The metabolic potential of soil microorganisms and enzymes in phosphorus-deficient KwaZulu-Natal grassland ecosystem soils

Abstract

The relationship between soil microbial communities and enzymes on the nutrient dynamics in phosphorus (P) deficient grassland ecosystem soils of KwaZulu-Natal province, South Africa was studied. The long-term veld fertilization trial (VFT) soils used for this study were fertilized with 366 kg of superphosphate per hectare per season (P) and non-superphosphate fertilizer (−P). The macronutrient concentrations in P-treated soil were higher and significantly different from that of −P treated soil (P ≤ 0.05), except for the N concentration. Culture-dependent techniques revealed that the bacterial dominance in both soil treatments was mainly represented by species belonging to the Proteobacteria phyla, including the genera; Pseudomonas, Sphingomonas, Caulobacter and Variovorax forming the N-cycling, N-fixing and P-cycling bacterial communities. The −P treated soil showed increased β-Glucosaminidase, acid phosphatase and alkaline phosphatase activities of 194.4 nmolh−1 g−1, 1424.8 nmolh−1 g−1 and 2487.0 nmolh−1 g−1 respectively compared to that of the P treated soil with 160.2 nmolh−1 g−1, 967.3 nmolh−1 g−1 and 1770.5 nmolh−1 g−1 respectively. Pseudomonas nitroreducens and Variovorax paradoxus as representatives of P-cycling bacteria were selected and screened to gain insight into the P-solubilization mechanisms of the bacteria, based on qualitative and quantitative assay. A maximum P-solubilization by P. nitroreducens was achieved under acidic pH (4.2), accompanied by acid phosphatase activity of 45.3 U/mL while that of V. paradoxus was obtained under acidic pH (4.9) condition, and accompanied by alkaline phosphatase activity of 94.0 U/mL after a 10-day incubation period of the liquid medium. This study reveals that soil microorganisms and enzymes play a significant role in the nutrient bioavailability in phosphorous-deficient soil. This information has demonstrated the selection of these microbial communities for potential biotechnological applications.