PsycheVolume 2012 (2012), Article ID 235840, 11 pagesdoi:10.1155/2012/235840

Research Article

Properties of Arboreal Ant and Ground-Termite Nests in relation to Their Nesting Sites and Location in a Tropical-Derived Savanna

B. C. Echezona,¹ C. A. Igwe,² and L. A. Attama¹

¹Department of Crop Science, University of Nigeria, 410001 Nsukka, Nigeria²Department of Soil Science, University of Nigeria, 410001 Nsukka, Nigeria

Received 11 July 2011; Revised 5 October 2011; Accepted 19 December 2011

Academic Editor: Panagiotis Milonas

Copyright © 2012 B. C. Echezona et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Ecosystem engineers such as ants and termites play an important role in the fertility of tropical soils. Physicochemical analyses were thus carried out on some arboreal ant nests collected from mango (Mangifera indica), bush mango (Irvingia gabonensis), kola (Cola nitida), newbouldia plant (Newbouldia laevis), and oil bean plant (Pentaclethra macrophylla) and on ground nest of termite, Odontotermes sudanensis Sjost. (Isoptera: Termitidae) in Nigeria. Arboreal nests, particularly those of M. indica, were significantly richer in the chemical constituents sampled, compared to those of ground-termite nests or adjacent unaffected soils. Available water capacity of nests from M. indica (60.0%) was significantly higher than those of other sites or locations sampled. While biogenic structures were sandy-loamy in texture, their corresponding adjacent soils were either sandy or sandy-loamy. Soils worked by ants and termites had greater proportions of silt-sized (17.9 versus 9.7) and clay-sized (19.2 versus 9.3) to the detriment of coarse-sized particles (51.2 versus 60.9) and fine-sand-sized particles (11.7 versus 20.1) relative to the adjacent soils. Generally, biogenic structures were about 348% richer in P than their corresponding adjacent soils; an attribute, which holds a strong promise in bioremediation and biofortification of soils especially during amendment.

1. Introduction

Tropical-derived savannah ecosystems are often dominated with patches of arboreal ant and epigeous termite nests (≈5 mounds/m²). Ants are ubiquitous, diverse, and abundant in tropical ecosystems and represent up to 80% of animal biomass [1]. Tree crops are of great economic significance in the forests of Nigeria, and the ants which inhabit those trees profoundly affect the ecosystem dynamics through the modification, maintenance, and/or creation of habitats for other organisms in the forest ecosystem [2].

Ecosystem engineers such as ants and termites play an important role in the fertility of tropical soils. Physicochemical analyses were thus carried out on some arboreal ant nests collected from mango (Mangifera indica), bush mango (Irvingia gabonensis), kola (Cola nitida), newbouldia plant (Newbouldia laevis), and oil bean plant (Pentaclethra macrophylla) and on ground nest of termite, Odontotermes sudanensis Sjost. (Isoptera: Termitidae) in Nigeria. Arboreal nests, particularly those of M. indica, were significantly richer in the chemical constituents sampled, compared to those of ground-termite nests or adjacent unaffected soils. Available water capacity of nests from M. indica (60.0%) was significantly higher than those of other sites or locations sampled. While biogenic structures were sandy-loamy in texture, their corresponding adjacent soils were either sandy or sandy-loamy. Soils worked by ants and termites had greater proportions of silt-sized (17.9 versus 9.7) and clay-sized (19.2 versus 9.3) to the detriment of coarse-sized particles (51.2 versus 60.9) and fine-sand-sized particles (11.7 versus 20.1) relative to the adjacent soils. Generally, biogenic structures were about 348% richer in P than their corresponding adjacent soils; an attribute, which holds a strong promise in bioremediation and biofortification of soils especially during amendment.

Ants and termites as decomposers have been reported as the basis for soil formation [3]. Some of the outcome of this decomposition is nutrient release to the soil worked upon by these organisms. Extensive works have been done on the nutrient recycling, soil formation, and soil structural modification by these animals. Lee and Foster [4] observed that the activities of ants and termites together with their abiotic physical and chemical processes regulate the soil fertility and counteract the physical and chemical processes of soil degradation. Leprun and Roy-Noël [5], Boyer [6], and Mahaney et al. [7] reported a remarkable increase in the mineralogical properties of mounds built by ants and termites. Jouquet et al. [8] and Holt and Lepage [9] found enrichment in mineral nutrients (e.g., NH₄⁺, NO₃⁻) and exchangeable cations (e.g., Ca²⁺, Mg²⁺, K⁺, and Na⁺) on biogenic (nest) structures of termites as compared to that of the surrounding soils. Konaté et al. [10], Macmahon et al. [11], Kristiansen et al. [12] and Jouquet et al. [13] also observed that through the impact on soil by termites, their biogenic structures can constitute patches in the landscape where the availability of soil nutrients for plants is improved. Nests of the harvester ant Pogonomyrmex barbatus typically contain higher concentrations of organic matter, nitrogen, and phosphorus than surrounding soils [14]. Comparative studies by Jouquet et al. [15] on nests made by ants and termites revealed some changes in their soil nutrient properties. They suggested that these changes could be due to greater litter associated with ant nest relative to termite nests, which also could be responsible for changes in the morphology and performance of plants and the composition of plant communities in any agroecology.

