Crop uptake and leaching of 15N applied in ruminant slurry with selectively labelled faeces and urine fractions

Two animal slurries either labelled with N-15 in the urine or in the faeces fraction, were produced by feeding a sheep with unlabelled and N-15-labelled hay and collecting faeces and urine separately. The slurries were applied (12 g total N m(-2)) to a coarse sand and a sandy loam soil confined in lysimeters and growing spring barley (Hordeum vulgare L). Reference lysimeters without slurry were supplied with (NH4NO3)-N-15-N-15 corresponding to the inorganic N applied with the slurries (6 g N m(-2)). In the second year, all lysimeters received unlabelled mineral fertilizer (6 g N m(-2)) and grew spring barley. N harvested in the two crops (grain + straw) and the loss of nitrate by leaching were determined. N-15 in the urine fraction was less available for crop uptake than mineral fertilizer N-15. Th, first barley crop on the sandy loam removed 49% of the N-15 applied in mineral fertilizer and 36% of that applied with urine. The availability of fertilizer N-15 (36%) and urine N-15 (32%) differed less on the coarse sand. Of the N-15 added with the faeces fraction, 12-14% was taken up by the barley crop on the two soils. N mineralized from faeces compensated for the reduced availability of urine N providing a similar or higher crop N uptake in manured lysimeters compared with mineral fertilized ones. About half of the total N uptake in the first crop originated from the N applied either as slurry or mineral fertilizer. The remaining N was derived from the soil N pool. Substantially smaller but similar proportions of N-15 from faeces, urine and fertilizer were found in the second crop. The similar recoveries indicated a slow mineralization rate of the residual faeces N since more faeces was left in the soil after the first crop. More N was lost by leaching from manured lysimeters but as a percentage of N applied, losses were similar to those from mineral fertilizer. During the first and second winter, 3-5% and 1-3%, respectively, of the N-15 in slurry and mineral fertilizer was leached as nitrate. Thus slurry N applied in spring just before sowing did not appear to be more prone to loss by nitrate leaching than N given in mineral fertilizer. Slurry N accounted for a higher proportion of the N leached, however, because more N was added in this treatment.