values (79.00–142.00 mg/kg) reflects consistent availability in the rooting zone, which is vital for enzymatic activity and chlorophyll synthesis. Despite moderate skewness (−0.41), the negative value hints at a slightly left-skewed distribution, implying the majority of plots had Fe levels above the mean.

Manganese (Mn) had a mean of 40.08 mg/kg, well above the sufficiency level of 1.0 mg/kg [32], but it did not exceed the toxicity threshold of 60 mg/kg according to Schafer [31]. This micronutrient is released quickly and hardly retained in the litter layer [43]. Mn also had a substantial variability (CV = 98.47%), confirming its heterogeneity in soils around sampled trees. Copper (Cu) had a mean of 0.66 mg/kg, just at the critical sufficiency level of 0.6 mg/kg according to Havlin et al. [30], with values ranging from 0.01 to 1.42 mg/kg. The relatively high variability (CV = 61.06%) and skewness (0.33) indicate that copper distribution was moderately inconsistent under Musanga cecropoides. Since Cu is tightly bound to organic matter [44], the differences in litterfall and decomposition rates may contribute to spatial fluctuations in Cu concentrations.

Zinc exhibited an unusually high mean of 4.99 mg/kg, exceeding the deficiency threshold of 0.5 mg/kg suggested by Lindsay and Norvell [32]. Perhaps this could be related to the rapid decomposition of organic matter or other soil and plant processes. The CV was also very high (164.56%), and the standard deviation (8.21 mg/kg) confirmed its extreme variability. The maximum value (24.31 mg/kg), skewness (1.57), and association with a CV of 164.56% may suggest that certain microsites beneath Musanga cecropoides are significantly enriched with Zn, certainly due to soil and plant processes [45].

Aluminum had a mean concentration of 294.33 mg/kg, characteristic of acidic tropical soils, and Musanga cecropoides can thrive under such conditions. Elevated Al levels may influence the mobility of other nutrients and affect root growth. Overall, Musanga cecropoides seems to influence the soil environment in ways that promote both enrichment and spatial heterogeneity of several micronutrients, particularly Zn and Mn. These patterns may reflect the species’ ability to alter microclimates, organic inputs, and microbial activity beneath its canopy, further influencing nutrient availability and transformation.

3.1.4. Descriptive Statistics for Soil Micronutrients Under Treculia africana in June 2022

Treculia africana is a non-leguminous, fast-growing, evergreen tree in the Moraceae family, mostly found in tropical, semi-deciduous forests and swampy or riparian environments. It can be used as a food plant and for various other traditional uses [46]. It has litter quality resulting from its broad, nutrient-dense leaves that are generally lower in lignin, allowing for rapid decomposition. Its root structure is fine-meshed and highly branched, optimized for large-volume soil exploration in moist, clay-heavy soils. However, it has strong associations with Ectomycorrhizal Fungi (EMF) and, to a lesser extent, Endomycorrhizal (AMF) fungi. Its association with these enzymes makes this species effective at chelating and releasing organically bound micronutrients (such as copper and zinc), making them more available for plant uptake [28].

The descriptive statistics for micronutrients in soils under Triculia africana in the secondary forest are presented in Table 5. The data shows that boron had a relatively high mean concentration of 0.50 mg/kg at the critical sufficiency level of 0.5 mg/kg, as reported by Havlin et al. [30]. This concentration was also the highest among all species studied in this secondary forest. This suggests a possible strong contribution of Triculia africana to boron accumulation in the soil, which may be due to its leaf litter quality and decomposition rate. The coefficient of variation (CV = 49.18%) indicates moderate variability, with values ranging from 0.01 to 0.84 mg/kg. The slight negative skewness (−0.29) suggests a distribution slightly biased toward higher boron values.

Iron (Fe) levels averaged 215.67 mg/kg, a value above the sufficient threshold of 4.5 mg/kg [29] and which also slightly exceeds the toxicity threshold of 200 mg/kg [30], therefore causing a risk of toxicity. The CV (33.48%) and standard deviation (72.21 mg/kg) show moderate variation. The skewness was minimal (0.02), pointing to a fairly symmetrical distribution of Fe in the soil. Iron availability is often linked to organic matter and redox conditions, both of which may be stabilized under the canopy of Triculia africana [45].

