Agri Kultuur June / Junie 2016 | Page 46

André Meyer & John Wooldridge ARC Infruitec-Nietvoorbij, Stellenbosch Joanna Dames Department of Biochemistry and Microbiology, Rhodes University A s in all natural ecosystems, soil microbial communities are active in agricultural soils where they perform such essential ecosystem-level functions as organic residue decomposition and the release of their contained nutrients to the rhizosphere. Soil microbes differ in terms of the types of carbonaceous substrate that they are able to utilise. This has led to the design of Biolog Eco Plates™ as a tool for ecological studies of microbial communities. This approach quantifies the complexity of individual microbial communities by assessing how many different carbonaceous substrates, of which 31 are presented on a single microtitre plate, they are able to utilise. These substrates not only have high relevance to soil microbial communities, but are also known to be contained within plant root exudates. When inoculated into the plate wells, each community gives a characteristic reaction pattern called a metabolic fingerprint. This enables scientists to compare microbial communities in terms of their metabolic diversity (known as the community metabolic diversity). Recently, scientists from the Agricultural Research Council (ARC) at Infruitec-Nietvoorbij in Stellenbosch in collaboration with Rhodes University, used the Biolog Eco Plate approach as an assay to detect and evaluate changes in the soil environment that stemmed from different orchard floor management practices. Deciduous fruit producers are under increasing pressure to adopt environmentally friendly soil surface management practices. However, evidence that organic practices result in commercial orchard soil ecosystems approximating more closely to natural ecosystems than conventional practices is lacking. This study, which was carried out on a ‘Cripp’s Pink’/M7 apple orchard in the Elgin area aimed to provide further information. Five treatments (Table 1) were applied to nine-tree plots in spring (September) 2003 when the trees were entering their third growth season. Applications continued until January 2011. The community metabolic diversity was determined by the Biolog method in composite soil samples from the 0-15 cm depth interval on both sides of the tree row, beneath the canopy drip line, in spring (September) 2008 and 2009, and in summer (January) 2010. Results showed that carbon substrate utilisation in the conventional treatments where chemical weed control and inorganic fertliser were applied in the tree row (T1 and T2), were low compared to the compost and mulch (organic) treatments (T4 and T5) in 2008 (Table 2). In the subsequent year (2009) the result was similar. In 2010, utilisation was lowest in T1. Averaged over all sampling dates, carbon substrate utilisation in T1 and T2 were lower than in T4, and utilisation in T1 was lower than in T3, T4 and T5. Over all treatments, utilisation increased in the sequence: spring 2008 < spring 2009 < summer 2010. The higher community metabolic diversity in the organic compared to the conventional treatments was ascribed to respectively higher and lower soil organic matter in these