Soil nitrogen contribution to grasslands yield in southern Chile its implications for nitrogen use efficiency nitrogen use efficiency

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(-1) N efficiency. This value was one third of that showed by the optimized treatment, which was 1.8 times 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.