Very special biogenic structures created by ants are the so-called ant gardens [2]. Jouquet et al. [2] also indicated that a clear agreement has been established between plant preferences for soil altered by termite activity and termite preferences for plant species growing on their own nests. Ants also have been reported to be capable of building nests of carton materials constructed around epiphytic roots thereby developing aggregation of artificial soil [16, 17]. Buckley [18] noted that direct positive effect of the engineering activity of ants lies in the development of a mutualistic relationships between the ants and the epiphytes, whereby the ants profit from the roots forming an integral part of the nest and increasing its structural stability and an abundant food source close to the nest. It could therefore be suggested that these soil engineers indirectly invade their own availability through the increase in colony fitness (i.e., better nourishment of nymphs, higher alate production, and survival) [2].

While the biogenic structures of these organisms have been shown to influence soil quality, microorganism activities, and plants, very little information is available about the differences in the nutrient status of the nest soils based on their nesting sites, biota, and soil locations. We therefore wanted to test the hypotheses that nest substrate (sites) does not influence nest soil properties and that nest soil properties are not a reflection of the initial soil properties when it was unaffected by soil engineers. The aims of this study therefore were to determine the physicochemical properties of the biogenic structures in relation to their nesting sites and biota nests and to compare the characteristics of these nests and their adjacent surface soil. Particle-size analyses of soils were used to assess the physical properties of the soils. The chemical properties of the soils were determined by estimating their C, N, organic matter, base saturation, cation exchange capacity (CEC), exchangeable bases, and P-contents, as well as their pH in both water and in KCl. To determine the availability of the chemical nutrients to plants and thus assess the relative importance of the various soil characteristics in the ecosystem, we also estimated the available water capacity (AWC) and the dispersion ratios (DR) of the various soils, in addition to the dispersibilities of their clay and silt fractions in both calgon and water. The specific objectives of this study thus were to(i)determine the effect of different tree hosts on the physiochemical properties of nests they inhabit,(ii)compare the physiochemical properties of biogenic structures with that of their adjacent unmodified soils,(iii)ascertain the interaction effects of the nesting sites and soil location on the physiochemical properties of their soils.

2. Materials and Methods

2.1. Study Site and Species Studied

Field samples of arboreal-ant and ground-termite nests were collected at Orba Nsukka in Udenu Local Government Area of Enugu State in Southeast Nigeria (06° 52′N, 07° 24′E; altitude 442 m above mean sea level). Nsukka is situated in a derived savannah belt with some relics of rainforest distributed in patches [19]. The soil is well-drained reddish-brown Typic Paleustult [20]. The annual bimodal rainfall is 1800 mm and spans from April to November of each year [21], with peaks around July and September. The mean monthly temperatures vary between 25°C and 32°C [22]. The study site is a grassy humid savannah with sparse shrub vegetations intermingled with palm trees and some other tree plants forming patches of thickets.

2.2. Field Sample Collection

Six biogenic structures (comprising five from different trees and one from ground soil termite nest—termitarium) were sampled for this study. The five trees were mango (Mangifera indica), bush mango (Irvingia gabonensis), kola (Cola nitida), newbouldia (Newbouldia laevis), and oil bean plant (Pentaclethra macrophylla). Biogenic structures (nests) collected from ants and termite nests and the adjacent surface soils (control) were regarded as two soil locations. This is to be able to compare the constituents of nest and unmodified surface soil. Ant arboreal nests were built by Camponotus acvapimensis Mayr. (Hymenoptera: Formicidae), while the termite nest was built by Odontotermes sudanensis Sjost. (Isoptera: Termitidae). Each ant arboreal nest sample was collected from two trees of the same tree species and about the same age and height to represent the two replications of each treatment. Ground-termite nest samples were taken in pairs from the same soil type (Typic Paleustult) also. Termite mounds were collected and excavated from 0 to 10 cm depth from actively forming nests from the open field. Adjacent soils (without any visible termite—or ant—activity) were sampled at the same depth (0–10 cm depth) and 6 m away from each habitat tree and termite mound. Sampling from adjacent unaffected soils was conducted in pairs from their surrounding environment. Nest samples were randomly collected from actively forming mound samples.

The entire arboreal carton ant nests were pried from their host trees, placed in plastic bags and taken to the laboratory for analysis. The nests were dissected by segmental shaving after being air-dried for seven days to remove the ants. The corresponding termite nests collected were also placed inside plastic bags and taken to the same laboratory after being air-dried for seven days. All samples were sieved with 2 mm mesh and used for laboratory analysis. Each laboratory result was read in triplicate for each sample.

