The use of 15N-enriched feed to label pig excreta for N cycling studies

Isotopic labelling can help improve our knowledge of the fate of manure N in agroecosystems. Our objective was to investigate the labelling dynamics of excreta N by feeding a pig with a 15N-enriched diet (2.808 atom % 15N) and to establish the implications of using the labelled excreta for N cycling studies. Pig urine and feces were collected and pooled each day for 20 d following the start of 15N-feeding. Each of the 20 excreta samples were analyzed for pH, dry matter content, C and N contents, and 15N distribution between the mineral and organic N pools. Sub-samples of each excreta sample were incubated for 84 d, and the 15N abundance of N mineralized after 7, 21 and 84 d of incubation was determined. The 15N concentration in pig excreta increased sharply during the first 3 d of 15N-feeding and slowly thereafter. The 15N concentration in excreta decreased rapidly when an unlabelled feed was served after 12 d of 15N-feeding. On the first day and after 9 d of 15N-feeding, the mineral and the organic N pools of the collected excreta had similar 15N content. However, from day 2 to 9 of 15N-feeding, the 15N abundance of excreta mineral N was 0.1 to 0.3 atom % lower than in the organic N pool. During incubation of the excreta samples, the 15N content of the mineralized N was 0.1 to 0.4 atom % lower after 84 d than after 21 d of incubation, indicating a heterogeneous distribution of 15N between the rapidly and the slowly mineralizable N pools of pig excreta. Despite some heterogeneity, the measured differences in 15N enrichment among the various excreta N pools were generally less than 15% for the first 9 d of 15N-feeding, and less than 5% afterwards. The labelled excreta were thus considered appropriate for short-term studies on the fate of manure N in the soil-plant system, especially for excreta collected after 9 d of 15N-feeding.