Effects on green manure crops on nitrogen loss and availability: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours At Lincoln University

Abstract

Nitrogen (N) is the nutrient required in the largest quantity by arable crops. However, it is also a difficult nutrient to manage due to its susceptibility to loss through leaching, particularly in winter. In certified organic cropping systems, the supply of N to crops is further restrained by restrictions on fertiliser inputs. These factors have lead to a renewed interest in the use of green manure crops for improving N-use efficiency in conventional and organic arable cropping systems. This study involved a comparison between three green manure crops (oats (Avena saliva), lupins (Lupinus auguslifolius), oats-lupins biculture) and a fallow for their ability to conserve N over winter and influence N availability to a subsequent crop. Intact soil monolith lysimeters (19cm diameter x 30cm deep) were taken from an established organic farm and the experiment included four replicates of each treatment. Nitrate leaching losses from the various treatments were measured over the green manure growth period (March - October). All the green manure crops reduced the amount of N that was leached over winter. Cumulative N leaching loss was similar for the three green manure treatments (4.1 - 4.9 kg N ha⁻¹), and these were significantly lower than N loss from the fallow treatment (8.4 kg N ha⁻¹ ). Nitrogen uptake in green manure herbage was 100, 162, and 126 kg N ha⁻¹ for the oats, lupins, and oats-lupins, respectively, with corresponding herbage C:N ratios of 18, 13, and 16. Annual ryegrass (Lolium mulliflorum) was sown three weeks after green manure incorporation to assess their impact on N availability. Dry matter yield and N uptake by ryegrass was significantly greater for the oats lupins (689 kg DM ha⁻¹ , 22 kg N ha⁻¹) than the fallow treatment (297 kg DM ha⁻¹ , 9 .4 kg N ha⁻¹ ). The findings of this study demonstrated that a green manure is an effective means of reducing N leaching loss over winter and improving N supply to a subsequent crop.