Decomposition is a microbial process that involves the breakdown of complex organic compounds into less complex inorganic materials (De Mesel et al., 2006; Yadav et al., 2018). This process is fundamental for the release of immobilized nutrients such as nitrogen from plant residues into the soil. Plant residues, including maize leaves and stalks, provide energy to the soil food web in form of organic carbon and nitrogen, and also protect the soil against rain and wind erosion. In addition, plant residues are composed of complex compounds primarily lignin, cellulose, and hemicellulose. Conversely, microbes have been found to breakdown these compounds into nutrients forms that are easily available for plant uptake (Chaves et al., 2004). The decomposition rate is usually influenced by environmental factors including moisture and temperature, C to N ratios in the plant residues, and the number/type of organisms and their interactions (Bengtsson et al., 1995; Ha et al., 2008). More importantly, the C to N ratio of plant residues has been shown to exhibit a significant effect on decomposition (USDA NRCS, 2011). For example, if wheat straw (C to N = 80:1), hairy vetch (C to N = 11:1), and alfalfa hay (C to N = 25:1) are incorporated into the soil, the vetch and alfalfa will be decomposed more rapidly than wheat straw. Likewise, young alfalfa hay (C to N = 13:1) will also decompose more quickly compared with rye straw (C to N = 82:1) and corn stover (C to N = 57:1) due to lower C to N ratio.
Free-living nematodes (FLN) also hold an integral position in decomposition (Neher, 2010; Yadav et al., 2018). These nematodes closely interact with soil microbes and contribute either directly or indirectly to N mineralization through accelerated turnover of microbial cells, modification of the microbial community, and addition of new substrates, thereby increasing the availability of N (Mekonen et al., 2017; Sultana and Bohra, 2012; Yadav et al., 2018). In the soil food web, the decomposition of organic matter can be divided into two energy channels, including a faster bacterial facilitated channel and a slower fungal mediated channel (Ferris et al., 2001). These two channels run simultaneously, and they are dynamic features in the soil ecosystem (Ruess and Ferris, 2004). The prevalence of a specific decomposition channel in the food web is primarily determined by the type of soil and nutrient forms of organic plant residues (Ferris and Matute, 2003; Ingham et al., 1985). According to Ingham et al. (1985), bacteria and fungi play a pivotal role in the soil food web decomposition, particularly mineralizing nutrients from forms otherwise unavailable for plant uptake.
Monitoring consumer organisms of bacteria and fungi (bacterivores and fungivores, respectively), as well as other nematode assemblages such as omnivores and predators, may serve as good indicators of changes that occur in the soil (Ruess and Ferris, 2004). For instance, it has been demonstrated that overgrazing of bacteria and fungi by nematodes (bacterivores and fungivores, respectively) can lower the overall activity of these decomposers (Ferris et al., 2012a). Interestingly, the soil food web structure consists of higher trophic groups (omnivores and predators), which exert a regulatory function on bacterivores and fungivores, thus increasing the cycling of nutrients (Ferris et al., 2012b; Yeates and Wardle, 1996). Neher (2001) observed that nematode excretion may provide approximately 19% of soluble nitrogen into the soil. Under field conditions, studies have estimated that bacterivores and predators contribute, directly or indirectly, around 8 to 19% of N mineralization in conventional and integrated farming systems, respectively (Beare, 1997; Neher, 2010).
Moreover, multiple reports have established that the contribution of nematodes to N mineralization is relatively higher than that of bacteria, and besides N mineralization, they are also capable of mineralizing many other soil nutrients, such as phosphorous, thus enhancing plant growth (Gebremikael et al., 2016; Pokharel, 2011; Wang et al., 2004). The purpose of this study was to assess the influence of decomposing maize residues on the abundance and diversity of free-living nematodes. In this context, we also evaluated the ecological role of these nematodes.
