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There is still disagreement about whether crop growth rate or soil nitrate concentration control nitrogen absorption by crops under field conditions. The influence of these factors on the control of N uptake rate was examined in the absence of water stress, using data on dry matter production, above-ground nitrogen accumulation and soil nitrate concentration from several N-fertilizer experiments on winter wheat, winter oilseed rape and maize. The results confirmed that crops can accumulate nitrogen far in excess of the ‘critical dilution curve’, which defines the minimum amount of nitrogen needed for maximal growth rate: the N concentration in plants could exceed the critical N concentration by 70 to 80% for the three species studied. The nitrate uptake rate index (NUI) was calculated as the ratio of actual and critical N uptake rates, at intervals of 1 week. NUI varied with nitrate concentration in the 0–30 cm soil layer according to a Michaelis–Menten equation (with one or two components). This response was compared with the kinetics of saturation of the nitrate uptake systems: the high affinity transport system (HATS) and the low affinity transport system (LATS). As a result, it is proposed that there is a critical N dilution curve delimiting two domains of N use by plants. This is linked to the two nitrate transport systems, with HATS working at low nitrate concentrations, below the critical dilution curve, and LATS at high nitrate concentrations, above the curve. NUI provides another method for calculating the actual nitrate uptake rate, which depends on the maximal crop growth rate (without N deficiency) and on the external nitrate concentration.

Integrated Control of Nitrate Uptake by Crop Growth Rate and Soil Nitrate Availability under Field Conditions