Gross rates of N mineralization associated with the decomposition of plant residues

Nitrogen-15 isotope dilution was used to determine gross rates of N mineralization immediately following the incorporation of crop residues in soil. Laboratory and field experiments were performed. Two crop residues were used: oil-seed rape stems and winter wheat straw. Incorporation of oil-seed rape residues into the soil resulted in an immediate increase in gross N mineralization rate from 0.87 to 2.2 mg kg−1 d−1. After 12 d, gross N mineralization in the presence of the residues had dropped to 0.79 mg kg−1 d−1, compared to 0.72 mg kg−1 d−1 in the control. Of the N in the oil-seed rape, 24% was mineralized in 12 d. In contrast, incorporation of winter wheat residues caused only a small immediate increase in gross N mineralization to 0.92 mg kg−1 d−1. Of the N in the winter wheat residues, 12% was mineralized in the 12 d following incorporation. Longer-term measurements indicated that the residue incorporation was still influencing N mineralization 54 d after incorporation with 19 and 11% of the mineralization flux resulting from oil-seed rape and winter wheat decomposition, respectively. Field measurements showed that 5 months after incorporation, oil-seed rape residues were contributing 13% of the mineralization flux while winter wheat residues were contributing 9%.