This study was carried out in two separate trials (March–July 2018 and October–February 2019) at Nyangati, Mwea Sub-county, Kenya (Longitude 0°36'45.5“S and Latitude 37°21'18.0“E). The region is classified in agroecosystem zone 3 at 1,202 m in lowland altitude. The soil type in the area is characterized as vertisols (Jaetzold et al., 2009), and receives two rainy seasons per year. The long rains (from March to May) average about 900 mm, whereas short rains (from October to November) average around 800 mm. The minimum temperature and average rainfall recorded in the study area are provided in Table 1, while soil physicochemical properties are given in Table 2.
Minimum temperature and rainfall data at Nyangati study site in Mwea Sub-county, Kenya.
Month | Minimum temperature (°C) | Average rainfall (mm) |
---|---|---|
March | 14.6 | 245 |
April | 14.7 | 619.2 |
May | 14.8 | 299.6 |
June | 13.3 | 78 |
July | 12.7 | 51.5 |
August | 12.3 | 18.8 |
September | 13.0 | 38.5 |
October | 14.6 | 51.5 |
November | 15.5 | 100.1 |
December | 14.7 | 109.2 |
January | 13.6 | 46.5 |
February | 21.6 | 21.6 |
Soil physicochemical characteristics at Nyangati study site in Mwea Sub-county, Kenya.
Soil property | Mean | Standard error |
---|---|---|
Calcium me % | 6.07 | 0.788 |
Clay % | 44.00 | 4.000 |
Copper ppm | 2.95 | 0.090 |
Iron ppm | 23.50 | 3.239 |
Magnesium me % | 3.63 | 0.135 |
Manganese me % | 0.97 | 0.114 |
Phosphorus ppm | 16.67 | 1.667 |
Potassium me % | 0.75 | 0.096 |
Sand % | 46.67 | 3.712 |
Silt % | 9.33 | 0.667 |
Sodium me % | 0.16 | 0.074 |
Soil pH | 5.85 | 0.057 |
Total Nitrogen % | 0.18 | 0.003 |
Total Organic Carbon % | 2.03 | 0.050 |
Zinc ppm | 0.20 | 0.029 |
Note: me = milliequivalent.
This work comprised of two experimental groups, consisting of decomposition plots (incorporated with maize residues at the rate of 5 tons/hectare, which is the rate commonly used by many Kenyan farmers) and control plots, where no maize residues were incorporated. The field plots were arranged in a randomized complete block design (RCBD), with each experimental group consisting of four replicates. Each plot measured 36 m2 (6 × 6 m) and was separated by a 1 m buffer zone between plots. In addition, each plot had six rows per plot. The maize residues were first chopped into small pieces (approximately 10 cm) to enhance the decomposition process and subsequently incorporated into the soil to a depth of 30 cm.
Soil samples were taken monthly for a period of 4 months after the incorporation of maize residue for two seasons (March–July 2018 and October–February 2019). In each plot, five soil cores (100 g each) were randomly collected from the inner rows at 0–20 cm depth, using a soil auger measuring 3.5 cm in diameter through a cross diagonal sampling pattern. During each sampling date, the five soil sub-samples were mixed gently and composited into a 500 g homogenous sample (Coyne et al., 2014).
Each soil sample (250 g derived from a 500 g composite sample) was placed in filter trays and nematodes were extracted and recovered for 48 h using a modified Baermann technique (Hooper et al., 2005). After extraction, nematodes were enumerated by counting and identified to genus level under a compound microscope using diagnostic keys. Afterwards, the FLN nematode genera were classified into four functional guilds, including bacterivores, predators, fungivores, and omnivores (Yeates et al., 1993). They were also assigned into their respective colonizer-persister group, from cp-1 to cp-5, where index values represent life course strategies that are attributed with either r- or K- characteristics (Ferris et al., 2001). Those nematodes assigned to cp-1 are referred to as colonizers that are usually characterized by high fecundity and small eggs. They also require enriched nutrients for favorable growth, have large population changes, and short generation times. Those assigned to cp-5 are known as persisters, which are characterized by few offspring, low fecundity, large body size, and great sensitivity to disturbances (Bongers and Bongers, 1998).
