The Potential and Coupling Effect of Compost and <i>Mucuna</i> for Quarry Site Restoration: A Study at the YongwaLimestone Quarry in Ghana

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2.2. Compost Production

Pig manure and cassava wastes were obtained from villages near the Quarry and composted at the plant nursery of the Quarry, applying an open-windrow composting technique ^[3]. The composting process involved heaping a mixture of pig manure and cassava wastes in a ratio of 2:1. That is, each composting windrow consisted of about 100 kg of pig manure and 50 kg of cassava waste. These were mixed and heaped into a cone (Figure 2). The composting windrow was watered occasionally to maintain appropriate moisture for microbial action. It was also turned twice a week to facilitate temperature distribution and uniform decomposition of organic material. It took one and half months for the compost to mature.

2.3. Soil Sampling

The site where the experiment was conducted was bare and somewhat rocky. With the aid of a shovel, initial soil samples (up to a depth of 5 cm) were collected from the site into polythene bags for soil analysis. The site was then divided into three plots, each with an approximate dimension of about 20 x 8 m. Thus, the initial soil samples were collected prior to dividing the site into plots. Final soil samples were collected from each plot approximately three months after treatment. All the soil samples were sent to the laboratory for analysis.

2.4. Field Treatment and Re-vegetation

There were two experimental plots (Plots 1 and 2) and one control plot (Plot 3) (Figure 3). The compost produced was applied to plant M. pruriens seeds on one experimental plot. On the other experimental plot, the Mucuna was planted without the application of compost. The control plot was left blank, without any intervention. As a creeping plant, the Mucuna provided potential for protection against erosion. It has the added advantage of being a leguminous plant capable of fixing atmospheric nitrogen into soil nitrogen ^{[5, 6, 7]}. Therefore the choice of Mucuna species for re-vegetation in combination with compost application, was anticipated would help improve the fertility of the soil and facilitate plant growth. This is critical for future food production on such re-vegetated lands, once decommissioning has taken place. The compost application was done at pointsof planting. The advantage of this approach is that it allows judicious use of the compost when reclaiming very large tract of land such as a quarry site. Three seeds of M. pruriens were planted at each point and planting was spaced at an interval of about half a meter. There were approximately 80 planting points per plot. The planting was done on 13^thJune, 2014. The Mucuna seeds were obtained from the Crop Research Institute of the Centre for Scientific and Industrial Research located at Fumesua, Kumasi, Ghana.

Figure 2. Composting windrows

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Figure 3. Experimental design

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2.5. Growth Rate of Mucuna and Biodiversity Assessment

The growth rate of the Mucuna species was evaluated using changes in leaf size with time. Twenty M. pruriens leaves were sampled from each of the experimental plots once a month on 13^th July, 13^th August and 13^th September, 2014 to evaluate the growth rate of the plant. The species richness of plants that colonised the experimental sites within the three months of planting the Mucuna species was noted and identified by plant taxonomists.

2.6. Soil Analysis

Soil samples were analysed in the laboratory for various soil fertility parameters including (nitrogen (N), phosphorous (P), potassium (K) and carbon (C)). The initial samples as well as samples from each plot were analysed in triplicates. One hundred grams (100g) of each soil sample was weighed and oven dried at a temperature of 105°C for 3 hours. This enabled maximum removal of moisture from the samples. Each sample was subsequently allowed to cool at room temperature (about 25 °C). The samples were then ground with a porcelain mortar and pestle and sieved through pores of diameter 0.1 mm. Total N was analysed using an Automated Kjeldahl Distillation System (model KA-ZDDW-II). Total P was determined by first acid digesting the samples ^[8] followed by colorimetric measurement applying spectrophotometer. Total K was determined after acid digestion with Ternary mixture (20 ml HClO₄:500 ml HNO₃:50 ml H₂SO₄) using ﬂame photometer. Organic carbon was determined by the wet oxidation method ^[9].