2.3. Laboratory Analysis

The particle-size analyses of soils, termite, and carton nests were determined by the hydrometer method as described by Gee and Bauder [23] using sodium hexametaphosphate (Calgon) dispersant while dieonised water was used separately to disperse soil mechanically only after soaking for 24 hours and distilled water separately as dispersants. The percentage clay-sized and silt-sized particles obtained using calgon were regarded as the total clay and total silt while those obtained with water alone were assumed to be water dispersible clay and silt.

Soil pH for soils and ant/termite affected soil in both 1 : 2.5 soil: 0.1 M KCl suspension and in a soil/water suspension ratio using a Beckman Zeromatic pH meter were determined. The soil organic carbon was determined by a modified acid dichromate oxidation procedure according to Walkey and Black [24] as described by Nelson and Sommers [25]. The percentage organic matter was calculated by multiplying values obtained by “Van Benmelin” factor of 1.724. The exchangeable cations and acidity were determined by the method described by Thomas [26]. The cation exchange capacity (CEC) was calculated as the total of all the exchangeable cations.

Total (Kjeldahl) nitrogen was measured with a block digester [27] and distilled using NaOH. Available P was determined using Bray and Kurtz [28] method. AWC was calculated by Klute [29] method.

Dispersion ratio (DR) was calculated as (WDSi + WDC)/(Tsilt + Tclay), where WDSi is the water dispersible silt, WDC is the water dispersible clay, Tsilt is the total silt and Tclay is the total clay.

The laboratory analyses results were read in triplicates (three times) for each sample.

2.4. Data Analysis

Treatments comprised factorial combinations of six nesting sites of ants and termites nests and two soil locations (adjacent soils and biogenic structures) arranged in a completely randomized design (CRD). These treatment combinations were each replicated two times. The laboratory results were read in triplicates (three times) for each sample making a total of six replications on the whole. Treatment effects were tested through analysis of variance (ANOVA) and differences between means were tested with Duncan’s New Multiple Range Test (DNMRT). Differences were only considered significant when values were lower than or equal to 0.05. Percent values were subjected to angular (inverse sine) transformation (), before analyses of variance were carried out on them.

3. Results

The AWC of ant nests from Cola, Irvingia, Newbouldia, and termitaria did not differ () with one another statistically, but were significantly (; ; ) lower than the AWC of ant nests collected from Mangifera (Table 1). The AWC of M. indica hosted nests (60%) was significantly higher than the AWC obtained from C. nitida (42.3%), I. gabonensis (30.4%), N. leavis (43.3%), termitaria (44.6%), and P. macrophylla (46.7%). Differences amongst the nesting site effects on dispersion ratios (DRs) of nest soils, calgon and water dispersible clay, clay + silt, and silt were not significant. However, there were evident trends of higher calgon dispersible clay + silt and silt and water dispersible clay and clay-silt on arboreal ant nests than on epigeous termite ground mounds. Similarly the AWC (53.8%), calgon dispersible clay (19.2%), clay + silt (36.5%), and silt (15.9%) and water dispersible clay (22.1%), clay + silt (32.8%), and silt (7.7%) were found to be significantly higher on biogenic structures compared to their corresponding adjacent surface soil values of 36.0%, 9.3%, 19.4%, 8.8%, 9.5%, 12.7%, and 3.3%, respectively (Table 2). Although the differences produced by the effect of soil location amongst the dispersion ratio (DR) were not statistically significant, the DR of the biogenic structure was relatively higher compared with those of their adjacent soils.

Table 1: Main effects of soil engineers nesting substrates (sites) on the mean available water capacity (AWC), dispersion ratio (DR), and calgon and water dispersibilities of the biogenetic structure’s clay, clay and silt, and silt.

Soil engineer
Nesting Site
AWC¹  (%)
DR
Calgon dispersed
Water dispersed

Clay
Clay + silt
Silt
Clay
Clay + silt
Silt

Ant
C. nitida
42.3ab
0.72a
12.5a
23.5a
10.6a
15.2a
17.8a
2.5a

Ant
I. gabonensis
30.4a
0.71a
16.0a
34.0a
17.6a
20.7a
24.8a
4.1a

Ant
M. indica
60.0c
0.76a
17.5a
35.0a
14.5a
16.2a
24.8a
8.8a

Ant
N. leavis
43.3ab
0.70a
10.5a
25.0a
11.1a
10.4a
19.8a
5.6a

Ant
P. macrophylla
46.7b
1.16a
15.5a
27.5a
11.6a
21.7a
33.7a
7.1a

Termite
Ground
44.6ab
0.71a
13.5a
22.5a
8.6a
10.7a
15.8a
5.1a

-value
4.08
2.11
2.10
0.94
1.16
0.91
0.93
1.60

d.f.
5
5
5
5
5
5
5
5

value
0.005
0.087
0.088
0.469
0.348
0.485
0.471
0.201

AWC: available water capacity and DR: dispersion ratio.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significantly different.

