Effect of seasonal distribution and rate of nitrogen fertilisation on canola production and soil bacterial communities in the Western Cape

Thesis(MScAgric)--Stellenbosch University, 2017.ENGLISH ABSTRACT: Canola (Brassica napus) was introduced into crop rotation systems of South Africa in 1994. Ever since, canola production is expanding as the benefits of canola is recognised. Canola has a higher nutrient demand than most other crops such as wheat and barley. Of these nutrients, nitrogen (N) comprise most of the production costs as it is applied at high rates. Currently, in South Africa N fertiliser guidelines for canola production is adopted from guidelines for wheat or from international literature. Losses of N is not only economically inefficient, but can also be detrimental to the environment and human health. Sustainable production necessitates a reduction of these losses and lower dependency on inorganic fertilisers without compromising high yields. The soil biological component renders ecosystem services such as nutrient cycling. Soil bacterial communities are principally involved in the cycling of N and could therefore determine the fate of fertilisers. Results of studies done on the effect of N fertilisation on soil bacterial communities lack consistency and is often contradictory and thus not well understood. The aim of this study was to evaluate different N fertilisation rates and seasonal distribution for canola production in South Africa, and to determine the effect of the fertiliser N on soil bacterial communities. The study was conducted during the 2016 production season under dryland conditions in the Western Cape. It was replicated at three different localities, representative of the important canola production regions, namely Langgewens Research Farm, Altona and Roodebloem Experimental Farm. Langgewens and Altona are situated in the Swartland region and Roodebloem in the southern Cape. Each of the trials were laid out as a randomised complete block design with six N fertilisation treatment-combinations including a control without added N. The treatment-combinations was replicated in four blocks. Two factors were evaluated, i.e. N fertiliser rate and distribution of N. Two N fertiliser rates (60 and 150 kg ha-1) were applied. Twenty kg ha-1 was applied at planting and the remainder was distributed at either only 30 days after emergence (DAE), 30 and 60 DAE or 30, 60 and 90 DAE. Results indicated that the increase in N fertilisation from 60 kg ha-1 to 150 kg ha-1 did not increase yields (P>0.05). Soil bacterial community changed through time (P0.05). Soil bacterial biodiversity and species richness decreased over time (P0.05). Die grondbakteriese gemeenskap het oor tyd verander maar die N-bemestingbehandelinge het geen effek (P>0.05) daarop gehad nie. Grondbakteriese biodiversiteit en spesiesrykheid het oor tyd by Langgewens afgeneem (P