Labile organic nitrogen transformations in clay and sandy-loam soils amended with 15N-labelled faba bean and wheat residues

Labile organic nitrogen (N) fractions are actively involved in short-term N mineralization, but the extent to which each fraction contributes to N mineralization is not fully understood. The objective of this study was to examine the flow of 15N-labelled faba bean (Vicia faba L.) and wheat (Triticum aestivum L.) residues through the soil microbial biomass N (MBN), water-extractable organic N (WEON), light fraction organic matter N (LFOMN), particulate organic matter N (POMN) and mineral N pools in sandy-loam and clay soils under controlled conditions. After 3 d, 17–30% of the residue 15N was recovered in the POMN fraction, with a greater proportion of the wheat than faba bean residue recovered as POM15N. This POM15N probably included undecomposed residues and LFOM15N. Net N mineralization was greater in faba bean- than wheat-amended soils and greater in the sandy-loam than the clay soil. The LFOM15N concentrations decreased throughout the study, while POM15N concentrations increased or remained constant for 28 d in the sandy-loam and until 56 d in the clay soil. This suggests possible encrustation of LFOMN with soil mineral particles causing increased densification and recovery in the POMN fraction. The subsequent decrease in POM15N concentrations corresponded with mineral 15N accumulation in the soils. Mineral 15N concentration after 112 d was positively related to the initial POM15N concentration (r = 0.78, P < 0.001) but not to the initial LFOM15N concentration (r = −0.48, P > 0.05). The WEON and MBN appeared as transient, intermediary pools. The results of this study suggest that mineralization of POMN is a major pathway through which mineral N is supplied in agricultural soils, with C/N ratio of crop residues and to a lesser extent soil properties, influencing the mineralization rate.