Manganese (Mn) concentration (2.34 mg/kg) was above the sufficiency level but below the toxicity level, with high variability. Copper (Cu) had a mean value of 0.44 mg/kg, close to the sufficiency level. This micronutrient is complexed by organic matter, reducing its losses [43]. Cu had a CV of 49.45%, suggesting moderate variability. The values were tightly clustered around the mean (0.01–0.83 mg/kg), and the distribution was nearly symmetrical (skewness = −0.05), implying consistent Cu levels in the soil. Zinc had a mean of 0.59 mg/kg, close to the deficiency threshold of 0.5 mg/kg suggested by Lindsay and Havlin [30,32]. It also had a high variability. Aluminum, a key indicator of soil acidity and weathering, averaged 433.50 mg/kg. The CV (36.44%) and moderate skewness (0.67) do not suggest extreme variability.

The distribution of Al across the plots (205.00–714.00 mg/kg) suggests that Triculia africana may influence Al dynamics, possibly through litter that contributes to soil acidification and mineral leaching [31]. In general, Triculia africana appears to contribute to the enrichment of several key soil micronutrients, especially boron and iron. However, the significant variability observed in Mn and Zn concentrations highlights the influence of soil and plant processes. The species’ dense canopy and litter quality may help regulate nutrient cycling, but the heterogeneous distribution underlines the complexity of soil-plant interactions in tropical secondary forests.

3.2. Analysis of Variance for the Effect of Soil Sampling Depth and Plant Species on Soil Micronutrients in the Secondary Forest in June 2022

Table 6 shows the analysis of variance for the effect of soil sampling depth and plant species on soil micronutrients in the secondary forest in June 2022. Soil depth (SD) only significantly affected the distribution of manganese (Mn) and aluminum (Al). Mn showed a highly significant difference across depths (p = 0.0014), with concentrations peaking at the surface layer (0–10 cm: 18.30 mg/kg) and declining sharply in subsurface layers (10–20 cm: 4.06 mg/kg; 20–30 cm: 4.81 mg/kg). These values were above the sufficiency level of 1.0 mg/kg [29] but not above the toxicity threshold of 60 mg/kg, according to Schafer et al. [31].

This pattern is ecologically consistent with the known behavior of Mn, which accumulates in surface soils due to organic matter inputs, litter decomposition, and root exudates that enhance Mn solubility and bioavailability in the upper horizons [31]. Moreover, microbial processes, particularly in moist tropical soils, can lead to a concentration gradient favoring Mn retention in topsoil [35].

Aluminum concentrations also varied significantly by depth (p = 0.0216), with the highest levels found in the deepest layer (20–30 cm: 443.80 mg/kg). This aligns with previous findings suggesting that weathering and leaching in acidic tropical soils lead to Al accumulation in lower horizons, where lower pH and less organic matter inhibit Al complexation [34]. Such elevated Al levels in subsurface soils may pose constraints for root development, especially in sensitive crop or tree species, due to aluminum toxicity [42]. Although B, Fe, Cu, and Zn did not show statistically significant changes across depths, their slight fluctuations suggest underlying trends possibly masked by inter-sample variability. The analysis also revealed that plant species (PS) were a major determinant of micronutrient concentrations, with significant effects observed for B (p = 0.0001), Fe (p = 0.0001), Mn (p = 0.0001), Zn (p = 0.0087), and Al (p = 0.0075).

These results may emphasize the role of vegetation in influencing soil chemistry through litter inputs, root turnover, canopy cover, and interactions with the microbial community [29]. However, they may also be due to other soil or plant processes, as this study is the first conducted in this secondary forest. Triculia africana exhibited the highest B and Fe concentrations of 0.50 mg/kg and 215.67 mg/kg, respectively. These values may suggest that this plant might be a nutrient-accumulating species that contributes significantly to micronutrient enrichment through leaf litter and root exudates. However, more studies will need to be conducted to confirm such ability. Its possible association with elevated Mn and Zn also points to a potential to rehabilitate nutrient-depleted soils [45]. Musanga cecropoides had a distinctive effect on Mn (40.08 mg/kg) and Zn (4.99 mg/kg). Although these concentrations were above the sufficiency threshold [32], it may be thought that Musanga cecropoides’s organic residues are particularly rich in these nutrients or that it promotes conditions favorable for their mobilization, although more studies are needed to confirm this possibility. This could relate to rapid litter turnover and higher microbial biomass under Musanga cecropoides stands, enhancing the mineralization of these elements.

