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

Abstract

Rice cultivation under submerged conditions may exhibit low N use efficiency, however, plants release available C sources into the rhizosphere and therewith control the dynamics of soil organic nitrogen, though at yet unknown rates. Here we studied the cycling of rhizosphere N through soil proteins, as affected by short- and long-term lowland rice cultivation. For this purpose, rice was grown in the greenhouse on soils formed under 50 and 2000 years paddy rice cultivation in China and labeled for three days during late tillering stage using 13CO2 (3000 ppmv). Bulk and compound-specific isotope analysis allowed for tracing the 13C-label in total soil organic matter (SOM) and peptide-bound amino acid enantiomers after 1, 3, 11 and 16 days after labeling, respectively. Free amino acids had been removed with 1M HCl prior to protein hydrolysis.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.