Data on average rainfall and minimum temperature (Table 1) during the field trials were obtained from Kenya Meteorological Department, Nairobi, Kenya. The soil physicochemical properties (calcium, clay, copper, iron, magnesium, manganese, phosphorus, potassium, sand, silt, sodium, pH, nitrogen, total organic carbon, and zinc) were analyzed at Kenya Agricultural and Livestock Research Organization, Nairobi, Kenya (Table 2).
Prior to the analysis, log transformation (Log (
Thirty free-living nematode genera assigned to four trophic levels were identified. The most predominant trophic guild in all decomposition plots in both seasons was bacterivores, followed by fungivores. We observed significant treatment effects (
Average number of nematodes in 250 g soil collected from control and decomposition plots during season one.
Taxa | Cp-class | Control Mean ± SE | Decomposition Mean ± SE |
|
|
---|---|---|---|---|---|
|
|||||
|
2 | 42 ± 9.5 | 58 ± 8.0 | 37.039 | <0.001*** |
|
2 | 16 ± 4.4 | 8 ± 3.5 | 9.153 | 0.005** |
|
4 | 31 ± 8.8 | 30 ± 5.7 | 2.331 | 0.137 |
|
1 | 5 ± 2.4 | 2 ± 1.1 | 1.327 | 0.258 |
|
2 | 107 ± 6.4 | 109 ± 9.9 | 0.106 | 0.747 |
|
2 | 52 ± 11.3 | 23 ± 3.8 | 0.082 | 0.777 |
|
2 | 0 ± 0.0 | 3 ± 2.0 | 1.946 | 0.173 |
|
2 | 91 ± 12.9 | 118 ± 21.1 | 2.92 | 0.098 |
|
2 | 10 ± 4.8 | 7 ± 3.6 | 0 | 0.996 |
|
2 | 120 ± 20.9 | 86 ± 13.4 | 1.927 | 0.175 |
|
2 | 26 ± 9.0 | 37 ± 7.6 | 7.016 | 0.013* |
|
3 | 18 ± 4.0 | 40 ± 11.0 | 2.586 | 0.118 |
|
1 | 39 ± 8.0 | 41 ± 12.6 | 0.223 | 0.64 |
|
2 | 0 ± 0.0 | 1 ± 1.3 | 0.272 | 0.325 |
|
2 | 31 ± 10.1 | 17 ± 4.2 | 0.679 | 0.417 |
|
|||||
|
2 | 16 ± 5.8 | 9 ± 3.2 | 1.088 | 0.305 |
|
2 | 143 ± 1.0 | 181 ± 19.0 | 2.479 | 0.126 |
|
2 | 13 ± 4.6 | 1 ± 1.3 | 49.301 | <0.001*** |
|
2 | 16 ± 2.2 | 9 ± 2.7 | 9.997 | 0.004** |
|
|||||
|
5 | 0 ± 0.0 | 4 ± 1.9 | 4.719 | 0.038* |
|
4 | 8 ± 2.4 | 3 ± 1.3 | 3.684 | 0.064 |
|
4 | 62 ± 4.0 | 65 ± 8.0 | 1.196 | 0.283 |
|
|||||
|
5 | 8 ± 2.4 | 10 ± 3.3 | 0.076 | 0.784 |
|
5 | 36 ± 6.9 | 42 ± 6.4 | 9.345 | 0.005** |
|
2 | 0 ± 0.0 | 6 ± 2.4 | 22.167 | <0.001*** |
Notes: Cp = colonizer-persister value. *P < 0.05; **P < 0.01; ***P < 0.001.
Average number of nematodes in 250 g soil collected from control and decomposition plots during season two.
