Is crop N demand more closely related to dry matter accumulation or leaf area expansion during vegetative growth?

Agricultural Production Systems Research Unit (APSRU), University of Queensland, School of Land and Food Sciences, Brisbane, Qld 4072, Australia
Field Crops Research (Impact Factor: 2.61). 01/2007; DOI: 10.1016/j.fcr.2006.05.009

ABSTRACT The critical crop nitrogen uptake is defined as the minimum nitrogen uptake necessary to achieve maximum biomass accumulation (W). Across a range of crops, the critical N uptake is related to W by a power function with a coefficient less than unity that suggests crop N uptake is co-regulated by both soil N supply and biomass accumulation. However, crop N demand is also often linearly related to the expansion of the leaf area index (LAI) during the vegetative growth period. This suggests that crop N demand could be also linked with LAI extension. In this paper, we develop theory to combine these two concepts within a common framework. The aim of this paper is to determine whether generic relationships between N uptake, biomass accumulation, and LAI expansion could be identified that would be robust across both species and environment types. To that end, we used the framework to analyze data on a range of species, including C3 and C4 ones and mono- and di-cotyledonous crops. All crops were grown in either temperate or tropical and subtropical environments without limitations on N supply. The relationship between N uptake and biomass was more robust, across environment types, than the relationship of LAI with biomass. In general, C3 species had a higher N uptake per unit biomass than C4 species, whereas dicotyledonous species tended to have higher LAI per unit biomass than monocotyledonous ones. Species differences in N uptake per unit biomass were partly associated with differences in LAI and N-partitioning. Consequently the critical leaf-N uptake per unit LAI (specific leaf nitrogen, SLN) was relatively constant across species at 1.8–2.0 g m−2, a value that was close to published data on the critical SLN of new leaves at the top of the canopy. Our results indicate that critical N uptake curves as a function of biomass accumulation may provide a robust platform for simulating N uptake of a species. However, if crop simulation models are to capture the genotypic and environmental control of crop N dynamics in a physiologically functional manner, plant growth has to be considered as the sum of a metabolic (e.g. leaves) and a structural (e.g. stems) compartment, each with its own demand for metabolic and structural N.

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    ABSTRACT: The fertilizer-nitrogen (N) requirement for wheat grown in the UK varies from field to field. Differences in the soil type, climate and cropping history result in differences in (i) the crops' demands for N, (ii) the supply of N from the soil (SNS) and (iii) the recovery of the fertilizer by the crops. These three components generally form the basis of systems for N recommendation. Three field experiments were set out to investigate the variation of the N requirement for wheat within fields and to explore the importance of variation in the crops' demands for N, SNS and fertilizer recovery in explaining the differences in the economic optima for N. The N optima were found to vary by >100 kg N/ha at two of the sites. At the other site, the yield response to N was small. Yields at the optimum rate of N varied spatially by c. 4 t/ha at each site. Soil N supply, which was estimated by the unfertilized crops' harvested N, varied spatially by 120, 75 and 60 kg/ha in the three experiments. Fertilizer recovery varied spatially from 30% to >100% at each of the sites. There were clear relationships between the SNS and the N optima at all the three sites. The expected relationship between the crop's demand for N and N optima was evident at only one of the three sites. There was no consistent relationship between the N recovery and the N optima. A consistent relationship emerged, however, between the optimal yield and SNS; areas with a greater yield potential tending to also supply more N from the soil. This moderated the expected effect of the SNS and the crop's demand for N on the N optima.
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