Downsizing nitrogen fertiliser input for improved canola nutrition, stomatal activity, and yield with sunnhemp cover crop, nano zinc copper micronutrients and enhanced soil health

Abstract

Canola (Brassica napus L.) represents the second most significant oilseed crop worldwide regarding oil production. However, the production challenge lies in the substantial demand for nitrogen fertiliser, which results in both environmental and economic losses. Agricultural practices surrounding the production of canola tend to compromise soil health, which is fundamental to enhancing crop productivity, by excessively applying nitrogen fertiliser for high yield outputs. This study presents a comprehensive analysis of the synergistic impact of incorporating sunnhemp cover crop, combined nano zinc and copper, alongside varying nitrogen fertiliser application rates, on key soil and agronomic parameters. The study took place at two distinct locations: Syferkuil and Ofcolaco, over the 2023 and 2024 growing seasons. Planted sunnhemp was slashed 60 days after planting and incorporated into the soil, followed by the establishment of winter canola in a split split-plot design with 16 treatments replicated 3 times. Soil parameters measured post-harvest included concentrations of total N, P, K, Mg, Cu and Zn; soil pH, organic carbon (OC), Mean weight diameter (MWD), bacterial and fungal populations, microbial activity (FDA), urease enzyme, active carbon (POxC), soil organic matter (SOM) and relative abundance of pathogenic fungi. Meanwhile agronomic parameters included canola grain, seed oil and protein yield, crude oil concentration, yield components, harvest index (HI), total dry matter (TDM), nitrogen uptake, chlorophyll content, leaf area index (LAI), leaf gases (photosynthetic (A) and transpiration (E) rate) and foliar copper and zinc concentrations. Statistical analysis for the measured parameters was conducted using JASP 0.19.3. The soil results from Ofcolaco in 2023 indicated enhancements in fungal populations, organic matter, and urease, with 2024 showing minor changes. Nitrogen fertilisation improved POxC (10 to 22%) and urease (31 to 111%) across both sites, though variations were noted by application rates. Treatment interactions improved notably in measured parameters but fluctuated by season and location. The study found several pathogenic fungi (Aspergillus quadrilineatus, Macrophomina tecta, Agroathelia rolfsii, Neopestalotiopsis clavispora, Fusarium longifundum, Fusarium oxysporum and Alternaria destruens) were not linked to specific treatments. The combination of sunnhemp cover crop, nano Zn and Cu, and N fertilisation at 60 and 120 kgN ha−1 demonstrated potential for boosting soil microbial activity while decreasing reliance on synthetic fertilisers in canola production. Cover crops enhanced total N (0.02 to 0.03 percentage points) and P (31 to 78%), while K declined (25%). Reductions from nano Zn and Cu were between 6 to 8% for P and 9% for K, while higher N fertiliser applications led to lower concentrations of N, P, and K, notably at 180 kgN ha-1. In contrast, 60 and 120 kgN ha⁻¹ improved nutrient concentrations and influenced Mg, Cu, Zn, OC, and pH variably. The findings reveal the potential of these integrated practices to bolster soil microbial activity and nutrient dynamics, thereby mitigating the dependency on synthetic fertilisers in canola cultivation. The agronomic parameters’ response in relation to the applied treatments indicated significant increases in grain yield (17 to 43%), oil yield (21 to 68 %), and protein yield (21 to 47%) with the use of cover crop, though variances were location specific. The addition of nano Zn and Cu increased grain yield by 15 to 17% and protein yield by 16 to 27%. Sunnhemp cover crop also improved TDM by 12 to 23%, nitrogen uptake by 44%, and LAI by 167%, with photosynthetic rates varying from 23 to 43%. The combination of nano Zn and Cu increased TDM by 15 to 33%, nitrogen uptake by 30 %, chlorophyll by 2.9%, and foliar Cu by 45%, though foliar Zn dropped by 15%. Nitrogen fertilisation at 120 kgN ha-1 enhanced oil yields, showing comparability to 180 kgN ha-1. The application of 180 kgN ha-1, also resulted in TDM increases of 41% to 70% and LAI improvements of 57 to 85%. Seasonal and spatial variation in the measured parameters was also noted. Moreover, the results underscore that judicious nitrogen fertilisation, particularly at 120 kgN ha-1, can produce comparable results to 180 kgN ha-1 without compromising environmental integrity. Overall, the integrated use of cover crops, nano Zn and Cu, and nitrogen fertiliser had positive impacts on growth and nitrogen uptake while enabling sustainable practices. This, research also contributes valuable insights into sustainable agricultural practices, demonstrating the potential for enhanced canola production and soil productivity through innovative nutrient management strategies while reducing reliance on nitrogen inputs