Depolymerization and mineralization – investigating N availability by a novel 15N tracing model

Abstract

Depolymerization of soil organic matter such as proteins and peptides into monomers (e.g. amino acids) is currently thought to be the rate limiting step for N availability in terrestrial N cycles. The mineralization of free amino acids (FAA), liberated by depolymerization of peptides, is an important fraction of the total N mineralization. Accurate assessment 10 of peptide depolymerization and FAA mineralization rates is important in order to gain a better understanding of the N cycle dynamics. Due to the short time span, soil disturbance and unnatural high FAA content during the first few hours after the labelling with the traditional 15N pool dilution experiments, analytical models might overestimate peptide depolymerization rate. In this paper, we present an extended numerical 15N tracing model Ntrace which incorporates the FAA pool and related N processes in order to 1) provide a more robust and coherent estimation of production and mineralization rates of FAAs; 2) 15 and 2) suggest an amino acid N use efficiency (NUEFAA) for soil microbes, which is a more realistic estimation of soil microbial NUE compared to the NUE estimated by analytical methods. We compare analytical and numerical approaches for two forest soils; suggest improvements of the experimental work for future studies; and conclude that: i) FAA mineralization might be as equally an important rate limiting step for gross N mineralization as peptide depolymerization rate is, because about half of all depolymerized peptide N is consecutively being mineralized; and that ii) FAA mineralization and FAA 20 immobilization rates should be used for assessing NUEFAA.