Nitrogen source effects on canola (Brassica napus L.) grown under conservation agriculture in South Africa

Abstract

Canola (Brassicanapus L.) growth and yield can be enhanced by, amongst others, choosing the correct rate, source, and timing of N fertilizer. Limited research exists on the effects of nitrogen (N) source on canola growth and yield properties when grown under conservation agriculture. This study compared the performance of different N fertilizer sources on canola biomass production, seed yield, and oil content. Field trials were conducted from 2016 to 2019 at five sites in the Western Cape Province of South Africa which is characterized by a Mediterranean climate. Nitrogen fertilizer source had no effect (p > .05) on biomass production or oil content at any site.Sufficient soil water levels and warm daily temperatures led to greater biomass production regardless of nitrogen source. Oil content was determined by the presence of rainfall during the flowering and seed filling stages. The effect of N source on yield varied among sites in accordance with timing of fertilizer application in relation to rainfall. The urea + urease inhibitor N source led to greater (p < .05) yields than the urea N source without the inhibitor at the Darling site, regardless of the soil water conditions. Mean yield values of all seasons and sites indicated that urea + urease inhibitor fertilizer to be the most reliable N source to ensure optimal yields over the long-term.

Transcriptomic Analysis of the Reduction in Seed Oil Content through Increased Nitrogen Application Rate in Rapeseed (Brassica napus L.)

“…However, Ibrahim et al (1989) reported that the seed oil content and percentage of major fatty acid components were not significantly affected by the nitrogen rates or times [18]. Crous et al (2021) revealed that the nitrogen fertilizer source did not affect the seed oil content, which was determined by the presence of precipitation during the flowering and seed filling stages [19]. The above results suggest that the reduction in rapeseed seed oil content caused by nitrogen application was generally consistent across the different investigations.…”

“…Here, we found that the expression of related genes was mainly high in the early development stages-namely 30-40 days after flowering-and then decreased. According to research, the development stage (30-40 days after flowering) is the rapid accumulation stage of seed oil, which will account for 70.1% to 93.5% of mature seed oil contents, moving into a slow accumulation period up to 50 days [19,41]. In this research, genes were more active at the early sampling time, D1, and then their activity gradually decreased at D2 and D3, which may be the result of the rapid synthesis and transformation of the seed oil.…”

Aligning Conservation Agriculture among various disciplines in South Africa

“…When possible, changes in soil nutrient dynamics and stratification are considered, and different fertiliser optima than for conventional tillage systems could be expected. For instance, producers in the Western Cape, where CA is well established, are still using nitrogen fertiliser guidelines that were developed under conventional tillage systems (Viljoen et al 2020;Crous et al 2021). These guidelines do not account for the potential supply of mineralised nitrogen from soil organic matter.…”

“…The stratification of nutrients, particularly soil organic matter, underscores the importance of interpreting data with caution, as samples taken at a shallow depth may falsely overestimate the benefits of soil nutrients and soil organic matter. There are confounding recommendations on how to take a soil sample for CA systems (Derpsch et al 2014;Olson and Al-Kaisi 2015). For instance, in soils with a highly stratified pH, a soil acidity problem is often not picked up in soil tests if soil samples are taken to 200 mm or deeper, diluting the stratification effect.…”