1Forestry Technology Department, Federal College of Forestry, Jos - Plateau, Nigeria, 2Science Laboratory Technology Department, Federal College of Forestry, Jos - Plateau, Nigeria, 3Pest Management Technology Department, Federal College of Forestry, Jos - Plateau, Nigeria, 4Agricultural Technology Department, Federal College of Forestry, Jos - Plateau, Nigeria
Forest vegetation cover engenders enhanced soil fertility and microbial community structure based on litter diversity change, which significantly affect metabolic quotient. Soil quality assessment is essential to monitor forest ecosystem stability. Thus, the effects of Albizia lebbeck plantation on some soil physicobiochemical properties after 13 years of planting were assessed in the Federal College of Forestry Jos. A 2 × 2 factorial experiment in a randomized complete block design (RCBD) was employed consisting of four treatments combinations in three replicates. There were significant % increases ranging from 26.7 to 51.4% (sand), 24% to 70% (cation exchangeable capacity), 31.90% to 61.25% (pH [H2O], and 44.70% to 62.45% (pH(KCl). Highest % increase of Na (285.7%), P (122.8%), and K (121.0%) was obtained from T1D1, T1D1, and T1D0, with variations in physicobiochemical properties (silt, clay, sand, bulk density, textural class, and porosity), chemical (Na, P, K, OC, and OM), heavy metal (Cr, Cu, and Mn). There were significant effects on soil enzymes (urease, phosphatase, and dehydrogenase), over the control (adjacent non-vegetated plot) at the investigated depths. Soil indices indicated moderate contamination of Cu, Mn, and Cr with no ecological threats under A. lebbeck cover, although Cr might pose a potential health risk. These findings revealed the nitrogen-fixing and bio-remediating capacities of the tree species, which could be exploited for agro-forestry system as green nitrogen source, while ensuring hazard-free and sustainable environment.
Keywords: Vegetation cover, heavy metals, physical properties, mineral elements, enzymes, Albizia lebbeck
In general, anthropogenic factors constitute the major drivers of forest degradation in the tropical forest. These activities affect the distribution and supply of soil nutrients as well as effecting biological transformations, leading to a decrease in the productivity capacity and alterations in the ecological function of the soil with a consequence of an increased biochemical activity. Forest soils are subject to fewer agricultural practices, yet forest vegetation impact on soil, and especially the impact of trees, differs in many ways. The impact of tree species on soil fertility and microbial community structure differs significantly with the type as well as metabolic requirements. In the tropical forest ecosystem, high soil acidity engendered by increased rainfall and temperature resulted in high microbial degradation of organics and nutrient leach usually leads to high C and Al, and lowered Ca and Mg. These coupled with the quantity and quality of litter composition can affect soil carbon availability and microbial utilization efficiency. Change in litter diversity proportionately affects metabolic quotient significantly.
The assessment of soil quality is essential to monitor forest ecosystem stability. Soil properties that change slowly overtime may not be useful tools for soil quality assessment, especially under drastic environment fluctuations. Consequently, soil properties that respond rapidly to environmental stress could be deployed for soil quality evaluation. Biological and biochemical soil properties have been found useful for their prompt responses to changes in the environment. These properties directly relate to population and activity of soil microbes and their enzymes as well as soil organic biomass. Soil or land cover, was described by the Organization for Economic Cooperation and Development, as the proportion of the total arable cropland under vegetative cover throughout the year, observed as one of a series of useful indicators for assessment the environmental performance of agriculture overtime. It was developed to report on the state of and trends in the impact of agriculture on the environment. As a forest soil management practices, forest vegetation or canopy cover confers partial protection and are less susceptible to degradation processes such as erosion, organic matter depletion, structural degradation and loss of fertility.
Some of the primary factors affecting soil quality are the management practices which include cultivation practices, length of time of fallowing, agroforestry practices, type of species and their planting densities, litter quality and quantity, etc. These have strong influence on forest soil fertility. This paper, therefore, focuses on the effect of 13 years of Albizia lebbeck plantation on some soil physicobiochemical properties in the federal college of forestry, Jos.
