Evidencing overwinter loss of residual organic and clay-fixed nitrogen from spring-applied, 15N-labelled pig slurry

In northern areas, it is assumed that soil residual mineral N measured at harvest represents the pool at risk of environmental loss during the non-growing season. Using N-15-tracing techniques we determined (i) the distribution of side-dressed pig slurry N in the mineral, organic and clay-fixed N pools of sandy loam and clay soils, and (ii) whether N-15 found in each individual pool at harvest is retained or lost during the non-growing season. Pig slurry labelled with N-15 was side-dressed to corn (four-leaf stage; 100 kg ha(-1) total N) and traced in soil (0-60 cm) and plants for 2 yr. Pig slurry contained 60% of its N as NH4. During the growing season, applied N accumulated in the clay-fixed N pool in the clay soil, and in the organic N pool in both soils. At harvest, less than 5% of side-dressed N was present as soil mineral N, whereas 60 to 65% was in the organic and clay-fixed pools. Although these forms are considered immobilized, one-third to one-half was lost during the non-growing season. This work provides evidence that the soil organic and clay-fixed N pools contribute to the loss of soil N during the non-growing season in cold areas, which should be included in future indicators for the risk of soil N loss.