The N:P stoichiometry of cereal, grain legume and oilseed crops
Introduction
Crop yield is frequently constrained by availability of major nutrients, including nitrogen and phosphorus. While approaches for the diagnosis and management of crop nutrition often target individual nutrients, there is an increasing interest in integrated nutrient management. Heady et al. (1955) developed early empirical models of crop and pasture responses to multiple nutrients (NPK). Angus et al. (1993), Hedge (1996), Witt et al. (1999), Prasad et al. (2002), de Varennes et al. (2002), Kho (2000) and Duivenbooden et al. (1996) are examples of more recent theoretical and experimental attempts to understand and manage crop responses to multiple nutrients.
N:P stoichiometry and co-limitation are two important ecological concepts which can contribute to understand N–P relationships in crops. N:P stoichiometry has been interpreted in the light of evolutionary constraints on the chemical composition of living organisms (Elser et al., 1996, Sterner and Elser, 2002, Ågren, 2004). Scaling up from biomolecules to ecosystems, N:P ratios have provided a valuable link between cellular, ecosystem and evolutionary processes. Furthermore, N:P ratios have been proposed as a diagnostic tool for nutrient limitations in natural vegetation (Koerselman and Meuleman, 1996, Verhoeven et al., 1996, Güsewell and Koerselman, 2002, Güsewell et al., 2003). Co-limitation is operationally identified when the response of the system to two or more factors is greater than its response to each factor in isolation, and has been recognised at levels of organisation from cells to ecosystems (Venterink et al., 2001, Flynn, 2002). Based on economic marginal analysis, Bloom et al. (1985) proposed that a high degree of resource co-limitation is a condition for maximum growth of stressed plants. Consistently with this theory, the optimum N:P for a species has been defined as the N:P ratio where its growth is equally limited by N and P (Sterner and Elser, 2002). Rastetter and Shaver (1992) modelled plant growth under multiple-element limitation upon the assumption of an optimal ratio of mineral elements in vegetation biomass.
Agronomic studies of crop N:P stoichiometry and co-limitation are scarce. Janssen (1998) proposed that nutrient uptake efficiency, i.e. the ratio of actual uptake to potential supply, and utilisation efficiency, i.e. the ratio of yield to actual uptake, require “N, P and K perfectly in balance to reach their maximum values”. For cereals, Duivenbooden et al. (1996) calculated an optimum N:P ≈ 7. In comprehensive experiments with rice, Witt et al. (1999) estimated balanced nutrient uptakes of 14.7 kg N and 2.6 kg P per tonne of grain; this corresponds with a ratio of 5.6. Mkamilo (2004) used N:P ratios in plant tissue to assess relative nutrient limitation in maize–sesame intercrops. Recent studies of crop responses to water and nitrogen availability support the concept of high degree of co-limitation as a condition for maximising growth of stressed wheat (Sadras, 2004, Sadras et al., 2004).
This paper investigates the N:P stoichiometry of cereal, grain legume and oilseed crops. The focus is on variability in N:P ratios of field crops, and N:P ratios of crops achieving maximum yield. Ågren (2004) defined “critical ratio” as the ratio where N and P are simultaneously limiting growth. According to the theory of co-limitation (Bloom et al., 1985, Sadras, 2004) this critical ratio should correspond with maximum yield. But the large capacity of plants for excess uptake of nutrients may affect the way in which N:P stoichiometry scales with growth and yield (Sterner and Elser, 2002, Ågren, 2004). Typical N:P ratios, nonetheless, are expected to reflect the dominant chemical compounds stored in grains, e.g. protein in grain legumes versus starch in cereals.
Section snippets
Method
CAB abstracts (1973–2004) were searched using combinations of key words including nitrogen, phosphorus, uptake, and yield. The retrieved abstracts were inspected, and papers reporting grain yield and crop uptake (i.e. shoot content) of N and P were used to build the data base summarised in Table 1. Only papers from field experiments were considered.
Grain yield and nutrient uptake
Although the relationships between grain yield and nutrient uptake are well established, this section outlines the specific relationships for the data set in Table 1 to provide an agronomic background for the N:P stoichiometry in Section 3.2.
Crops summarised in Table 1 grew under contrasting soil, weather and management conditions. This caused a large variation in crop yield and uptake of nutrients (Table 2). Nutrient availability, as affected by soils and fertiliser rate, was a major source of
Concluding remarks
This study characterised the variability of N:P ratios in grain crops, and showed consistent differences among crop types. Non-linear functions relating N uptake and P uptake for crops achieving maximum yield described the way in which N:P stoichiometry scales with grain yield. Variation in protein concentration was a major source of instability in N:P ratios, particularly in grain legumes. Grain phytate concentration is a potentially important source of N:P variation that deserves further
Acknowledgements
I thank Luis Aguirrezábal, Daniel Cogliatti and Steve Milroy for valuable comments on the manuscript, Mikaela Lawrence and her colleagues at CSIRO Library for their kind assistance with the literature search, and the Grains Research and Development Corporation of Australia for financial support.
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