Resumen de Soil nitrogen contribution to grasslands and its implication for nitrogen use efficiency

I Lobos Ortega, M. Alfaro, J Martinez-Lagos

• In pasture systems of southern Chile, nitrogen (N) fertilization is mostly carried out without considering soil N supply, while seasonal N plant uptake is partially accounted for. These aspects are fundamental to correctly decide N fertilization. The aim of this work was to determine soil N contribution to grasslands yield in southern Chile and its implication for N use efficiency. Three treatments were distributed in a completely randomized block design with four replicates. A negative control treatment without N application and a positive control without N deficiency (450 kg N ha-1) were considered. An optimized treatment was also used, so that the total amount of mineral N applied as fertilizer was only that necessary to account for the arithmetic difference among plant uptake and N soil supply, resulting in the application of 171 kg N ha-1 only. Soil mineralization was high (241-934 kg N ha-1) and strongly affected by soil available N (VIP > 0.8). The 450 kg N ha-1 treatment produced 1,726 kg DM more than the 171 kg N ha-1 treatment. Nevertheless, this increase was associated to the application of an extra 279 kg N ha-1, so that the extra yield was produced at a 6 kg DM kg-1N efficiency. This value was one third of that showed by the optimized treatment, which was1.8times more efficient in the use of N than the 450 kg N ha-1 treatment, on average. Results suggest that it is possible to adjust pasture’s N fertilization considering soil N contribution via mineralization, and that this improves resources efficiency while maintaining pasture productivity.