Impacts of organic and conventional crop management on diversity and activity of free-living nitrogen fixing bacteria and total bacteria are subsidiary to temporal effects

A three year field study (2007-2009) of the diversity and numbers of the total and metabolically active free-living diazotophic bacteria and total bacterial communities in organic and conventionally managed agricultural soil was conducted at the Nafferton Factorial Systems Comparison (NFSC) study, in northeast England. The result demonstrated that there was no consistent effect of either organic or conventional soil management across the three years on the diversity or quantity of either diazotrophic or total bacterial communities. However, ordination analyses carried out on data from each individual year showed that factors associated with the different fertility management measures including availability of nitrogen species, organic carbon and pH, did exert significant effects on the structure of both diazotrophic and total bacterial communities. It appeared that the dominant drivers of qualitative and quantitative changes in both communities were annual and seasonal effects. Moreover, regression analyses showed activity of both communities was significantly affected by soil temperature and climatic conditions. The diazotrophic community showed no significant change in diversity across the three years, however, the total bacterial community significantly increased in diversity year on year. Diversity was always greatest during March for both diazotrophic and total bacterial communities. Quantitative analyses using qPCR of each community indicated that metabolically active diazotrophs were highest in year 1 but the population significantly declined in year 2 before recovering somewhat in the final year. The total bacterial population in contrast increased significantly each year. Seasonal effects were less consistent in this quantitative study

Evidence of variability in the structure and recruitment of rhizospheric and endophytic bacterial communities associated with arable sweet sorghum (Sorghum bicolor (L) Moench)

Sorghum is the second most cultivated crop in Africa and is a staple food source of many African communities. Exploiting the associated plant growth promoting bacteria (PGPB) has potential as an agricultural biotechnology strategy to enhance sorghum growth, yield and nutritional properties. Here we use Terminal-Restriction Fragment Length Polymorphism (TRFLP) and Denaturing Gradient Gel Electrophoresis (DGGE) to evaluate the factors that potentially shape rhizospheric and endophytic bacterial communities associated with sorghum farmed in South Africa. Microbial diversity was typically higher in the rhizosphere and rhizoplane compared to the endophytic zones (root, shoot and stem). Geographical location was one of the main drivers in describing microbial community assemblages found in rhizospheric and endophytic sorghum-linked niches. NO3-N, total nitrogen and pH were clearly identified as the main abiotic factors shaping sorghum-associated soil communities. Our results also suggest that specific bacterial taxa with potential N-fixing capacities (Acetobacter sp., Azospirillum sp., Pantoea sp., Bacillus sp. and cyanobacteria) are consistently detected in sorghum-created rhizospheric and endophytic environments, irrespective of environmental factor effects.The South African National Research Foundation (NRF)http://link.springer.com/journal/11104hb2016Genetic