Effect of long-term stubble management on yield and nitrogen-uptake efficiency of wheat topdressed with urea in north-eastern Victoria

Abstract

Use of urea fertiliser for cereal cropping in south eastern Australia has increased rapidly in recent years to arrest a general decline in grain protein and to increase yields. In conservation cropping systems, crop stubbles provide a source of carbon, which has the potential to retain a portion of the fertiliser nitrogen in the soil. The impact of fertiliser nitrogen was compared under 4 stubble management regimes for efficiency of nitrogen uptake by a wheat crop in a long-term cereal–grain legume rotation. The experiment was established on a duplex red-brown earth in 1985 to compare stubble retention (standing, shredded, incorporated) with stubble burning. In 1995, wheat following a failed lupin crop was topdressed with urea fertiliser at 50 kg nitrogen per hectare to split plots of each stubble treatment at the third-leaf stage of growth. The urea significantly increased nitrogen uptake by wheat grown on burnt stubbles and increased grain yield by 1 t/ha. Nitrogen applied to wheat grown on stubbles retained above-ground increased yield by 0.5 t/ha, whereas there was no significant yield increase from nitrogen when stubble was incorporated due to less transfer of dry matter to grain. Efficiency of urea-nitrogen uptake in grain was reduced under stubble retention. The total grain nitrogen uptake in response to stubble burning increased by 17.6 kg/ha, which was equivalent to a conversion efficiency of 35%, compared with only 26, 24 and 16% of the applied 50 kg nitrogen per hectare for stubble standing, shredding and incorporation treatments, respectively. Soil organic carbon and total nitrogen levels were 1 and 0.1%, respectively, irrespective of stubble treatment. Added urea increased microbial decomposition of cellulose in calico cloth buried beneath stubbles retained above-ground by 30%, compared with stubble incorporated or burnt treatments. These results suggest that where low levels of available nitrogen exist in cropping systems that use stubble retention, higher nitrogen inputs may be needed, due to less efficient uptake of nitrogen from urea fertiliser.