Impacts of summer fallow weeds on soil nitrogen and wheat in the southern, western and northern Australian grain regions.

Abstract

National research to quantify nitrogen use by summer fallow weeds and the impact this has on subsequent wheat crops has been conducted across the Australian grain region during the past three and a half years. Identical, simple design, long term experimental protocols were followed at three sites. Three fallow and three cropping phases were studied. Soil nitrogen was measured incrementally and tallied for the depth of the profile at the start and end of each fallow period. Weed biomass was measured during the fallows. Each year the sites were recropped to wheat with grain yields and proteins recorded. No specific treatments were applied during the cropping phases. Where summer weeds were permitted to flourish, nitrogen was moved out of the profile to the surface where it was rendered unavailable to following wheat crops. The amount of nitrogen affected depended on the fallow weed biomass. Similarly, the degree of negative impact in the wheat depended on the amount of nitrogen consumed and moved by these weeds. Weed dry matter levels above 2500 kg ha resulted in net decreases in the inherent soil nitrogen. All weed levels consumed some degree of the nitrogen mineralised during the fallow. Wheat grain yield losses of up to 40% occurred when fallow weed biomass was measured at around 3000 kg ha. Weedy fallow treatments resulted in grain protein reductions of 3–5% on average across all sites.