Understanding the role of soil microbes and their associated extracellular enzymes in long-term grassland experiments presents an opportunity for testing relevant ecological questions on grassland nutrient dynamics and functioning. Veld fertilizer trials initiated in 1951 in South Africa were used to assess soil functional microbial diversity and their metabolic activities in the nutrient-poor grassland soils. Phosphorus and liming trials used for this specific study comprised of superphosphate (336 kg ha−1) and dolomitic lime (2250 kg ha−1) (P + L), superphosphate (336 kg ha−1) (+ P) and control trials. These soils were analyzed for their nutrient concentrations, pH, total cations and exchange acidity, microflora and extracellular enzyme activities. The analysed soil characteristics showed significant differences except nitrogen (N) and organic carbon (C) concentrations showing no significant differences. P-solubilizing, N-cycling and N-fixing microbial diversity varied among the different soil treatments. β-glucosaminidase enzyme activity was high in control soils compared to P-fertilized and limed soils. Alkaline phosphatase showed increased activity in P-fertilized soils, whereas acid phosphatase showed increased activity in control soils. Therefore, the application of superphosphate and liming influences the relative abundance of bacterial communities with nutrient cycling and fixing functions which account for nutrient bioavailability in acidic and nutrient stressed grassland ecosystem soils.

Soils in grasslands and savannas of southern Africa are acidic and nutrient-poor. Legume plants, such as Vachellia nilotica and alien invasive Leucaena leucocephala, are a major component of the vegetation there. Vachellia nilotica can establish in drought-prone environments, and is invasive in high rainfall areas. Leucaena leucocephala is an emerging invasive in South Africa and is ranked among the world’s 100 most invasive alien species. Alien plants can invade native habitats through their adaptability to low-resource soils, and thus can out-compete and displace native vegetation. We investigated the effects of phosphorus (P) deficiency and soil acidity on legume-microbe symbiosis, nitrogen (N) nutrition and carbon (C) growth costs of these two legumes in grassland soils. We used as inoculum and growth substrate soils collected from a long-term (> 65 years) nutrient and lime addition trial, the Veld Fertilizer Trial (VFT), located at Ukulinga Research Farm near Pietermaritzburg in South Africa. We used soils from three VFT treatments: soils fertilized with superphosphate (336 kg. ha -1) applied once per year (+P), soils fertilized with superphosphate (336 kg. ha -1) applied once per year with dolomitic lime (2250 kg. ha -1) applied once every 5 years (P+L), and soils with no superphosphate and no dolomitic lime applications (Control). Seeds of V. nilotica and L. leucocephala were germinated and grown independently in these soils in green house conditions and harvested after 125 days for measurement of growth, legume-microbe symbiosis, N nutrition and C growth costs. Results showed that the two legumes had different growth adaptations. Vachellia nilotica grown in control soils and +P soils nodulated with various Burkholderia spp. while L. leucocephala did not nodulate in all soil treatments. Both legumes utilised for growth both atmospheric and soil derived N across all treatments thereby decreasing C growth costs. Vachellia nilotica grown in +P soils accumulated the most biomass and N nutrition. Leucaena leucocephala maximised specific N assimilation rates by investing in below ground biomass accumulation in control soils. This shows that L. leucocephala possesses traits that are successful in acquiring nutrients by investing in below ground biomass and relying on utilisation of N from both the soil and the atmosphere.

PurposeThis study investigated the effects of long-term phosphorus (P) fertilization and liming on soil microbe diversity and extracellular soil enzyme activities in acidic and nutrient-poor grassland soils in KwaZulu-Natal (KZN). MethodsExperimental soil samples were collected from the Veld Fertilizer Trials (VFT) initiated in 1951 at Ukulinga Research Farm in Pietermaritzburg, South Africa. P fertilizer and liming treatments comprised of superphosphate (336 kg. ha-1) applied once a year and dolomitic lime (2250 kg. ha-1) applied every 5 years (P+L), superphosphate (336 kg. ha-1) applied once a year (+P) and soils with no superphosphate and no dolomitic lime (-P-L). These soils were analyzed for their nutrient concentration, pH, total cation concentrations and exchange acidity, microflora and extracellular enzyme activities. ResultsSoil nutrition, pH, total cations and exchange acidity showed significant differences between the soil treatments. However, nitrogen (N) and organic carbon (C) concentrations showed no significant differences between the soil treatments. P-solubilizing, N-cycling and N-fixing microbial diversity varied among the different soil treatments. Various nutrient cycling bacteria were identified across soil treatments with the most abundant strains in the Pseudomonas genera. β-glucosaminidase enzyme activity was high in acidic soils compared to P-fertilized and limed soils. However, N reductase activity showed no significant differences between the soil treatments. Alkaline phosphatase showed increased activity in P-fertilized soils, whereas acid phosphatase showed increased activity in acidic soils. Conclusion:These experimental results show that soil microbes and soil enzymes play an integral role in nutrient dynamics in acidic and nutrient stressed grassland ecosystem soils.