The efficiency of nitrogen in cattle manures when applied to a double-annual forage cropping system
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Using cattle manure (CM) fertilization, it is challenging to optimize nitrogen use. Our work aimed to assess nitrogen efficiencies, in a six-year experiment which involved three biennial rotations of four crops: oat-sorghum (first year) and ryegrass-maize (second year). This rotation was maintained
in a rainfed humid Mediterranean area of Spain. Fertilization treatments included: control (no-N), 250 kg mineral N ha-1year-1 (250MN), three CM rates (170, 250 and 500 kg N ha-1year-1), and four treatments where the two lowest CM rates were complemented with either 80 or 160 kg mineral N ha-1 year-1. The treatments were distributed randomly in each of three blocks. Maximum dry-matter yield (~44-49 t ha-1 rotation-1) was achieved in the third rotation, and only the control and the 170CM yielded significantly less. Inside the limitations of the EU Nitrate Directive, the N steady state of 170CM always requires a mineral complement (80 kg N ha-1) to maximize N agronomic efficiency. The maximum nitrogen fertilizer replacement value (250CM vs. 250MN) was 0.67, without significant differences between the two treatments in other N related efficiency indexes, which indicates that plants took advantage of N residual effects. Nitrogen losses by leaching in the 250CM were around 5-7% of the N applied. This reinforces the sustainability of manure recycling in long cropping seasons.