Taxa | Cp |
Control Mean ± SE | Decomposition Mean ± SE |
|
|
---|---|---|---|---|---|
|
|||||
|
2 | 39 ± 7.5 | 58 ± 12.8 | 6.604 | 0.015* |
|
2 | 42 ± 8.0 | 27 ± 6.0 | 1.434 | 0.241 |
|
4 | 8 ± 2.4 | 10 ± 3.9 | 0.059 | 0.81 |
|
2 | 151 ± 10.4 | 280 ± 26.9 | 25.737 | <0.001*** |
|
2 | 42 ± 13.5 | 34 ± 12.1 | 0.466 | 0.5 |
|
2 | 5 ± 2.4 | 9 ± 3.2 | 4.225 | 0.049* |
|
2 | 101 ± 19.1 | 164 ± 23.8 | 48.172 | <0.001*** |
|
2 | 16 ± 3.5 | 8 ± 3.0 | 7.391 | 0.0108* |
|
2 | 57 ± 10.8 | 129 ± 14.8 | 68.576 | <0.001*** |
|
1 | 0 ± 0.0 | 14 ± 4.9 | 37.891 | <0.001*** |
|
1 | 3 ± 1.2 | 5 ± 2.0 | 0.486 | 0.491 |
|
2 | 10 ± 3.5 | 27 ± 4.7 | 96.111 | <0.001*** |
|
3 | 16 ± 3.5 | 24 ± 4.2 | 3.989 | 0.055 |
|
1 | 21 ± 7.0 | 73 ± 34.4 | 2.592 | 0.118 |
|
2 | 3 ± 1.2 | 3 ± 1.5 | 0.207 | 0.652 |
|
|||||
|
2 | 29 ± 6.1 | 98 ± 29.5 | 13.911 | <0.001*** |
|
2 | 164 ± 20.9 | 320 ± 47.2 | 18.84 | <0.001*** |
|
2 | 21 ± 4.8 | 16 ± 6.0 | 2.041 | 0.163 |
|
|||||
|
5 | 0 ± 0.0 | 1 ± 1.3 | 1 | 0.325 |
|
4 | 26 ± 4.2 | 22 ± 5.2 | 1.34 | 0.256 |
|
5 | 0 ± 0.0 | 1 ± 0.7 | 1 | 0.325 |
|
4 | 39 ± 7.1 | 47 ± 7.1 | 2.561 | 0.12 |
|
4 | 23 ± 3.5 | 10 ± 2.9 | 13.006 | 0.001** |
|
4 | 0 ± 0.0 | 3 ± 1.5 | 3.649 | 0.066 |
|
|||||
|
5 | 8 ± 1.5 | 8 ± 2.0 | 0.051 | 0.823 |
|
4 | 5 ± 1.5 | 4 ± 2.7 | 4.741 | 0.037* |
|
5 | 78 ± 10.3 | 54 ± 9.9 | 5.483 | 0.026* |
.
There were no differences in ecological indicators (MI, MI2-5) and functional indicators (CI, BI, EI, and SI) in season one at
Nematode ecological and functional indices in control and decomposition plots during season 1 and 2.
Control | Decomposition | |||
---|---|---|---|---|
Index name | Mean ± SE | Mean ± SE |
|
|
|
||||
Maturity Index (MI) | 2.41a ± 0.038 | 2.43a ± 0.040 | 0.38 | 0.542 |
Maturity Index (MI2-5) | 2.47a ± 0.044 | 2.49a ± 0.036 | 0.409 | 0.528 |
Channel Index (CI) | 60.35a ± 5.95 | 65.66a ± 5.948 | 0.985 | 0.329 |
Basal Index (BI) | 37.82a ± 2.016 | 35.72a ± 1.652 | 1.452 | 0.238 |
Enrichment Index (EI) | 34.52a ± 1.455 | 33.03a ± 2.472 | 0.511 | 0.481 |
Structure Index (SI) | 53.19a ± 2.537 | 55.73a ± 2.224 | 1.435 | 0.24 |
|
||||
Maturity Index (MI) | 2.54a ± 0.033 | 2.26b ± 0.036 | 64.715 | <0.001*** |
Maturity Index (MI2-5) | 2.57a ± 0.031 | 2.33b ± 0.029 | 68.996 | <0.001*** |
Channel Index (CI) | 81.01a ± 4.923 | 68.37b ± 5.864 | 12.359 | <0.001*** |
Basal Index (BI) | 34.44b ± 1.267 | 42.47a ± 1.495 | 65.944 | <0.001*** |
Enrichment Index (EI) | 28.14b ± 2.519 | 35.06a ± 2.941 | 15.409 | <0.001*** |
Structure Index (SI) | 59.91a ± 1.535 | 42.21b ± 2.689 | 41.205 | <0.001*** |
Metabolic footprints in control and decomposition plots during season 1 and 2.
