Department of Agronomy

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Browsing Department of Agronomy by Author "Becker, Frederick"

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    Effect of seasonal distribution and rate of nitrogen fertilisation on canola production and soil bacterial communities in the Western Cape
    (Stellenbosch : Stellenbosch University, 2017-12) Becker, Frederick; Le Roux, M. R.; Swanepoel, Pieter Andreas; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    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 (P<0.05), but fertilisation treatments had no effect (P>0.05). Soil bacterial biodiversity and species richness decreased over time (P<0.05) at Langgewens. It is therefore recommended to apply 60 kg ha-1, split into three increments, i.e., 20 kg ha-1 at planting and the remainder in two equal applications at 30 and 60 DAE. These applications can vary in amount and timing due to weather conditions of the specific growing season.
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    High nitrogen rates do not increase canola yield and may affect soil bacterial functioning
    (American Society of Agronomy, 2019) Becker, Frederick; MacLaren, Chloe; Brink, Casper J.; Jacobs, Karin; Le Roux, Marcellous R.; Swanepoel, Pieter Andreas
    Nitrogen fertilization has a fundamental role in agricultural productivity. However, injudicious N applications to crops are common. It is important to ensure the minimum N required for satisfactory crop growth is applied but that excess amounts are avoided due to potential impacts on agroecosystem functioning. Nitrogen at 0, 60, and 150 kg ha-1 was applied as limestone ammonium nitrate to plots arranged in a randomized complete block design, on three farms to determine the impact of rate and temporal distribution of fertilizer on canola (Brassica napus L.) production in South Africa, and the effect of N fertilizer application on the composition and diversity of soil bacterial communities. The amount and distribution of N had only minor effects on canola growth (P < 0.05) and no effects on yield or harvest index. Splitting fertilizer into two or three applications throughout the season resulted in more mineral N available in the soil later in the season. Increasing the N rate from 60 to 150 kg ha-1 had a significant impact on bacterial community composition. The lower rate favored bacteria that are more able to break down N-containing carbon sources. No effects of fertilizer amount or distribution were observed on either N fixation potential (number of nifH gene copies) or bacterial community diversity. Overall, a low rate of N fertilizer split into multiple applications is recommended for canola production, as higher rates do not increase yield and may have a detrimental impact on soil carbon and nitrogen cycling.