Table 2: Main effect of soil locations on the mean available water capacity (AWC), dispersion ratio (DR), calgon and water dispersed clay, clay/silt, and silt of different soil locations.

Soil location
AWC¹  (%)
DR
Calgon dispersed
Water dispersed

Clay
Clay + silt
Silt
Clay
Clay + silt
silt

Adjacent soil
36.0a
0.69a
9.3a
19.4a
8.8a
9.5a
12.7a
3.3a

Biogenic structures
53.8b
0.89a
19.2b
36.5b
15.9b
22.1b
32.8i
7.7a

-value
961.23
6.07
39.68
108.0
87.6
287.41
164.28
480.00

d.f.
1
1
1
1
1
1
1
1

value
<0.001
0.220
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001

AWC: available water capacity and DR: dispersion ratio.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significantly different.

Except for the total dispersible silt which differed significantly when the effect of the nesting sites was combined with the soil types, the AWC, DR, and other calgon or water dispersible soil fractions did not show such statistical differences () under comparable combined treatment effects (Table 3). This is such that ant nests of biogenic structure on I. gabonensis contained significantly (; ; ) higher calgon dispersible silt (28.6) than other nest structures or adjacent soil, while the adjacent soil around C. nitida (5.6), I. gabonensis (6.6), M. indica (13.6), N. leavis (8.5), P. macrophylla (9.6), and the termitaria contained significantly (; ; ) lower calgon dispersible silt than other soils. However, there were glaring trends of numerically higher AWC, DR, and soil dispersibilities amongst the nest structures collected from any of the substrates than their corresponding adjacent soils.

Table 3: Interaction effects of soil engineers’ nesting substrates (sites) and soil locations on the values of some soil properties of biogenic structures and their adjacent soils.

Soil engineer
Nesting site
Soil location
AWC¹ (%)
DR
Calgon dispersed
Water dispersed

Clay
Clay + silt
Silt
Clay
Clay + silt
Silt

Ant
C. nitida
Adj. soil
30.3a
0.65a
6.5a
12.0a
5.6a
7.2a
7.8a
0.56a

B. structure
54.2a
0.68a
19.6a
13.0a
15.6b
23.2a
27.8a
4.56a

Ant
I. gabonensis
Adj. soil
31.0a
0.68a
6.5a
13.0a
6.6a
7.2a
8.8a
1.56a

B. structure
29.7a
0.74a
26.5a
55.0a
28.6c
34.2a
40.8a
6.56a

Ant
M. indica
Adj. soil
40.0a
0.75a
14.5a
33.0a
13.6ab
15.2a
20.8a
6.06a

B. structure
83.9a
0.75a
21.5a
37.0a
15.5b
17.2a
28.8a
11.56a

Ant
N. leavis
Adj. Soil
36.2a
0.63a
6.5a
15.0a
8.6a
7.2a
8.8a
1.56a

B. structure
50.9a
0.77a
16.6a
35.0a
13.6ab
13.7a
30.8a
9.56a

Ant
P. macrophylla
Adj. soil
39.9a
0.73a
13.5a
23.0a
9.6ab
11.2a
16.7a
5.56a

B. structure
53.5a
1.60a
18.5a
32.0a
13.6ab
32.2a
50.8a
8.56a

Termite
Ground
Adj. soil
38.4a
0.72a
11.5a
20.0a
8.6ab
9.2a
13.8a
4.56a

B. structure
50.8a
0.70a
16.5a
25.0a
8.6ab
12.2a
17.8a
5.56a

-value
0.92
2.44
0.68
2.62
3.75
0.55
0.89
2.14

d.f.
5
5
5
5
5
5
5
5

value
0.489
0.066
0.641
0.053
0.013
0.737
0.502
0.099

AWC: available water capacity; DR: dispersion ratio; B. structure: biogenic structure; Adj. soil: adjacent soil.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significant different.

