Fate of N in soil amended with 15N-labelled residues of winter cereals combined with an organic N fertilizer

In organic farming winter cereals, as cover crops, provide nutrients, when they return to soil as crop residues. The fate of N deriving from two decomposing 15N-labelled winter cereals, with or without the supply of an organic N fertiliser, was studied in a field experiment. The stabilisation of residues N in soil aggregates and the portion lost from the system were also evaluated. Barley or triticale residues were incubated alone or with hydrolysed leather (L) in mesocosms over one year. The residues derived N was measured in soil and in soil aggregate size fractions >250m (macroaggregates, macro); 53-250m (microaggregates, micro); <53m (silt and clay, s+c) obtained by wet sieving. Barley degradation (77% of the initial amount) was faster than triticale degradation (55%). The L slowed down the barley degradation (72%) and speeded that of triticale (64%). Greater amount of residues N was in macro and micro than in s+c. The presence of L reduced the residues N stabilised in the finest fractions. The losses of barley N were reduced by the L addition, those from triticale were increased. The fate of residues N was affected by the L application that influenced the residues mineralisation, the stabilisation and the losses of their derived N.