The study was carried out in Federal College of Jos Plateau state. The area lies between the southern limit of guinea savanna ecological zone. The soil is sandy, light to dark in color, the soil is well drained and well aerated. It lies between latitude 7°–11° N and longitude 7–25 E with an annual rainfall of about 1460 mm–4800 mm. Temperature ranges from 10 to 32°C with an altitude of 1200 mm above sea level.[9,10] The experimental site was a 13-year fallowed post-agroforestry system of alley cropping of Irish potato (Solanum tuberosum L.) within A. lebbeck plantation. The site was divided into two transects of 200 m2 (i.e. 10 m × 20 m) each. The transects were subdivided into five plots of 4 m × 10 m each. Triplicate soil samples were collected at two different depths: 0–30 cm and 30–60 cm from each plot per transect. The replicate samples for each depth were made into composite, air dried, crushed, passed through a 2 mm sieve, and analyzed using standard procedure. Similarly, triplicate samples were collected from the adjacent non-forested plot (200 m2) of 5 m away from forest stands. The experiment was a 3 × 2 factorial, which comprised three fallowed transects (two forested and an adjacent plot, as control: TO, T1, and T2) and soil depths (0–30 cm and 30–60 cm: DO and D1) making up six treatment combinations (TODO, TOD1, T1DO, T1D1, T2DO, and T2D1), applied in five replicate plots, executed using RCBD.
The soil physical parameters were assessed based on standard procedures as described in Table 1.
Table 1: Soil parameters and their methods of assessment
The enzyme activities of dehydrogenases, urease, and phosphatase were determined using the methods of Alef and Nannipieri. The enzyme activity was measured using PerkinElmer Lambda 25 spectrophotometer. The absorbance for dehydrogenase activity was measured at a wavelength of 485 nm (nanometer). The absorbance for urease and phosphatase was measured at a wavelength of 410 nm, followed by the following calculation:
An = As⨯C⨯Vol. of extract /W⨯Vf ⨯5 / Va
Where: An = absorbance of test sample; As = absorbance of standard solution
C = concentration of standard solution; W = weight of soil used
Vf = total volume of extract; Va = volume of extract analyzed.
This was obtained from the formula below based on.
Mw = (Phos + Deh + Ure⨯10-1)⨯%C
Where: Phos = phosphatase; Deh = Dehydrogenase; Ure = Urease.
The following soil indices were determined.
This was obtained from the formula below, according to Sabba.
If contamination is ≥1 which is moderately contaminated.
This was obtained from the formula below, according to Sabba.
If a HQ >0.2 is obtained, a risk to human health potentially exits.
This was obtained from the formula below, according to Efroymson et al.
QV = Quality value (background/pre-contamination concentration); CI = Heavy metal concentration in the soil fractions; ERF <O = Potential ecological threat; ERF >O = No threat.
Data obtain were subjected to analysis of variance (ANOVA) to determine their significance while significant means were separated using LSD at 5%.
There were significant variations in the effects of A. lebbeck cover after 13 years on the soil textural properties and soil depth. Sand fractions were higher at a depth of 0–30 cm in transect 1 (T0D0 [72.20], T0D1 [74.00], and C0D0 [75.10]) than those of 30–60 cm depth in transect 2 (TID0 [69.10], TID1 [67.20], and C1D0 [74.20]), respectively. The contrary was the case for silt and clay, whereby the transect 2 had higher values than transect 1, as shown in Table 1. The effect on bulk density (BD), was in the order of T0D0 > T0D1 > TID0 > TID1 > T2D0 > T2D1.. Furthermore, soils from non-forested site (control) had significant higher porosity values at lower depth than the treatments [Table 2]. These showed that the soil under the vegetation cover was texturally sandy clay, with silt and clay proportions increased with depth, as the sand fraction reduces in all the transects.
