The N:P stoichiometry of cereal, grain legume and oilseed crops

Abstract

A data base including grain yield, nitrogen uptake ( N u) and phosphorus uptake ( P u) was compiled to investigate the N:P stoichiometry of cereal ( n = 759), grain legume ( n = 413) and oilseed species ( n = 421). Actual ratios and slopes of functions relating nitrogen uptake and phosphorus uptake were used to characterise N:P stoichiometry. The focus is on variability in N:P ratios of field crops, and N:P stoichiometry of crops achieving maximum yield. N:P ratios varied between ∼20, the maximum for legumes and oilseed crops, and ∼1.5, the minimum for cereals. By definition, N:P ratios are a direct function of N uptake and an inverse function of P uptake. The expected association between N:P ratio and N uptake was not evident except for grain legumes whereas the expected relationship between N:P and P uptake was verified for all three crop types. This highlights the role of P uptake as the main source of variability in N:P stoichiometry. The relationship between N uptake ( N u) and P uptake ( P u) for crops achieving maximum yield in each experiment was markedly non-linear. Functions of the form N u = N max P u/( P u # + P u) described the way in which N:P stoichiometry scaled with grain yield. Maximum uptake of nitrogen ( N max) was similar for all three crop types (≈240 kg N ha −1), but it was achieved via different avenues, i.e. high yield and low grain protein concentration for cereals compared to lower yield and higher grain protein concentration in oilseed and legume crops. Phosphorus uptake at half N max ( P u # ) ranged from 27 kg P ha −1 for oilseeds to 14 kg P ha −1 for legumes. The N:P ratio at P u # was 4.5 for oilseed crops, 5.6 for cereals and 8.7 for legumes. For cereals and oilseeds, over 40% of crops attaining maximum yield had N:P ratios in a relatively narrow range between 4 and 6. Variation in grain protein concentration was a major source of instability in N:P ratios of legumes. Being at the base of agro-ecological food webs, the N:P stoichiometry of crops has implications for both decomposers and consumers, and stoichiometric concepts might be of interest in fertiliser management and modelling. Variability in N:P stoichiometry related to plant storage products, however, restricts its application.