High nitrogen rates do not increase canola yield and may affect soil bacterial functioning

Abstract

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 < .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 C 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 C and N cycling.

Benefits Are Limited with High Nitrogen Fertiliser Rates in Kikuyu-Ryegrass Pasture Systems

“…Other authors have reported higher urease activity in treatments where no N was applied [45] or when high amounts of organic material were added to the soil [46,47] compared to chemically fertilised plots [48]. This supports the postulation that the urease enzyme is unnecessary in the presence of mineral forms of N. Applying lower amounts of N helps to maintain the ability of soil microbes to supply plant-available N [49]. The suppression of urease activity, which is indicative of reduced microbial activity, was more evident at N rates above 40 kg N ha −1 grazing −1 in the current study.…”

“…The suppression of urease activity, which is indicative of reduced microbial activity, was more evident at N rates above 40 kg N ha −1 grazing −1 in the current study. Becker et al [49] also found that microbes that immobilise N and release soil carbon are favoured when high rates of N are applied to canola (Brassica napus).…”

Utilisation of cover crops: implications for conservation agriculture systems in a mediterranean climate region of South Africa

“…As a result of the nutrient carry-over, fertiliser use can potentially be reduced, particularly N fertilisers. Reduced use of fertilisers will have a positive effect on ecosystem function and agricultural pro tability (Becker et al 2020).…”

Environmental Impact of Rotationally Grazed Pastures at Different Management Intensities in South Africa

“…Oversupply of N through fertilization can be detrimental to bacteria involved in mineralization of N from soil organic matter [ 50 ]. Sustained plant production is dependent on the potential of the soil to supply plant-available N through effective N cycling.…”