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

Becker, Frederick (2017-12)

Thesis(MScAgric)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Kanola (Brassica napus) was vir die eerste keer in 1994 in wisselboustelsels in Suid-Afrika verbou. Sedertdien het produksie aanhou toeneem soos die voordele van kanola erken word. Kanola het ‘n hoër natuurlike behoefte aan voedingstowwe in vergelyking met ander gewasse, soos koring en gars. Stikstof (N) is hoofsaaklik die voedingstof wat in die grootste hoeveelhede toegedien word en maak dus die meeste van die kanolaproduksiekoste uit. Stikstofbemestingsriglyne vir kanolaproduksie in Suid-Afrika word hoosaaklik vanaf riglyne vir koringproduksie of internasionale literatuur bepaal. Verliese van N is nie net ‘n ekonomiese verlies nie, maar kan ook implikasies op die omgewing of gesondheid van mense hê. Vir volhoubare landbouproduksie moet hierdie verliese beperk word en landboupraktyke minder afhanklik van anorganiese misstowwe vir hoër opbrengste wees. Die biologiese komponent in gronde lewer verskeie eksosisteem-dienste bv. voedingstofsirkulering. Bakteriese gemeenskappe in gronde is betrokke by die sirkulering van N en kan dus die sukses van N-bemesting bepaal. Die effek van N-bemesting op grondbakteriese gemeenskappe word nie goed verstaan nie, as gevolg van gebrek in konsekwente of dikwels teenstrydige resultate van sulke studies. Die doel van hierdie studie was om N-bemestingspeil en verspreiding daarvan op kanolaproduksie in Suid-Afrika te evalueer, en om die effek van die N-kunsmis op grondbakteriese gemeenskappe te bepaal. Die studie is gedurende die 2016 produksieseisoen onder droëlandtoestande in die Wes-Kaap uitgevoer. Dit is op drie verskillende lokaliteite herhaal om die belangrike kanolaproduksiestreke te verteenwoordig, naamlik Langgewens Navorsingsplaas, Altona en Roodebloem proefplaas. Langgewens en Altona is in die Swartland area en Roodebloem in die Suid-Kaap geleë. Elke proef was in 'n ewekansige blokontwerp met ses N-bemestingsbehandeling-kombinasies uitgelê, plus een kontrole wat geen N ontvang het nie. Die behandeling-kombinasies was herhaal in vier blokke. Twee N bemestings hoeveelhede (60 en 150 kg ha-1) was toegedien. Twintig kg ha-1 is toegedien tydens vestiging en die oorblywende N is by slegs 30 dae na opkoms (DNO), 30 en 60 DNO of 30, 60 en 90 DNO toegedien. Die verhoging in N bemesting vanaf 60 kg ha-1 na 150 kg ha-1 het nie die opbrengs verhoog nie (P>0.05). 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 <0.05). Daar word dus aanbeveel om N teen ‘n totale hoeveelheid van 60 kg ha-1 toe te dien. Dit behels 20 kg ha-1 tydens vestiging en die oorblywende N, in twee gelyke kopbemestings teen 30 en 60 DNO. Hierdie toedienings kan wissel in hoeveelheid en tyd van toediening weens weersomstandighede van die spesifieke groeiseisoen.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102770
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