Cycling of rice rhizodeposits through peptide-bound amino acid enantiomers in soils under 50 and 2000 years of paddy management

Amino acid concentrations were similar in field (101-114 g C kg Corg−1) as in greenhouse soil samples (90-124 g C kg Corg−1), even though the latter completely consisted of rhizosphere soil. After one day the 13C label was already found in SOM (2.8-5.8 mg 13Cexcess per kg soil), peaking to a maximum of 10.3 mg kg−1 at the end of the labeling at day 3. About 15% of this 13C was recovered in amino acids, mainly in the L-forms, and particularly in l-alanine (max. 180 ± 5 mg kg−1 amino acid-C, day 3), which was likely representative for root exudates. Only the excess 13C in l-alanine but not in bulk SOM showed significant temporal dynamics, indicating incorporation of rhizodeposits into the peptide-bound soil amino acids. The bacterial residue marker d-glutamic acid showed a retarded incorporation of the 13C signal, without being affected by the duration of paddy management.