Table 2: Effects of 13 years Albizia lebbeck plantation on the soil physical properties
The forested plots had significant (P < 0.05) higher % base than the control adjacent plot, with highest base saturation of 4.22% recorded in 30–60 cm depth of transect 2. The non-forested soil recorded the highest pH (H2O) of 6.52 at 0–30 cm depth and pH (KCL) of 5.72 (at 30–60 cm depth). All soil samples from forested stands of A. lebbeck had a higher cation exchangeable capacity (CEC) values than those of non-forested at both soil depths. However, significant (P < 0.05) higher values T0D1 (7.45), T1D1 (8.37), and T2D1 (6.11) were recorded at depth 30–60 cm, than T0D0 (6.60), T1D0 (6.37), and T2D0 (5.55) at 0–30 cm depth [Table 3]. The analysis of variance indicated a significant impact (P < 0.05) of the effects of the plantation on these soil parameters.
Table 3: Effects of 13 years Albizia lebbeck plantation on the soil chemical properties
The soil samples from 13 years old A. lebbeck plantation gave higher values for sodium (Na) and potassium (K) over the non-forested transect, at both depths. However, higher values were recorded for soil depth 30–60 cm than 0–30 cm. Conversely, there were significant higher values obtained for nitrogen (N), phosphorus (P), organic carbon (C), and organic matter (OM) from soil of non-forested plot, with higher values recorded at 0–30 cm depth. There were variations in recorded values of carbon: nitrogen ratios. Nevertheless, higher values were recorded for soil samples at 30–60 cm depth than 0–30 cm depth, except at transect 2 [Table 4]. Analysis of variance (ANOVA) of data obtained on these mineral elements showed significant (P < 0.05) effects of vegetation cover and soil depths on these soil parameters, except for N and P (P > 0.05) for land use and soil depth, and Na for soil depth 0–30 cm only.
Table 4: Effects of 13 years of plantation of Albizia lebbeck on the soil mineral elements
The effects of A. lebbeck plantation cover indicated significant higher values Cu, Mn and Cr over the non-forested (control), at both soil depths. However, soil samples from 30 to 60 cm depth had higher concentrations of these heavy metals than those of the 0–30 cm depth. There was a significant difference (P < 0.05) of the effects of vegetation cover and soil depth on contents of these heavy metals of the sampled soils [Table 5].
Table 5: Effects of 13 years old plantation of Albizia lebbeck on the soil heavy metal contents
There was a general increase in the activities of dehydrogenase, urease, and phosphatase enzymes of soils under forest cover over those of the non-forested irrespective of the soil depth. The highest dehydrogenase activities T1D1 (10.10), T0D1 (8.66), and T2D1 (7.04) were recorded at 30–60 cm over the values at 0–30 cm depth. The same trend was observed for urease and phosphatase. In general, all the microbial enzyme activities were found to be highest in the sample soils in the order transect 1 > transect 2 > transect 3 at both soil depths. The analysis of variance of data on these parameters indicated significant effect (P < 0.05) of the plantation on the soil enzymes. Furthermore, the effects were affected by soil depth [Table 6].
Table 6: Effects of 13 years old plantation of Albizia lebbeck on the soil enzymes
From Table 6, it was observed that the effects of 13 years of A. lebbeck plantation had increased % sand, from 26.7 to 51.4%. There were reductions in %silt (41.5%, [T1D0] as the highest % reduction), 46.0% (T1D0) for % clay. Cation exchange capacity recorded % reduction (2.40–17.90%) at depth 30 cm and % increases (from 24% to 70%) at depth 60 cm [Table 7]. There were general increases in pH(H2O) values ranging from 31.90% to 61.25% while a range of 44.70–62.45% was recorded as % increase for pH(KCl) due to 13 years of plantation. There were increases in concentrations of mineral elements such as Na, P, and K, with 285.7% (T1D1), 122.8% (T1D1), and 121.0% (T1D0) as the highest, respectively. The concentration of nitrogen also increased in a range of 2.09–9.09% at depth 0–30 cm and 0.00–14.28% at 30–60 cm depth [Table 8].
