Early share of ¹⁵N-labelled fertilizer between trees and crop in young temperate alley-cropping system

Nitrogen (N) partitioning between trees and crops in alley-cropping systems (AC) may increase the N fertilizer use, but more investigation is needed to elucidate it for young trees. The aim of this study was to explore the share of N between crops and three tree species after a few years of tree establishment in AC. Using N-15 labeling (10 atm% as (NH4)-N-15,(NO3)-N-15 form), we traced the fate of N fertilizer in wheat associated with four-year-old trees at Ramecourt alley-cropping experimental site. The N-15 abundance of tree leaves and crop whole-plant was compared between samples collected in AC, in sole-crop control (CC) and in unlabeled and unfertilized pure-forest control plots (FC). The wheat delta N-15 increased from 0.7 +/- 0.6 parts per thousand to 994 +/- 474 parts per thousand before and 12 days after labeling and remained stable until harvest. For natural abundance, no significant difference in wheat delta N-15 was observed between AC and CC. The same result was observed for labeled treatment. The percentage of wheat N derived from fertilizer was on average 27 +/- 17% and no effect of type of system or associated tree species was observed. Tree foliar delta N-15 was multiplied by 1.5, 1.8 and 4 for willow, hornbeam and wild cherry respectively compared to unlabeled trees in AC. The percentage of tree foliar N derived from fertilizer was on average 0.04 +/- 0.04% and no significant difference was observed between tree species. The wheat associated with trees allocated significantly more tracer N to ears than wheat in monocrop. From this study, we succeeded in quantifying the beginning of tree/crop sharing of N fertilizer using labeling method and evidenced tree species effect. Young trees in AC contributed to valorize the N fertilizer without significant effect on global wheat uptake and favored its valorization in wheat grain.