Milettia laurentii and Entandrophragma utile showed moderate effects, particularly in contributing to Cu and Fe dynamics. Their modulating capacity (in the case of legumes like Milettia laurentii) might have improved micronutrient mobilization and retention. Hevea brasiliensis, a non-native plantation species, showed the lowest values for most nutrients, which may reflect its lower contribution to nutrient cycling, possibly due to slow-decomposing litter or reduced microbial association in comparison to native species. In fact, its litter is of moderate quality, but it has an active rhizosphere. However, this species is very sensitive to management.

There was a significant interaction between soil depth and plant species (SD × PS) for Mn (p = 0.0001), suggesting a synergistic relationship where the effect of one variable is dependent on the level of the other. In Figure 3, it seems like Mn concentration is always higher at 0–10 cm depth. The interaction also shows that, except for the soil under Entandrophragma utile, which has a similar level of Mn in the 0–10 and 10–20 cm layers, all other plant species’ soils had their highest concentration of Mn in the 0–10 cm surface layer, although this is a little difficult to see in the figure because of the scale.

The soil under Hevea brasiliensis had exceptionally high concentrations of this micronutrient. In Table 4, it seems like the higher values are for Musanga cecropoides, with a risk of toxicity [30]. For the other nutrients, the lack of significant interaction suggests that their availability is driven more independently by either species or depth, without strong synergistic effects between the two [31].

3.3. Analysis of Variance for the Effect of Soil Sampling Depth and Plant Species on Soil Micronutrients in the Secondary Forest in June 2023

Table 7 presents the results of the analysis of variance for the effect of soil sampling depth and plant species on soil micronutrients in the secondary forest in June 2023. In 2023, only Mn exhibited a statistically significant response to variation in depth (p = 0.0166). The highest concentration of Mn was observed in the topsoil layer (0–10 cm), averaging 5.45 mg/kg, while substantially lower values were recorded at both 10–20 cm (1.70 mg/kg) and 20–30 cm (1.35 mg/kg). Mn levels were, therefore, at an elevated level in the top layer (0–10 cm) but sufficient at 10–20 cm [29].

This pattern may suggest that manganese availability is largely concentrated in the surface layer, perhaps due to enhanced microbial activity and organic matter content [31]. Although other nutrients such as zinc (Zn) and Copper (Cu) also showed some numerical variation with depth, their differences were not statistically significant (p = 0.2609 and p = 0.4916, respectively). For instance, Zn decreased slightly from 1.56 mg/kg in the top layer to 0.96 mg/kg and 0.76 mg/kg in the lower layers, while Cu increased slightly with depth, suggesting different cycling behaviors.

Similarly, boron, iron, and aluminum did not show any significant differences with soil depth, indicating a relatively uniform vertical distribution of these nutrients in the secondary forest profile. This may be attributed to factors such as slower leaching, chemical stabilization by organic matter, or limited biological redistribution. In contrast, plant species (PS) had a more pronounced influence on certain micronutrients. Statistically significant differences were observed for both Fe and Zn, with p-values of 0.003 and 0.0037, respectively.

For iron, Millettia laurentii recorded the highest mean concentration at 199.25 mg/kg, followed closely by Treculia africana and Musanga cecropoides (192.67 and 188.33 mg/kg, respectively). In contrast, Entandrophragma utile and Hevea brasiliensis had significantly lower Fe concentrations, indicating that these latter species may contribute less to iron enrichment or mobilization in the soil. The higher Fe under Millettia laurentii and Treculia africana may result from differences in litter quality, microbial association, root exudates that enhance Fe solubility, or other soil and plant processes. Regarding zinc, the soil under Treculia africana was clearly distinct, with a mean Zn concentration of 2.76 mg/kg—more than double that of the next closest species (Musanga cecropoides, 1.10 mg/kg). The other three species, including Hevea, Millettia, and Entandrophragma, had considerably lower Zn levels (ranging from 0.40 to 0.76 mg/kg). This substantial difference suggests that Treculia africana may play a disproportionately important role in enhancing Zn availability in forest soils, potentially through its litter input, root–microbe interactions, or nutrient release patterns during decomposition [31].

Although the other nutrients—boron, manganese, copper, and aluminum—did not show statistically significant variation among species, the numerical patterns still offer important ecological insights. Treculia africana, for instance, consistently exhibited the highest mean values for boron and manganese, suggesting that it may foster microenvironments enriched in these elements, even if variation among species was not strong enough to be statistically conclusive. Lastly, the interaction effects between soil depth and plant species were not statistically significant for any of the micronutrients. The absence of significant interactions reinforces the notion that species and depth contribute separately to the observed patterns in micronutrient distribution.