Table 7: Comparative effects of 13 years Albizia lebbeck plantation on physical properties
Table 8: Comparative effects of 13 years Albizia lebbeck plantation on chemical properties
The CF computed was compared with bench mark (standard) and it indicated that the soil was moderately contaminated with the heavy metals (Cu, Mn, and Cr) in all the transects and at both depths, 0–30 cm and 30–60 cm. All the ERF values calculated were greater than the benchmark for safety (0.0) at both depths, thus, posing no environmental threats. The values of HQ calculated for copper (Cu) and manganese (Mn) were less than the benchmark of 0.2, posing no hazard threat. On the contrary, however, the values for chromium (Cr) were >0.2 for all treatments, indicating potential threat to human health [Table 9].
Table 9: Soil indices determination for heavy metals
The effects of 13 years A. lebbeck cover were determined on biochemical potential soil fertility. It revealed that the soil under forest cover had higher values and indication better biochemical potential soil fertility (MW). This propensity for fertility was highest on transect 2 at 0–30 cm depth and was in the order of 2.69(T1 D0) > 2.51(T0D0) > 2.44(T1 D1) > 2.43(T0 D1) > 2.23(T2 D1) > 2.22(T2 D0) [Table 10]. Computation of CF indicated that the soil is moderated with the heavy metals (Cu, Mn, and Cr), with no potential ecological threat. Computed HQ poses no threat to human health, but Cr content indicated potential health risk.
Table 10: Biochemical potential soil fertility index (MW)
The observed soil physical properties corroborated the findings of Habtamu et al. that the clay and silts fractions increasing with both land use and soil depth while sand fraction decreased. Furthermore, the BD was highly affected by a combined effect of vegetation cover and soil depth. The textural class was sandy loam Renella et al. found that enzyme inhibition was greater in sandy than in fine-textured soils because the clay fraction protects soil enzyme activity. Geiger et al. proposed that clay surfaces interact with both enzymes and metals and ultimately reduce the toxicity of metals. Clay and mineral contents have been reported to strongly affect extracellular enzyme activity in soil. According to Bi et al., soils under forest cover could become looser with surface soil BD decreasing, depending on the species, while the porosity increases with the development of underground root systems. All these contributed to improve the soil structure and the ability to hold water, which is of great significance for the subsidence land in the semi-arid and arid regions, as well as for subsequent ecological succession.
The variations in the CEC although depth dependent were similar to the findings of Gonnety et al., obtained, from soil under Chromolaena odorata-based fallow which displayed the highest values of exchangeable base contents, CEC, and the lowest C: N ratio, which are characteristics of good quality soil. Soils under plantations of leguminous plant species have the tendency in reducing the soil pH and improving soil electrical conductivity. The pH is considered the main chemical parameter controlling the bioavailability of heavy metals in the soil. This might be due to the accumulation of organic acids secreted or discharged from microorganisms, animals, plant roots, and leaf litters on the surface soil.
Increased mineral contents of the soils under study have been attributed to improved soil electrical conductivity, resulting from the accumulation of K, Na, and Mg ions released from the plant roots. A. lebbeck has been regarded as a nitrogen fixer, which can increase soil N and improve soil fertility by N-fixing bacteria in rhizosphere. Bi et al. investigated the effects of seabuckthorn (Hippophaer hamnoides) on soil amelioration, found it useful for the sustainable development of the damaged ecosystem. During growth of plants, mineral elements are absorbed by root systems, stored in the surface soil in the form of organic matters, and then quickly decomposed by microbes.
Heavy metal dynamics in soils are complex and metal bioavailability depends on a variety of factors including the properties of both the metal and soil environment such as the pH, soil organic matter, soil texture, redox potential, and temperature. Season and climatic conditions can also cause an enhanced or reduced mobility. The soil pH is generally the most important factor controlling partitioning behavior of heavy metals in soil. In general, metal sorption to soil is low at low pH (<5.0) and increases as soil pH increases due to the effects of pH on variable-charged sorption sites. Soil pH had significant positive correlation with concentrations of As, Cd, Cr, Cu, Mn, Se, and Zn.
