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The efficient use by crops of nitrogen from manures is an agronomic and environmental issue, mainly in double-annual forage cropping systems linked to livestock production.A six-year trial was conducted for a biennial rotation of four forage crops: oat-sorghum (first year) and ryegrass-maize (second year) in a humid Mediter ranean area. Ten fertilization treatments were introduced: a control (without N);two minerals equivalent to 250 kg N ha-1yr-1 applied at sowing or as sidedressing; dairy cattle manure at a rate of 170, 250 and 500 kg N ha-1yr-1 and four treatments where the two lowest manure rates were supplemented with 80 or 160 kg mineral N ha -1 yr -1.They were distributed according to a randomized block design with three blocks. The highest N mineral soil content was found in the summer of the third rotation, in plots where no manure was applied. The yearly incorporation of manure reduced, in successive cropping seasons, the amount of additional mineral N n eeded as sidedressing to achieve the highest yields.Besides, in the last two years, there was no need for mineral N application for the manure rate of 250 kg N ha -1yr-1. This amount always covered the oat-sorghum N uptake. In the ryegrass-maize sequence uptakes were as high as 3 kg N ha-1yr-1.In the medium term, the intermediate manure rate (250 kg N ha -1 yr -1) optimizes nutrient recycling within the farming system, and it should be considered in the analysis ofthresholds for N of organic orig in to be applied to systems with high N demand.

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. This research was financially supported by the following institutions involved in the improvement of fertilization practices in La Garrotxa, north-east Spain: Garrotxa Volcanic Zone Natural Park, Consortium SIGMA, IRTA-Mas Badia Foundation and Ministry of Agriculture, Livestock, Fisheries and Food (Generalitat de Catalunya, Catalonia, Spain).