Table 8 shows the combined analysis of variance for the effect of the year of sampling (YS), soil sampling depth (SD), and plant species (PS) on soil micronutrients in the secondary forest in June 2022 and 2023. The effect of the year of sampling on micronutrients is prevalent for B (p = 0.0001) and Mn (p = 0.0008) concentrations, which remained significantly higher in 2022 than in 2023. However, Cu (p = 0.0196) and Al (0.0001) concentrations significantly increased in 2023 compared to 2022. Fe and Zn did not significantly change.

The effect of soil depth (SD) on soil micronutrients was significant only for Mn (p = 0.0001), which remained highest in the topsoil layer (0–10 cm). In magnitude, a similar trend was observed for Fe and Zn, although it was not significantly different.

The soils under the plant species we investigated significantly differed in their B, Fe, Mn (p = 0.0001), and Zn (p = 0.05) concentrations. Compared to other plant species, the soil under Treculia africana had significantly the highest concentrations of B (0.37 mg/kg), although it was below the critical sufficiency level (0.5 mg/kg) as reported by Havlin et al. [30]. In addition, the same soil under Treculia africana (204.17 mg/kg), together with the soil under Milettia laurentii (204.63 mg/kg), also had the highest Fe levels.

These concentrations exceeded the sufficient threshold of 4.5 mg/kg [30], and they also slightly exceeded the toxicity threshold of 200 mg/kg [31]. Furthermore, the soil under Musanga cecropoides had significantly the highest concentrations of Mn (21.21 mg/kg) and Zn (3.04 mg/kg). Significant YS × PS interactions were found for all soil micronutrients, including Mn, which also had significant YS × SD (p = 0.0349), SD × PS (p = 0.0001), and YS × SD × PS (p = 0.0001). The YS × PS interaction is shown in Figure 4 for B, Fe, and Cu. The YS × PS interactions for Mn and Al are discussed, but the figure is not shown.

The soil under Hevea braziliensis had the same concentration of boron in 2022 and 2023, while that under the Musanga Cecropoides canopy had its highest boron concentration in 2023. In addition to that, all the soils under Milettia laurenti, Entendphragma utile, and Treculia africana had higher concentrations of boron in 2022 than in 2023. Similarly to B, iron (Fe) also had a higher concentration in the soils under Entendphragma utile and Treculia africana in 2023. Like for B again, the soil under Hevea braziliensis had the same concentration of Fe in both years. Finally, for Fe, the soil under Musanga Cecropoides followed the same trend as for B in 2023, while Milettia laurentii had the same concentration of Fe in both years.

Manganese (Mn) had a higher concentration in 2022 in the soil under Musanga cecropoides, while the other soils under Hevea braziliensis, Entendphragma utile, and Treculia africana had higher Mn in 2023, except for Milettia laurentii, which had the same level of Mn in 2022 and 2023, like for Fe. The results for Cu were the same as those for Mn. Finally, the soil under all plant species had higher aluminum in 2023 than in 2022.

The SD × PS interaction confirmed that soil nutrients were mostly concentrated in 0–10 cm, with Musanga cecropoides having the highest concentration, followed by Treculia africana, Entendphragma utile, and Hevea braziliensis. The soil under Milettia laurentii seemed to have equal concentration in all soil depths, but it was slightly higher in the 10–20 cm layer. Finally, the YS × SD interaction shows that sampling year 2022 had the highest micronutrient concentrations in all depths compared to 2023, with the 0–10 cm being superior to the other two depths.

4. Conclusions

This study suggests that many of the micronutrients studied in this secondary forest at Masako Forest Reserve seem to be concentrated in the topsoil (0–10 cm). It also suggested that plant species such as Treculia africana, Millettia laurentii, and Musanga cecropoides may play active roles in enhancing micronutrients in soil in which they grow, especially iron (Fe) and zinc (Zn) distributions. However, these results need to be confirmed with future investigations. In fact, to our knowledge, this is one of the first studies to explicitly quantify the species-driven spatial heterogeneity of micronutrients across the soil profile in the secondary forests of Masako Forest Reserve. For policymakers, integrating these species-specific nutrient dynamics into regional conservation policies (such as REDD+) will ensure that forest restoration efforts not only sequester carbon but also restore the biochemical vitality of tropical soils.

Limitations of This Study

A potential limitation of our study is that we did not isolate the 0–5 cm depth, which may exhibit sharper nutrient gradients due to recent litter fall. Future studies at Masako Forest Reserve would benefit from splitting the topsoil into 0–5 cm and 5–10 cm increments to detect ultra-surface stratification.