Ecology of Soil Biodata and Its Roles in Biodegradation
CHAPTER ONE
Significance of the Study
- This study will help the populace to know what ecology of soil biota is all about.
- The role of soil biota in the ecosystem
- It will also help people to know the group of micro organisms called soil biota.
- It will expose to people how soil biota affect degradation of organic materials in the soil.
CHAPTER TWO
CLASSIFICATION OF ORGANISMS
The modern studies have shown that living organisms can be broadly classified into two radically different kinds, prokaryotes (less complex cell structure) and eukaryotes (organisms with true nucleus), which further divided into many other forms. The free-living components of soil biota are bacteria, fungi, algae, actinomyotes and the fauna (Sharmilia et al., 2008) (Tables 1 and 2). According to Edmundo, (2007) soil organisms have been classified on the basis of body width into microflora (1-100µm, e.g bacteria, and fungi), micro fauna (3-120 µm, e.g protozoa, and nematodes), mesofauna (80 µm – 2 mm, e.g collembolan, acari) and macro fauna (500 µm-50 mm,
e.g earthworms, termites) and vascular plants. Sharmilia et al. (2008) stated that organism may be grouped either on the basis of body width viz; micro, meso and macro organisms or on the basis of functional groups viz, mycophagous/herbivores, omnivores and predators, period of soil inhabitation habitat preference or biological activity.
They state that feeding and locomotion are the other two main activities that divide the organism in the different groups. Based on locomotion the soil animals can be distinguished as burrowing ones from the other that move on the soil surface or through the pore spaces/ channels/cavities in the soil. In terms of feeding activity the soil animals can be classified into five major groups. They also classified living organisms on the basis of kingdom. All living things can be classified into one of the five fundamental kingdoms of life namely; Moriera, Protista, Fungi, Plantae, Animalia and are well represented in soil ecosystem,
(i) Kingdom Moriera: includes prokaryotes-single cell organisms that do not possess nucleus e.g bacteria, actinomycetes and blue green algae.
(ii) Kingdom Protista: include single cell organisms that do possess nucleus e.g nucleated algae and slime modules.
(iii) Kingdom fungi: These non-motile eukaryotes lack fragella and developed spores like yeast, moulds, and mushrooms.
(iv) Kingdom Plantae: these eukaryotes develop from embryos and use chlorophyll like mosses and vascular plants.
(v) Kingdom Animalia: the multicellular eukaryotes develop from a blastula (a halo ball of cells).
ACTION OF DIFFERENT ORGANISMS
Soil organisms are very important in agriculture because they mediate many beneficial processes that include recycling of plant nutrients: nutrients like nitrogen, phosphorus and sulphur which occur mostly as organic compounds (in manures, compost, crop residues, soil organic matter e.t.c) that are not available for plant uptake. During decomposition, soil organisms break these compounds and convert the nutrients into inorganic forms that plant can uptake through their root systems. Meeting some crop nutrients requirements through recycling reduces the need for fertilizers (Newton and Chantal, 2010). According to Adekunle and Dafiwhare (2011), microbes (bacteria, Achaea, fungi and protozoa) are very important in all processes related to soil function. The microbial constituents of soil are entirely responsible for breakdown of organic matter and degradation of toxic molecules. Microorganisms are also responsible for the mineralization process in the forest ecosystem. They act on the humus to release carbon dioxide (CO2), water and nutrients which could be absorbed directly by plants. The actions of microbes were summarized by Hoff et al. (2004) to include degradation of complex nutrient sources extra-cellular, transportation of simple nutrients across cell membranes for metabolic processes and tolerating or deactivation of compounds that could inhibit fungal growth. According to Rigobelis and Nahas, (2004) the most important soil nutrient supply to the forest soil environment is the one derived from litter decomposition by action of organism under condition of high air temperature and soil moisture content. These organisms mobilized the chemical elements in the litter and make them re-absorbable by plant roots. They are able to perform these because of their ability to obtain nutrients through absorption.