Control | Decomposition | |||
---|---|---|---|---|
Metabolic footprints | Mean ± SE | Mean ± SE | F-values |
|
|
||||
Composite footprint | 5.67a ± 3.444 | 5.77a ± 3.421 | 2.482 | 0.126 |
Omnivore footprint | 4.55a ± 1.750 | 4.35a ± 2.558 | 0.825 | 0.371 |
Structure footprint | 4.99a ± 2.345 | 5.07a ± 2.702 | 1.272 | 0.268 |
Predator footprint | 3.15b ± 2.127 | 3.82a ± 2.110 | 19.678 | <0.001*** |
Fungivore footprint | 2.91a ± 1.243 | 2.94a ± 1.101 | 0.126 | 0.725 |
Enrichment footprint | 4.326a ± 2.797 | 4.196a ± 3.262 | 0.556 | 0.462 |
Bacterivore footprint | 4.734a ± 3.267 | 4.918a ± 3.303 | 2.441 | 0.129 |
|
||||
Composite footprint | 5.68b ± 3.242 | 5.95a ± 4.301 | 11.018 | 0.002** |
Omnivore footprint | 4.23a ± 2.479 | 4.33a ± 2.657 | 0.679 | 0.417 |
Structure footprint | 5.22a ± 2.314 | 5.05b ± 2.870 | 4.500 | 0.042* |
Predator footprint | 4.48a ± 2.485 | 3.87b ± 2.515 | 5.894 | 0.021* |
Fungivore footprint | 2.89b ± 1.260 | 3.52a ± 1.972 | 21.683 | <0.001*** |
Enrichment footprint | 3.51b ± 2.755 | 4.43a ± 4.249 | 27.425 | <0.001*** |
Bacterivore footprint | 4.39b ± 2.945 | 5.10a ± 4.269 | 30.782 | <0.001*** |
Decomposition is a valuable process for restoring energy fluctuations in terrestrial environments. Recycling plant residues accompanied by the breakdown of these materials into nutrients by belowground decomposer systems enhances plant life and levels of soil fertility (Ruess and Ferris, 2004). This study showed seasonal variations in the population of bacterivores, omnivores, fungivores, and predators across all treatments. The most abundant bacterivore genera in the decomposition plots compared to the control belonged to the family Cephalobidae (
The fungivorous taxa
The Channel index (CI), reflects the nature of the decomposition of organic matter either through fungal or bacterial energy channels (Ferris et al., 2001). Lower CI values < 50% reflect bacterial energy pathways while CI > 50% indicate fungal decomposition activity (Ferris et al., 2001). For both seasons, findings from this study depict CI values that range between ~66 and ~68 in all decomposition plots. This designates the dominance of fungal decomposition activity in decomposing maize residues. The dominance of fungal energy channel may have been favored by maize residues perhaps due to its high C to N ratio, and also because fungi are able to degrade more complex polyaromatic compounds such as cellulose and lignin, thus providing more food resources to fungivores (Rosenbrock et al., 1995; Ruess and Ferris, 2004).
Maturity indices, (MI) and (MI2-5), are indicators of structural complexity and conditions of a succession of soil food web environments as reflected by various nematode community assemblages (Ferris and Bongers, 2009). Our results revealed that the values of MI and MI2-5 in both decomposition and control plots ranged between 2 and 3.5. This implies low to intermediate soil food web maturity, balanced organic matter decomposition, and medium soil food web structure in both seasons. A relatively high population of fungivores increased the level of the basal index (BI) in decomposition plots (season two), while in season one, it was not significant (
Structure index (SI) was designed to estimate the complexity of the soil food web structure based on higher trophic levels (cp-3, cp-4, and cp-5) (Ferris et al., 2001). Our findings demonstrated that SI was significantly (
In summary, this study demonstrates that maize residues influenced the abundance and diversity of free-living nematodes, especially the enrichment opportunist bacterivores which were lower compared with the general opportunist bacterivores and fungivores. Notably, the bacterivore,