Abstract
Due to the asymptotic nature of the crop yield response curve to fertilizer N supply, the nitrogen use efficiency (NUE, yield per unit of fertilizer applied) of crops declines as the crop N nutrition becomes less limiting. Therefore, it is difficult to directly compare the NUE of crops according to genotype-by-environment-by-management interactions in the absence of any indication of crop N status. The determination of the nitrogen nutrition index (NNI) allows the estimation of crop N status independently of the N fertilizer application rate. Moreover, the theory of N dilution in crops indicates clearly that crop N uptake is coregulated by (i) soil N availability and (ii) plant growth rate capacity. Thus, according to genotype-by-environment-by-management interactions leading to variation in potential plant growth capacity, N demand for a given soil N supply condition would be different; consequently, the NUE of the crop would be dissimilar. We demonstrate that NUE depends on the crop potential growth rate and N status defined by the crop NNI. Thus, providing proper context to NUE changes needs to be achieved by considering comparisons with similar crop mass and NNI to avoid any misinterpretation. The latter needs to be considered not only when analyzing genotype-by-environment-by-management interactions for NUE but for other resource use efficiency inputs such as water use efficiency (colimitation N–water) under field conditions.
Highlights
Nitrogen (N) is a critical factor limiting agricultural productivity, in addition to the supply of water and other nutrients such as phosphorus
The N dilution process in plants growing in a dense canopy is the consequence of the two adaptive mechanisms to competition for light: (i) a shade avoidance mechanisms determined by the photo-morphogenetic response of plants to changes in light quality associated with a light extinction profile within the canopy [33,34], and (ii) an optimization of N allocation within the plant to well-illuminated leaf layers for maximizing radiation use efficiency [35,36,37]
This improved approach permits an evaluation of different scenarios: (i) an increase in relative yield (Y/Ymax) as the nutrition index (NNI) int improves for a certain comparison of genotypes (G1 vs. G2; Figure 10), with a capacity to match their own N demand in situations of low N supply, (ii) a similar relative yield with a reduced NNI int, portraying the capacity of a genotype to maintain yield despite a low N
Summary
Nitrogen (N) is a critical factor limiting agricultural productivity, in addition to the supply of water and other nutrients such as phosphorus. For the last five decades, the external application of mineral N fertilizers increased sevenfold while agricultural food production only doubled [1]. The objective of sustainable N fertilization management should be to increase the synchrony between crop N supply and crop N demand in order to constrain N losses. The conventional method using an analysis of the yield response to the application of mineral N fertilizer, with the goal of defining the “optimum” N availability for the crop to achieve. Due to large uncertainties in the estimation of (i) the quantity of available N supplied by soil, and (ii) the crop N demand associated with the attainable yield in various conditions (related to soil and weather variations), crop fertilization management often leads to excess application rates due to risk aversion by farmers of not matching crop N demand [8]. The current questions dealing with sustainable development, climate change, quality of environment, and food security are strongly associated with the use efficiency of N fertilizers in the current farming systems [10]
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