Stabilization and plant uptake of N from 15N-labelled pea residue 16.5 years after incorporation in soil

The decline of N from 15N-labelled mature pea residues was followed in unplanted soil over 16.5 yr. Eight years after residue incorporation, 24% of the residue 15N input was still present in the soil and, after 16.5 yr, 16% of the residue 15N input remained. A double exponential model successfully described the decay of N from 15N-labelled pea residues. The total residual 15N declined with average decay constants of 1.45 yr−1 for the 30 d to 1 yr period and of 0.07 yr−1 for the 1–16 yr period. Sixteen years following incorporation of the residues, indicator plants growing in residues-amended soils were obtaining 1.7% of their N from residue N. This is, to our knowledge, the longest study on decay of N in soils from 15N-labelled crop residues. The current study thus provides a unique data set for our empirical understanding of N-dynamics in agricultural systems, which is a prerequisite to parameterize and validate N-simulation models.