Effects of green manures on nitrogen loss and availability in an organic cropping system

Abstract

In certified organic cropping systems, the supply of nitrogen (N) to crops is limited by restrictions on fertiliser inputs, although green manure crops can be used to improve N retention and supply. This study involved a comparison between three green manure crops (oats (Avena sativa), lupins (Lupinus angustifolius), and oats‐lupins mix) and a fallow for their ability to conserve N over winter and influence N availability to a subsequent crop. Intact soil monolith lysimeters (19 cm diameter × 30 cm deep) were taken from an established organic farm and the experiment included four replicates of the three green manure crops and a fallow treatment. Nitrate‐N leaching losses from the various treatments were measured over a 150‐day period (March‐October). The green manure crops reduced the amount of N that was leached over winter. Cumulative N leaching losses were similar from 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). Amounts of N taken up by the green manure crops were 100, 162, and 126 kg N ha–1 for the oats, lupins, and oats‐lupins, respectively. Annual ryegrass (Lolium multiflorum) was sown 3 weeks after green manure incorporation to assess their impact on N availability. Dry matter yield and N uptake by ryegrass were significantly greater for the green manure‐amended soil (454–689 kg DM ha–1, 17–22 kg N ha–1) than for the fallow treatment (297 kg DM ha–1, 9.4 kg N ha–1). The findings of this study demonstrated that inclusion of a green manure is an effective means of reducing N leaching loss over winter and improving N supply in organic cropping systems.