Adjusting nitrogen fertilization to spatial variations in growth conditions in silvopastoral systems for improved nitrogen use efficiency

Abstract Grass swards in silvopastoral systems have a lower herbage production near trees than open grassland. This is related to a lower nitrogen (N) uptake in an area close to the tree lines. The N use efficiency for the whole field can then be improved when N input is spatially adjusted and the overall amount reduced. We performed a 2-year field experiment to gain insights into this idea for making savings on N inputs by studying the response of the grass sward to fertilizer N input in relation to the distance from trees. We hypothesized an interaction between N input rate and position to the tree line on grassland herbage production and N uptake and, accordingly, N use efficiency. The field trial was carried out in a silvopastoral system in Germany consisting of the factors year (2019 and 2020), position (2.5, 6.5 and 24.5 m distances to tree line), annual N application rate (0, 15 and 30 g N m −2 ) and harvest date (four harvests per year). We found significant interactions between position × N rate for all target variables, and usually also in interactions with the harvest date or year. The N application close to trees at a distance < 6.5 m had in most cases no significant influence on herbage production but significantly increased herbage production by up to 35% at positions further away. Omitting the N fertilization in a corridor of 6.5 m near the trees would contribute directly to the saving of fertilizer N without a negative trade-off in herbage production.