The percent C and N, SOM, base saturation, exchangeable bases, and pH values differed significantly amongst the various soil engineering host substrates (Table 4). The carbon contents of kola (7.32%), bush mango (9.14%), mango (6.98%), newbouldia (7.01%), and oil bean (8.27%) were significantly (; ; ) higher than the C content of termitarium (1.23%). The same trend also followed in Mg²⁺. N was significantly (; ; ) higher in oil bean compared to samples, while Ca²⁺ was higher in Kola (5.15 meq/100 g sample) and newbouldia (6.0 meq/100 g sample) compared to others. K⁺ was significantly higher (; ; ) in Irvingia (1.74 meq/100 g sample), mango (1.69 meq/100 g sample), and oil bean (1.78 meq/100 g samples) relative to other soil engineer nests. The soil pH in H₂O was lower in kola (6.38), Irvingia (6.75), mango (6.28), and termitaria (6.60) compared to other nesting sites. Although SOM, the base saturation, CEC, P, and particle-size distributions did not differ between ant and termite nests, there were clear trends of numerically higher values in the ant nests compared to the termite nests. Based on Black [30] classifications, the values of organic C obtained amongst the ant nests were high (between 6.98 and 9.14%) compared to termite nests (1.23%). Similarly, SOM content of N. leavis was significantly higher (; ; ) than other nests, except those from I. gabonensis (15.8%). Base saturation of C. nitida was significantly (; ; ) higher than those of other nests, except those of Irvingia (36.3%). The N-contents of between 0.37 and 1.28% obtained amongst ant nests were classified as very high compared to 0.19% obtained from termite nests according to Metson [31] classification. Again, the SOM of between 12% and 16.5% from ant nests was considered high compared to SOM of 2.5% obtained from termite nests according to Metson [31]. Similarly, by FAO [32] classification, all the tree-ant nests except those of P. macrophylla with base saturations of between 23 and 41% would be regarded as having medium fertility as opposed to ground-termite nests and P. macrophylla nests with base saturation of 20% each and classified as belonging to soils of low fertility. Similarly, both the tree-ant nests and ground-termite nests have high phosphorus content (23.6–54.2 mg/kg) judging by Enwezor et al. [33] classification. The exchangeable Ca²⁺ ranged from low to moderately high in tree-ant nest when compared to ground-termite nest with very low Ca²⁺ [30]. Also K⁺ was high in tree-ant nests relative to the very low value obtained in the ground-termite nests using the classification of Black [30]. Conversely, Mg²⁺ content of tree-ant nests was moderately high compared to the termite nest which was very low after Black [30] classification. Except for N. leavis ant nests with neutral (pH = 7.28) soil reaction, all the ant-and termite-nest soils were slightly acidic with a pH range of between 6.2 and 6.8. The particle-size distribution of the various soil fractions showed that the ecosystem engineers’ nests were all of sandy-loamy soil, while their corresponding adjacent soils were predominantly sandy soils.

Table 4: Main effects of soil engineers’ nesting substrate (site) on the mean values of some physicochemical properties of their biogenic structures.

Soil engineer
Nesting site
C¹ (%)
N¹ (%)
SOM¹ (%)
Base¹ Sat. (%)
CEC
Exchangeable bases
pH value
(mg/kg)
Particle size (%)¹
Textural class

Ca²⁺
Mg²⁺
K⁺
Na⁺
H₂O
KCl
Clay
Silt
Fine sand
C. Sand

Ant
C. nitida
7.32b
0.37ab
12.6b
40.9c
27.9a
5.15 + I
5.20e
0.59a
0.51a
6.38ab
6.33b
53.9c
12.5a
11.0a
17.0a
59.5a
SL

Ant
I. gabonensis
9.14b
0.88b
15.8bc
36.3bc
45.3a
2.80a
5.75f
1.74b
0.51a
6.75ab
6.30b
54.2c
16.0a
18.0a
11.5a
54.5a
SL

Ant
M. indica
6.98b
0.46ab
12.0b
22.7ab
39.2a
2.15a
1.95bc
1.69b
0.46a
6.28a
5.68a
43.0b
17.5a
15.0a
10.2a
57.2a
SL

Ant
N. leavis
7.01b
0.41ab
16.5c
36.2b
45.5a
6.08b
4.50d
1.03a
0.54a
7.28b
6.35b
47.0b
10.5a
13.5a
16.8a
55.5a
SL

Ant
P. macrophylla
8.27b
1.28b
14.4b
20.0a
36.1a
2.15a
2.30c
1.78b
0.49a
6.80b
6.18b
42.8b
15.5a
12.0a
19.1a
53.2a
SL

Termite
Ground
1.23a
0.19a
2.5a
20.9a
36.2a
1.45a
1.00a
0.20a
0.45a
6.60ab
5.63a
23.6a
13.5a
9.0a
20.5a
56.5a
SL

-value
11.94
15.28
9.66
3.83
1.86
38.48
20.05
47.66
2.14
12.99
8.49
5.28
0.92
2.62
0.68
0.55

d.f.
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5

value
<0.001
<0.001
<0.001
0.012
0.142
<0.001
<0.001
<0.001
0.098
<0.001
<0.001
0.002
0.489
0.053
0.641
0.731

SOM: soil organic matter and SL: sandy loam; C. Sand: coarse sand; P: phosphorus. C: carbon; N: nitrogen; CEC: cation exchange capacity.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significantly different.

The result of the soil location effect on chemical compositions showed that biogenic structures contain significantly higher percentages of organic carbon (12.6%), nitrogen (1.10%), organic matter (23.3%), base saturation (33.9%), CEC (51.4%), Ca²⁺ (4.92%), Mg²⁺ (5.2%), K⁺ (2.21%), Na²⁺ (0.53%), and phosphorus (44.1%) than their respective adjacent soil nutrient contents of 0.74%, 0.10%, 1.30%, 25.4%, 1.67%, 1.70%, 0.13%, 0.46%, and 16.1% (Table 5). Similarly, both the pH and particle-size distribution of clay-sized (19.20%) and silt-sized (17.9%) particles in the structures were significantly higher for the nest structures compared with those of their adjacent surface soils of 9.3% and 9.7%, respectively. Both the particle sizes of coarse and fine sand fractions as well as the soil reactions in both H₂O and KCl of the nest structures did not differ significantly with those of their adjacent soil.

