Soil nutrition, microbial composition and associated soil enzyme activities in KwaZulu-Natal grasslands and savannah ecosystems soils
Introduction
The poor nutrient status of soils across the African continent has posed a serious threat to plant growth and sustainable agricultural practices (Goldman, 1995; Henao and Baanante, 1999). The ever-increasing population worldwide with stronger demand for agricultural products, drive us to increase the soil fertility of the already present agricultural land or to identify new regions that are suitable for agricultural practices. African savannahs and grasslands represent putative crop areas, however, studies on geochemistry of most South African ecosystems are reported to be nutrient-poor and relatively acidic (Nandwa, 2001; Mafongoya et al., 2006). The decreased pH in these soils and decreased cation-exchange capacity reduce the availability of nutrients such as potassium (K+), calcium (Ca2+) and ammonium (NH4+) (Aprile and Lorandi, 2012). These acidic conditions also lead to the sequestration of soil nutrients like phosphorus (P), that is rendered insoluble through binding with cations (Sharma et al., 2013). However, soil amelioration or even soil resetting can be achieved by the enhancement of soil-borne microorganisms. Actually, some microorganisms have been reported to solubilize nutrients increasing their availability to plants (Latha et al., 2011; Pérez-García et al., 2011).
The soil microbiota consists of N2-fixing Rhizobia, Ectomycorrhiza, Arbuscular Mycorrhiza, Pseudomonas, Ochrobactrum, Bacillus, Paenibacillus, Klebsiella, Lysinibacillus, and Actinomycetes among others (Martínez-Hidalgo and Hirsch, 2017). All of these microorganisms are potential plant growth promoters (Martínez-Hidalgo and Hirsch, 2017). For instance, Bacillus has been isolated from root nodules and shown to solubilize phosphate and synthesize hydrolytic enzymes, polyamines, and lipopeptides (Maymon et al., 2015). On the other hand, Ectomycorrhiza and Arbuscular Mycorrhiza increase plant below ground surface area to maximize nutrient uptake (Wardle et al., 2004). Some Pseudomonas, Ochrobactrum, and Klebsiella species have the ability to fix nitrogen through associative symbiotic and/or endophytic relationship in combination with certain non-legume plants (Franche et al., 2009). The rate of nitrogen fixation is dependent on many external factors that include competition among the soil microbial species, soil moisture, temperature, plant sanction for specific bacteria and pH (Nandwa, 2001; Stopnisek et al., 2014). The combined activity of the microbiota and the environment might be responsible for the increased soil enzyme activity that is characteristic in most soils. Moreover, soil quality can be determined by measuring several enzymatic activities as a subrogate of the microbial diversity in them. Soil microbial activities are partly due to the enzymes originating from organisms and are of importance for the decomposition of many labile organic substrates, activating biogeochemical cycling. Their activity reflects the functional diversity and activity of the microorganisms involved in decomposition processes (Sinsabaugh et al., 2008) at the time that they guarantee the correct soil functioning. Soil microorganisms produce extracellular enzymes that hydrolyze and transform polymeric compounds into readily available nutrients that can be assimilated by plants and microorganisms (Lucas et al., 2008). These extracellular enzymes are responsible for the mineralization and cycling of terrestrial nitrogen (N), phosphate (P) and carbon (C) and can be grouped accordingly, although some enzymes can play a role in more than just one cycle. Furthermore, these enzymes are key in preventing oxidative degradation caused by reactive oxygen species (Nanda et al., 2010). Catalase is one of the enzymes known to be indicative of soil oxidative stress tolerance and is known to reduce hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2) (Angaji et al., 2012). Enzymes involved in C cycling include β-D-cellobiohydrolase, dehydrogenase, and β-D-glucosidase. They achieve this by releasing saccharides from glycosides and catalysis of the degradation of cellulose and cellotetraose into cellobiose that can be further transformed into glucose (Henriksson et al., 1998). Apart from catalase, there are other important oxidoreductase enzymes like laccase, lignin peroxidase (LiP) and manganese peroxidase (MnP), which protect against oxidative stress (Lundell et al., 2010). Their primary functions include lignin transformation and H2O2 reduction into H2O. On the other hand, enzymes like asparaginase and β-D-glucosaminidase convert asparagine into aspartic acid and ammonia (NH3), and hydrolyze chito-oligosaccharides (Hill et al., 1967; Mega et al., 1972). This has an overall effect on N mineralization and increased N assimilation in plants. While phosphatases are responsible for P cycling and mineralization through the hydrolysis of phosphoric acid monoester to produce a phosphate anion (Turner et al., 2002). Plants growing in grassland and savannah ecosystem soils are subjected to these collective effects of soil geochemistry and microflora composition might employ different survival strategies and show varying growth patterns.
The aim of this study was to analyze the potential of four different soils for agriculture, based on their nutrition and fertility status. Soils from four geographic distinct locations were collected in KwaZulu-Natal (South Africa) region and analyzed for their macro and micro nutrients, bacterial and fungal composition and their enzyme activities. Soil fertility was related to microbial composition and their activities.
Section snippets
Total soil nutrient analysis and spore count
Soil samples collected from four different locations at varying altitudes in KwaZulu-Natal (KZN) province, South Africa were sent to the KwaZulu-Natal Department of Agriculture and Rural Development's Analytical Services Unit at Cedara College of Agriculture, South Africa, for total soil nutrient and cation concentrations, cation exchange acidity and pH analysis. The four soil collection sites were: Ashburton, midlands KZN (elevation 670 m; 29.3855° S; 30.2642° E), Hluhluwe, northern KZN
Soil geochemistry
Ashburton soil (Fig. 1A) had a black appearance while soils from Hluhluwe (Fig. 1B), Bergville (Fig. 1C) and Izingolweni (Fig. 1D) had black/dark brown, brown and red-brown colour, respectively. Microscopic enumeration of Arbuscular mycorrhizal spores showed no significant difference (p > 0.05) between Izingolweni (11.6 spores/100 g), Bergville (15.8 spores/100 g of soil) and Ashburton (13.4 spores/100 g), while Hluhluwe (5.2 spores/100 g) was significantly different (p < 0.05) from the other
Discussion
This study was aimed at assessing the properties of KZN soils to infer their potential effects on the growth and development of grassland and savannah ecosystem plants and to evaluate their potentiality for sustainable agriculture.
Bergville soils proved to be the richest in macronutrients. Sufficient soil nutrients guarantee plant growth and the right balance between below and above biomass production (Goldman, 1995; McCauley et al., 2009). This way, Bergville soils would rend better crops and
Conclusions
Grassland and savannah ecosystem soil may support the establishment and growth of these ecosystem plants and sustainable agriculture practices. Given that these different soils have different diversity of microbes and various soil enzymes that can solubilize bound nutrients. However, external inputs such as liming of soils in accordance with plant sufficiency range might be required to maximize yield potential.
The following is the supplementary data related to this article.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the National Research Foundation, South Africa, for funding this work (NRF grant no. UID 113576). We acknowledge the Environmental Microbiology/Biotechnology research lab at the School of Life Sciences, University of KwaZulu-Natal, Durban, for their research facilities. We want to thank José Ramos-Buzón for having painted the graphical abstract.
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