Nitrogen (N) is one of the most limiting nutrients for cereal production, especially in wheat, which is one of the main crops cultivated globally. To achieve high yields, wheat requires a certain amount of nitrogen (N), as N deficiency can lead to a decrease in yield and thus reduce income for farmers. In contrast, excessive applications of N fertilizer can be detrimental to both terrestrial and aquatic environments. To optimize N fertilizer applications in wheat, a three-year field experiment was conducted to evaluate the impact of different N fertilization strategies on various N-related physiological and agronomic traits. Moreover, to optimize N utilization efficiency while maintaining crop productivity, a mixture of five winter wheat varieties was used to mitigate the possible impact of environmental constraints. These strategies were based on a simultaneous increase in N fertilization and N fertilizer fractionation at key stages of plant development in a soil conservation agriculture (SCA) system in which legumes were grown prior to the cultivation of the main crop. In this SCA system, we observed that 200 kgN·ha⁻¹ was optimal for both N use efficiency (NUE) and aerial and grain biomass production. Moreover, we found that at this level of N fertilization, of the application strategies, a 40%/40%/20% split application at full tillering, at the first node, and at booting, respectively, appeared to be the best option for the highest plant productivity.

Nitrogenous compounds of soil organic matter constitute a major N reservoir on Earth. Both the world food protein supply produced by agriculture and the global contamination by reactive nitrogen species rely on the dynamics of these compounds. To investigate their dynamics, we used both natural¹³C labeling and accelerator mass spectrometry (AMS)¹⁴C dating of the α-carboxyl amino carbon, which is specific of the amino acid fraction that was extracted from bulk soil organic matter by ninhydrin hydrolysis. We applied this isotopic approach to investigate the age of carboxyl carbon in a maize-cultivated Cambisol chronosequence. Based on a few measurements, we demonstrate the feasibility of this new compound-specific method of investigation of soil carbon dynamics. We show that soil organic matter amino acids can be split into two very distinct dynamic compartments: the majority having a mean age of a few years and a minority having a mean carbon age of several millennia. The latter fraction can be either strongly stabilized in soils, or can arise from microbial utilization of old carbon resources, as predicted by the priming effect theory.