Table 5: Main effect of soil locations on the mean values of some physicochemical properties of different soils types.

Soil location
C¹ (%)
N¹ (%)
SOM¹ (%)
Base¹ sat. (%)
CEC
Exchangeable bases
pH value
(mg/kg)
Particle size (%)¹
Textural class

Ca²⁺
Mg²⁺
K⁺
Na²⁺
H₂O
KCl
Clay
Silt
Fine sand
Coarse sand

Adj. soil
0.74a
0.10a
1.30a
25.1a
25.4a
1.67a
1.70a
0.13a
0.46a
6.66a
5.94a
16.1a
9.3a
9.7a
20.1a
60.9a
LS

B. structure
12.57b
1.10b
23.30b
33.9b
51.4b
4.92b
5.20b
2.21b
0.53b
6.85b
6.21b
72.1b
19.2b
17.9i
11.7a
51.2a
SL

-value
321.86
143.75
271.07
6.07
51.52
175.53
96.79
686.77
13.45
7.07
7.40
197.69
16.53
19.31
1.74
3.12
—

d.f.
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
—

value
<0.001
<0.001
<0.001
0.022
<0.001
<0.001
<0.001
<0.001
0.001
0.014
0.012
<0.001
<0.001
<0.001
0.200
0.091
—

SOM: soil organic matter; LS: loamy soil; SL: sandy loam; B. structure: biogenic structure; Adj. soil: adjacent soil; P: phosphorus. C: carbon; N: nitrogen; CEC: cation exchange capacity.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significantly different.

Differences in the combined effects of the nest substrates of soil engineers and soil locations differed significantly with respect to all the physiochemical properties assessed, except Na⁺ content and coarse sand distribution (Table 6). There was a consistent trend of these constituent being higher amongst biogenic structure of ants and termites in the different substrates as opposed to their corresponding adjacent soils. Conversely, in all the chemical properties assessed, the unmodified adjacent soils did not show significant differences amongst the different nest substrates as the nest soils to their microhabitat. Furthermore, the interaction effects of nest sites and soil locations were only significant for C, N, SOM, base saturation. CEC, exchangeable bases (Ca²⁺, K⁺, and Mg²⁺), soil-pH, clay, and silt contents. This is such that they were significantly higher in the various ant-tree nests (biogenic structures) than in ground-termite mound nest and their corresponding surrounding adjacent soils, except amongst the termitaria (Table 6).

Table 6: Interaction effects of soil engineers’ nesting substrates (sites) and soil locations on the mean values of some physicochemical properties of soil engineers’ biogenic structures and their adjacent soils.

Soil engineer
Nesting site
Soils locations
C¹ (%)
N¹ (%)
SOM¹ (%)
Base¹ sat. (%)
CEC
Exchangeable bases
pH value
(mg/kg)
Particle size (%)¹
Textural class

Ca²⁺
Mg²⁺
K⁺
Na²⁺
H₂O
KCl
Clay
Silt
Fine sand
Coarse sand

Ant
C. nitida
Adj. soil
0.6a
0.09a
1.1a
36.2b
12.2a
1.9sb
1.30a
0.12a
0.47a
6.8sb
6.2sb
17.9s
6.0a
6.0a
33.5a
54.5a
S

B. structure
14.0b
0.65a
24.2b
45.6b
43.6c
8.4c
9.10e
1.06b
0.54a
6.8sb
6.5b
90.0b
19.0b
16.0sb
0.5a
64.5a
SL

Ant
I. gabonensis
Adj. soil
0.5a
0.08a
0.8a
48.9b
18.0a
1.7ab
4.00c
0.14a
0.48a
6.7sb
6.3b
10.9a
6.0a
7.0a
23.0a
64.0a
S

B. structure
17.8b
1.69b
30.7c
23.6a
72.6d
3.9b
7.50d
3.33c
0.54a
6.8sb
6.4b
97.5b
26.0b
29.0b
0.0a
45.0a
SCL

Ant
M. indica
Adj. soil
0.7a
0.11a
1.2a
8.9a
36.6bc
1.1a
1.10a
1.10b
0.41a
6.7sb
5.7sb
9.5a
14.0a
14.0sb
7.0a
65.0a
SL

B. structure
13.3b
0.82a
22.9b
36.4b
41.8c
3.2ab
2.80b
3.27c
0.51a
6.4sb
5.7sb
76.6b
21.0b
16.0sb
13.5a
49.5a
SCL

Ant
N. leavis
Adj. soil
0.7a
0.08a
1.4a
26.2a
25.0a
2.5ab
1.70a
0.17a
0.51a
7.4b
6.4b
18.8a
6.0a
11.0a
20.0a
63.0a
S