3. Results and Discussion

3.1. Growth Rate of Mucuna and Colonisation of Plots

The Mucuna species grew at a faster rate on the plot where the compost was applied, using change in leaf size as a measure of growth rate (Figure 4). The growth rate of the Mucuna on the plot with compost was about twice its growth rate on the plot without compost. There was a gradual increase in plant species richness according to the order: control plot < plot of Mucuna without compost < plot of Mucuna with compost (Figure 5). That is, the rate of colonisation of the plots was least on the control plot and highest on the plot planted with Mucuna applying compost amendment. While only one species of plant was observed on the control plot, five (5) different plant species colonised the plot where the Mucuna was planted without using compost. There were eleven (11) different species that colonised the plot where the Mucuna was planted with compost amendment (Table 1). The control plot was colonised by only Axonopus sp., a grass that also thrived on the experimental plots. It is worth noting that several of the colonisers have economic benefits. For instance, Talinumtriangulare is a commonly eaten vegetable in Ghana. Aqueous and ethanol extracts of Tridaxprocumbens have been shown to possess anti-malaria properties^[10], while Euphorbia hirta is reportedly potent against various types of pathogenic bacteria and plasmodium, which causes malaria ^{[11, 12]}.

Figure 4. Changes in leaf size of Mucuna species on plots with and without compost over a period of three months

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Table 1. Plant species that colonised experimental and control plots

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Tables index

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3.2. Effects of Treatments on Soil Fertility

Presumably, the plot treatment affected soil characteristics, which in turn influenced the diversity of plant species that colonised the plots. Soil analysis was done to ascertain this hypothesis. The soil was generally alkaline, irrespective of treatment, with pH range approximately 8.3 – 8.6. (Table 2). But Mucuna species has also been reported as tolerant to acidic soils ^[13]. The Mucuna, thus, could be useful in treating soils in a broad pH range. The nutrient content (N, P, K, C) of the soil was expectedly low, considering that the top soil was removed and it was left with a somewhat rocky surface. The treatment with compost however showed a relatively increased content in soil nitrogen, phosphorus and organic carbon. Electrical conductivity was also relatively high in the compost treated soil and may portend increased mineral content. The concentrations of potassium and organic carbon in the initial soil (i.e. before treatment) were not different from the contents of the respective nutrients in the soil from the plot of Mucuna without compost. This suggests that treatment of the soil by planting Mucuna without any further treatment did not vary potassium and organic carbon in the soil. However, when compost was applied to cultivate the Mucuna, the concentration of potassium and organic carbon increased significantly, p<0.001 in each case.

Figure 5. Image of site before experiment and plant species diversity attracted to experimental and control plots over a period of three months (A = before experiment; B = control plot; C = plot of Mucuna without compost; D = Plot of Mucuna with compost)

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Comparing the level of nitrogen in the soil before the experiment (initial soil samples) to the levels in soil from the control plot after three months, it appeared that the control plot got naturally enriched with nitrogen from sources that cannot be immediately explained (Table 2). Planting of the Mucuna without application of compost marginally increased the nitrogen content above the levels in the control soil. The soil appeared further enriched with nitrogen when the Mucuna was planted with compost. The variations in soil nitrogen among plots was statistically significant (p=0.002). It is assumed that there was a double dose of nitrogen enrichment on the plot where Mucuna was cultivated in the presence of compost. The compost has the ability to enrich the soil with nitrogen ^[14], while the Mucuna, as a leguminous plant, was expected to also fix atmospheric nitrogen into the soil ^[7]. Thus, although the Mucuna has the ability to fix nitrogen into soil, cultivating it with compost amendment provided further enrichment of soil nitrogen, as well as relative increase in soil phosphorus and organic carbon (Table 2). This relative increase in soil fertility perhaps influenced the growth rate of the Mucuna with compost amendment (Figure 4).

Table 2. Soil characteristics at experimental site before and after treatment

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