Separation of Pig Slurry and Plant Utilization and Loss of Nitrogen‐15‐labeled Slurry Nitrogen

Separation of slurry by centrifugation concentrates nutrients and facilitates the transport of surplus nutrients away from livestock farms. Nitrogen‐15 labeled pig slurries containing urine 15 N or fecal 15 N were produced to compare the utilization and fate of N in separated and unseparated pig slurry. There was good agreement between the mineral fertilizer equivalence (MFE) of manure fractions estimated from 15 N uptake and by a traditional non‐isotopic method, but the 15 N method was more precise. The weighted utilization of N in separated slurry was similar to the N utilization in unseparated slurry. The MFE of slurry total N was 75 to 79% after incorporation before sowing spring barley ( Hordeum vulgare L.) and 59 to 64% after surface application in winter wheat ( Tritium aestivum L.). Both the origin of the slurry N and the fractionation influenced the N availability. The uptake of urinary 15 N in the first barley crop was 35 to 53% and the uptake of fecal 15 N 21 to 44% with the lowest availability of 15 N in the dry‐matter‐rich fraction (DMR). The uptake of 15 N in the following cover crop and barley crop was low (1–4.5%). The residual N effect of the manures in the year after application (MFE) was equivalent to 1% for the liquid fraction, 3% for the slurry, and 5% for the DMR. The amount of 15 N remaining in soil 15 mo after application was 30 to 53% for urinary N and 44 to 61% for fecal N. It is concluded that the overall utilization of N is unaffected by slurry separation, but the separation facilitates a better distribution of nutrients.