B. structure
13.2b
0.79a
31.5c
46.1b
66.0d
9.7c
7.30d
1.89bc
0.58a
7.2b
6.4b
75.2b
15.5b
21.0b
14.5a
49.0a
SL

Ant
P. macrophylla
Adj. soil
1.0a
0.14a
1.7a
16.8a
28.6b
1.7ab
1.40a
0.18a
0.48a
6.1a
5.4a
13.4a
13.0a
10.0a
22.0a
55.0a
SL

B. structure
15.6b
2.41b
27.2b
23.2a
43.6c
2.6ab
3.20b
3.37c
0.51a
7.5b
7.0b
72.1b
18.0b
14.0sb
16.5a
51.5a
SL

Termite
Ground
Adj. soil
1.0a
0.11a
1.7a
13.5a
31.8b
1.1a
0.70a
0.08a
0.40a
6.1a
5.9sb
25.9a
11.0a
10.0a
15.0a
64.0a
LS

B. structure
1.5a
0.27a
3.4a
28.4b
40.6a
1.8ab
1.30a
0.33a
0.51a
6.4sb
5.4a
21.4a
16.0b
9.0a
26.0a
49.0a
SL

-value
12.96
14.55
10.31
4.20
4.97
22.20
9.86
44.89
0.34
11.91
9.67
10.45
4.77
9.66
2.62
0.68
—

d.f.
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
—

value
0.001
0.001
0.001
0.008
0.003
0.001
0.001
0.001
0.883
0.001
0.001
0.001
0.004
0.001
0.053
0.641
—

AWC: available water capacity; DR: dispersion ratio; S: sand; SL: sandy loam; SCL: sandy clay loam; LS: loamy sand; B. structure: biogenic structure;

P: phosphorus. C: carbon; N: nitrogen; CEC: cation exchange capacity.

¹Results of angular transformed data presented in the original scale.

Values within a column followed by the same letter are not significantly different.

4. Discussion

Soil engineers, notably ants and termites, have been reported to play important roles in the soil fertility in tropical ecosystems because of their impact on the soils they work on [2, 9, 13]. In the present study, however, efforts were made to ascertain whether these modifications significantly differed between biogenic structures of different soil engineers (ants and termites) and also if the modifications within biogenic structures were different for different nesting sites (trees) or locations.

Results of the laboratory assays however showed that the physicochemical properties of the ant biogenic structures were glaringly higher than those of the termites. Tree ants generally are exposed to more diverse range of plant litter diets and foraging materials than ground termites due to their proximity to dense litter falls from tree hosts. There is therefore always an increased resource access associated with ant nest construction relative to termite nest construction. This could therefore be harnessed in biofortifications of our impoverished tropical agricultural soils. Again, not only the soils of nests worked upon by the ecosystem engineers precisely contained higher values of the constituents assessed than their corresponding adjacent soils, but also their nesting sites played a major role in their constituents. Nesting sites differed as to their AWC, DR, and their dispersibilities in both calgon and water but their differences were not significant except for the AWC. Ant nests on M. indica consistently had higher AWC relative to other ant plant nests or epigeous termite ground nest. The higher AWC of ant nests from M. indica biogenic structures suggest higher tendency of ensuring more water in available form for crop development than those of other soils locations or nesting sites, which makes it still a good promise for biofortification. The process of soil dispersion in either water or calgon which is a function of the soil organic matter content [34] and an index of nutrient availability to plants did not differ either between the nesting sites or between ant nests and termite nests.

Soil texture on the other hand was not significantly affected in the termite nest compared with the ant nests, but was considerably modified in the biogenic structures compared to their corresponding adjacent unmodified soil. Thus, the nests and their corresponding adjacent soils differed as to their textural classes. The soil engineers ensured greater mineralization of clay and silt which was found to be greatly higher in the biogenic structures than in the adjacent soils resulting in differences in their textural classes. All the soils affected by ants and termites had a greater proportion of clay and silt to the detriment of coarse and fine sand. This finding suggests that the ecosystem engineers prefer the selection of finer materials such as clay and silt for building of its structures, thus supporting the result of the ability of termites and ants to select building materials when presented with different physical size materials but not clay type [2, 35, 36]. Termites have been observed to favour finer particles in their mound constructions which match their ecological, physiological, and behavioural needs [35]. In this study, however, there was a clear distinction between the physical size distribution of soils worked by ants and those by termites. The greater proportion of clay and silt as opposed to sand on soils worked by ant and termites showed the greater impact of clay and silt than sand on soils worked upon by ants and termites in the course of their building activities.