CHAPTER THREE
MATERIALS AND METHODS
Soil and maize roots
Soil was collected from 5 to 30 cm depth at the INRA Experimental Station in Estre´ es-Mons, France. The soil had a silty loam texture (17.8% clay, 77.3% silt, 3.8% sand), with 0.95% organic C, and a pH (H2O) of 7.6. The soil was air-dried to a moisture content of 120 mg g—1 dry soil for two days, and then immediately sieved to 2– 3.15 mm. All visible organic residues were removed by hand after sieving. The soil was stored at 15 ◦C for a week prior to incubation. Four maize (Zea mays L.) lines (F2, F2bm1, F292, F292bm3), and two maize hybrids (Mexxal and Colombus) were studied. All genotypes were grown in experimental fields at the INRA Experi- mental Station in Lusignan and harvested at physiological maturity. Only the roots were kept for experiments. These roots were washed with a 50 g l—1 sodium metaphosphate solution for 24 h, rinsed with deionised water to remove soil particles, and then dried for one week at 30 ◦C. Calibrated roots of 2–3 mm diameter were selected for the study and represented nearly half of the maize root biomass sampled.
For incubations in sterile conditions, root and soil samples in hermetically sealed plastic bags were sterilized by 45 kGy gamma irradiation from a 60Co source (Ionisos, Dagneux, France). After sterilization, the soil was stored at 4 ◦C for eight weeks until the were determined using an adaptation of the method proposed by Kamphake et al. (1967). Ammonium ions were determined following the method described by Krom (1980).
CHAPTER FOUR
RESULTS
Chemical characteristics of maize roots
The C content of the roots from the different maize genotypes varied significantly and was higher in F2 and F2bm1 than in the other genotypes (Table 1). The variations in N content followed the same patterns, the total root N concentration being significantly higher in F2 and F2bm1 than in F292, F292bm3, Mexxal and Colombus (P ≤ 0.05). No mineral N was detected in the roots (data not shown). The C to N ratio was therefore significantly lower in F2bm1 than in F2, and this of F2 was significantly lower than the other genotypes (Table 1) (P ≤ 0.05). The NDS soluble fraction accounted for less than 20% of the dry matter whatever the geno- type (Table 1) i.e. >80% of the root dry matter were cell walls.
CHAPTER FIVE
DISCUSSION
Impact of colonizing micro-organisms on subsequent root decomposition in soil
The impact of colonizing micro-organisms on maize root decomposition in soil was quantified by sterilizing the soil and/or residues by gamma irradiation.Gamma irradiation is generally assumed to produce less damaging effect on soil properties than do autoclaving, chemical fumigation or cobalt-60 irradiation methods (Wolf et al., 1989; Trevors,1996). The dose used in the present study (45 kGy) was considerably above that observed in normal soil environments ensuring the elimination of most micro organisms unless some specific bacteria and enzymatic activities (denitrifying potential, potential production of CO2 and b-glucosidase, as an example of extra-cellular activity) that have been shown to persist (Jackson et al., 1967; Coleman and Macfadyen, 1966; Lensi et al., 1991). This is why the C mineralization rates measured in glucose- amended sterilized soil were higher than those of sterilized soil alone (Fig. 3). However, this hydrolytic activity could not be maintained due to the absence, in sterilized soil, of micro-organ- isms to produce enzymes. Therefore the C mineralization rates measured in sterilized soil mixed with glucose always decreased from the beginning of incubation onwards. C mineralization rates in sterilized soil were higher when non- sterilized, rather than sterilized, root residues were added (Fig. 4). This indicates that the micro-organisms within the non-sterilized root residues, whatever the genotype, were active and able to mineralize organic carbon, in agreement with electron microscope observations. However, the amount of mineralized C was always less than in non-sterilized soil, indicating that the soil microbial communities developing on decaying roots were more efficient in decomposing root residues than the initial root-colonizing micro- organisms. This might be because an adequate succession of microbial communities, as observed during the decomposition of plant residues in soil, was absent (e.g. Liebich et al., 2007). The large decrease and similarity of the C mineralization rates observed after 3 weeks in non-sterilized residues mixed with either sterilized or non-sterilized soil, showed that after initial colonization, the decomposition process was limited by the chemical characteristics of the maize roots rather than by the origin of the decomposers. Plant residue degradability is generally considered to decrease during the course of decomposition (Minderman, 1968). The decomposing material consists of an increasing proportion of recalcitrant components originating from the residue or from degradation products of microbial origin or otherwise. The decomposition patterns were fairly soon driven by the intrinsic chemical characteristics of the maize roots, i.e., from day 20 of incubation, and the total amount of mineralized C measured when non-sterilized residues were placed in sterilized or non sterilized soil was inversely related to the amount of Klason lignin in the cell walls (R2 ¼ 0.72, data not shown).
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