In the current study, it was observed that the heavy metal concentrations had decreased for Cu and Mn after 13 years of establishment. These reductions have been attributed microbial activities. Soil fungi play an important role in reduction of heavy metal concentration in soil. Such variation could also be affected by vegetation succession process and toxic effect of accumulated heavy metal in the plants growing on plantation. According to Mishra et al., despite the fact that these trace elements are fundamentally needed for plant growth, their increased concentrations in soils may pose serious hazards to the plants, as it inhibits the establishment and growth of the plants. Heavy metals inhibit growth of microorganisms in the soil, which play an important role in the decomposition of organic matter and N fixation, thus soil with high concentration of heavy metals are often poor in organic C and N. Addition of organic matter decreases the bioavailability of heavy metals in soil and facilitates establishment of vegetation on such sites. Natural process such as precipitation and steady decrease in heavy metal concentrations with increasing age of plantations (as observed in this study) indicates that either the metals are accumulated in the vegetative parts of different plant species or they could form complexes with other minerals through chemical reaction. In some plant species, heavy metals absorbed by the roots form complexes which are unavailable for translocation and thus sequestered in the plant tissues.
Wyszkowska et al. investigated the effects of Cu on soil enzymes (dehydrogenase, urease, acid phosphatase, and alkaline phosphatase) and its interactions with other heavy metals (Mn, Ni, Pb, Cu, and Cr). They found that the activity of dehydrogenase was greater in heavy loamy sand, while the activities of other enzymes were higher in light silt clay. In another words, enzyme inhibition due to heavy metals was greater in heavy loamy sand than in light silt clay (except in the case of dehydrogenase).
The dehydrogenase enzyme activity is commonly used as an indicator of biological activity in soils. This enzyme is considered to exist as an integral part of intact cells but does not accumulate extracellularly in the soil. Dehydrogenase enzyme is known to oxidize soil organic matter by transferring protons and electrons from substrates to acceptors.
Urease activity in soils is influenced by many factors. These include cropping history, organic matter content of the soil, soil depth, soil amendments, heavy metals, and environmental factors such as temperatures. For example, studies have shown that urease was very sensitive to toxic concentrations of heavy metals. In general, urease activity increases with increasing temperature. It is suggested that higher temperatures increase the activity coefficient of this enzyme. Therefore, it is recommended that urea be applied at times of the day when temperatures are low. Since urease plays a key role in the hydrolysis of urea fertilizer, it is important to uncover other unknown factors that may reduce the efficiency of this enzyme in the ecosystem. Studies have shown that food crops cultivated on soils found in the area could be contaminated with heavy metals and therefore could expose consumers of such food to serious health hazards.[45,46] Heavy metals affect many characteristics of soils, including their biological properties Huang and Shindo, Khan et al. concluded that heavy metals have an inhibitory influence on soil enzyme.
The present study demonstrated a significant influence of increase soil enzymes activity on the biological soil fertility index [Table 8]. A high biochemical soil fertility index value indicates the possibility of generating high perennial legume cultivation yield and maintaining good soil culture. The enzymes in the soil mainly come from the microbes, excrements (from human and animals) and plant roots, and the increase of their activities reflects improvement in soil qualities such as physical and chemical properties, which are indicative of soil fertility.
The study shows that 13 years of A. lebbeck cover have significant effects on some soil physicobiochemical properties. The plantation cover effect increases in mineral in N, P, K, org. C, and organic matter, also increased the activities of dehydrogenase, urease, and phosphatase enzymes, which would enhance plant growth and boost soil fertility. The Cu and Mn contents indicated on treats on human and environmental health. However, the concentration of Cr seemed slightly above minimum benchmark, which could pose serious treats to human and environmental health.
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