Laboratory analyses of the various samples also showed that both the biota nesting sites and the soil locations richly influenced the SOM, base saturation, CEC, and soil-pH and significantly modified C- and N-contents and their exchangeable bases. It suggests therefore that there was more enrichment of C and N and exchangeable bases (Ca²⁺, K⁺, Mg²⁺, and Na⁺) in antnests most especially in antnests built on I. gabonensis and P. macrophylla compared with the termitarium, as nest structures generally were to unaffected adjacent soils. Tree antnests were richer in C, N, SOM, and exchangeable bases (Ca²⁺, K⁺, Mg²⁺, and Na⁺) compared with the termite nests. This result counteracts our first hypothesis as nesting sites have been shown to influence some of the nest soil properties. Therefore, better nest soil enrichment will be assured when nests are harnessed from I. gabonensis, N. leavis, and P. macrophylla trees. The ability of ants and termites to work and move through the soil and to build organomineral structures with specific physical, chemical, and microbiological properties has been well documented [15, 37]. The higher mineral contents of the ant-nests relative to the termite nests and the surrounding soils could be explained by (i) the relative intensities with which the two organisms impact on the soil, (ii) the differential efficiencies of the two organisms in the mineralization of the various nutrients, (iii) the quantum of food (organic matter or litter material) available, or their diversity (structural, biochemical, or biological attributes), and (iv) the soil conditions or fertility where the nest soils were formed. That the adjacent soils of the different nest hosts did not differ significantly in this study goes to excluding soil conditions as possible explanation for the differences in the ant and termite nest’s chemical content and thus supports the hypothesis that nest soil properties are not a reflection of the initial soil properties when they were unaffected by soil engineers. Jouquet et al. [8, 13] suggests availability of more diversified litter materials at the disposal of the ants than at the disposal of termites as the probable cause of this difference. The quantity of litter materials to be found deposited around trees were likely to be higher than the sparse grassy vegetations reminiscence of a derived savannah ecosystem from where the termite mounds were excavated. Although it has been reported that tropical ant diversity positively correlates with plant structures [38], it was still not clear if litter diversity affects litter nesting assemblages and compositions [39]. From the stand point of this result, ants may be considered to be more efficient in the mineralization of organic nutrients than termites. Enhancement of soil nutrient concentrations according to Wagner [40] may be of general importance in understanding how plants benefit from interaction with ants, especially if ants are more likely to nest near plants bearing extrafloral nectaries.

Also the less abundance of organic matter in soils worked by termites may be attributed to the microenvironment where the mounds are located. Jones et al. [41] observed that termite mounds are usually exposed to intensive sunlight which may reduce the activity of microbes involved in the decompositions of organic matter they contain.

All the ant-nest soils irrespective of tree host contain very significantly higher phosphorus content than termite nests. Also the biogenic structures of either ant or termites irrespective of their nesting sites were found to contain significantly higher P-content than those of their corresponding adjacent surface soils. Ant tree nests were between 81 and 130% richer in P-content than termite ground nests. Similarly, the biogenic structures generally were about 348% richer in P-content than their corresponding adjacent soils. The richer P-content of ant-nested structures of trees relative to the ground termite mounds could still be explained by the higher SOM content of the former as opposed to the latter and the efficient mineralization potential of ants over termites. The rich P-content of the nest soil especially the tree nests could enable them be recommended in the biofortification of acidic tropical soils deficient in phosphorus induced by Fe- and/or Al-toxicity problems.

5. Conclusion

The physicochemical properties of arboreal ant nests, the ground-termite nests, and their corresponding adjacent soils were studied in addition to the influence of nesting sites on nest soil characteristics. Nesting site influenced nest soil characteristics. Ant nests pried from tree tops especially those from M. indica were rich in most of the chemical properties assessed compared to those of epigeous ground-termite nests or the adjacent surface soil samples. Holistically, soils altered by soil engineers, notably ants and termites, were also predominantly greater in these basic nutrients relative to their adjacent unmodified soil, whereas soils collected from trees and ground nests were either sandy-loam or sandy-clay-loam in texture; those of their corresponding adjacent surface soils were mainly sandy for soils near trees and loamy-sand for soils near termitaria.

The higher influence of ants than termites on the maintenance of ecosystem heterogeneity through its soil bio-perturbation effects was therefore revealed in this work. Ants in addition to termites have also been seen to influence soil properties by making resources available for other organisms. These findings and more could therefore be harnessed by culturing these organisms and protecting some of these prospective nesting sites like M. indica, P. macrophylla, and I. gabonensis capable of hosting nests of high mineral content. Such nests could be harnessed for a variety of bio-remediation and biofortification purposes for human use. Purposes where the soil engineers could be utilized include landfills to decompose waste, improvement of soils by composting materials, detoxification of hazardous substances, and the production of biomass of animal feed and biochemical. Besides being fed upon by animals, solid wastes constituting environmental hazards are decomposed and readily converted into useful forms for soil improvement. The soil engineers-mediated chemical changes of soil, commonly represented mainly by a shift in pH towards neutral from acidity and an increase in nutrient content, helps in soil detoxification.

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