Plant preference for NH4+ versus NO3− at different growth stages in an alpine agroecosystem

It remains unclear if crops can change their preference for NH4+ or NO3− during their growth stages in alpine habits. Here, we conducted a field experiment using 15N-labelled NH4+ and NO3− to examine the preference for mineral N forms by four crop species (highland barley, forage oat, alfalfa and common vetch) at three growth stages in a Tibetan cropland. Twenty-four hours after 15N labeling, plant and soil samples (0-15 cm) were collected for determining NH4+ and NO3+ as well as plant N content and its 15N/14N ratios. Soil NO3− concentration was significantly higher than NH4+ throughout the growth stages for legumes. For grasses, soil NH4+ concentration was higher than NO3− at early stages but lower at later stages. The N uptake efficiency (NUE) for alfalfa, forage oat, and common vetch decreased at the early stage, but increased later; for highland barley it decreased throughout the growth stage. The legumes preferred NO3− throughout their growth while the grasses preferred NH4+ at the early stages but switched the preference for NO3− later. We conclude that the crop N uptake preference can change during its different growing stages.