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 N-15-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 N-15 was recovered in the POMN fraction, with a greater proportion of the wheat than faba bean residue recovered as (POMN)-N-15. This (POMN)-N-15 probably included undecomposed residues and (LFOMN)-N-15. 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 (POMN)-N-15 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 (POMN)-N-15 concentrations corresponded with mineral 15N accumulation in the soils. Mineral N-15 concentration after 112 d was positively related to the initial (POMN)-N-15 concentration (r = 0.78, P < 0.001) but not to the initial (LFOMN)-N-15 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. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.