Contribution of summer rainfall and nitrogen to the yield and water use efficiency of wheat in Mediterranean-type environments of South Australia

In a worldwide context of agricultural intensification, cropping systems in Mediterranean-type environments have been reducing the frequency of long-fallow in a shift to continuous cropping. The focus of this paper is the short summer fallow between successive winter grain crops in environments of South Australia with winter-dominant rainfall. Our aims were to (i) estimate wheat yield response to summer rainfall, (ii) explain yield responses in terms of capture and efficiency in the use of radiation and water, (iii) explore the incidence of the amount and disposition of stubble on storage of summer rainfall, and (iv) assess the interaction between summer rainfall and nitrogen supply. We used trickle irrigation to manipulate summer water supply in two factorial experiments combining water and stubble treatments, and three factorials combining water and nitrogen supply. Addition of 50-100 mm of water increased soil water content at sowing between 8 and 46 mm compared to controls that only received the background summer rainfall (10-74 mm). Yield gain from additional water input in summer declined from 1.1 t ha(-1) to zero when the yield of controls increased from 2.0 to 7.8 t ha(-1). Where yield response was related to a single resource, water or nitrogen, capture of radiation and water were major drivers of growth and yield response. Where yield response was related to the interaction between water and nitrogen, both capture and efficiency in the use of resources were important. Amount (0-5 t ha(-1)) and disposition of stubble (standing or flat) did not affect the amount or distribution of water in soils, and had no effect on grain yield. High nitrogen rate was critical to capture the benefits of additional summer water and reciprocally high water supply was required to capture the benefits of nitrogen fertilisation; this highlights the resource co-limitation for wheat production in these environments. In a water x nitrogen factorial, crops with either low nitrogen or no additional summer water supply had radiation use efficiency similar to 1.6 g MJ(-1), biomass per unit evapotranspiration similar to 33 kg ha(-1) mm(-1) and yield per unit evapotranspiration similar to 15 kg ha(-1) which increased to 1.9 g MJ(-1), 40 kg ha(-1) mm(-1) and 18 kg ha(-1) mm(-1) in crops with both additional water and high nitrogen. Across experiments and treatments, grain number accounted for 88% of the variation in yield. Grain number was proportional to crop growth rate between stem elongation and anthesis; crops with high nitrogen produced 116 +/- 5.0 grains per unit crop growth rate and their low nitrogen counterparts 99 +/- 4.6. Building evidence from Mediterranean environments worldwide highlights the importance of grain number as the main source of variation in yield and therefore the critical period between stem elongation and anthesis where both water and nitrogen supply are critical. (C) 2011 Elsevier B.V. All rights reserved.