Articles | Volume 2, issue 3
SOIL, 2, 433–442, 2016
SOIL, 2, 433–442, 2016

Original research article 06 Sep 2016

Original research article | 06 Sep 2016

Simultaneous quantification of depolymerization and mineralization rates by a novel 15N tracing model

Louise C. Andresen1, Anna-Karin Björsne1, Samuel Bodé2, Leif Klemedtsson1, Pascal Boeckx2, and Tobias Rütting1 Louise C. Andresen et al.
  • 1Department of Earth Sciences, University of Gothenburg, 405 30 Gothenburg, Sweden
  • 2Isotope Bioscience Laboratory, ISOFYS, Ghent University, 9000 Ghent, Belgium

Abstract. The depolymerization of soil organic matter, such as proteins and (oligo-)peptides, into monomers (e.g. amino acids) is currently considered to be the rate-limiting step for nitrogen (N) availability in terrestrial ecosystems. The mineralization of free amino acids (FAAs), liberated by the depolymerization of peptides, is an important fraction of the total mineralization of organic N. Hence, the accurate assessment of peptide depolymerization and FAA mineralization rates is important in order to gain a better process-based understanding of the soil N cycle. In this paper, we present an extended numerical 15N tracing model Ntrace, which incorporates the FAA pool and related N processes in order to provide a more robust and simultaneous quantification of depolymerization and gross mineralization rates of FAAs and soil organic N. We discuss analytical and numerical approaches for two forest soils, suggest improvements of the experimental work for future studies, and conclude that (i) when about half of all depolymerized peptide N is directly mineralized, FAA mineralization can be as important a rate-limiting step for total gross N mineralization as peptide depolymerization rate; (ii) gross FAA mineralization and FAA immobilization rates can be used to develop FAA use efficiency (NUEFAA), which can reveal microbial N or carbon (C) limitation.

Short summary
In soil the constant transport of nitrogen (N) containing compounds from soil organic matter and debris out into the soil water, is controlled by soil microbes and enzymes that literally cut down polymers (such as proteins) into single amino acids (AA), hereafter microbes consume AAs and excrete ammonium back to the soil. We developed a method for analysing N turnover and flow of organic N, based on parallel 15N tracing experiments. The numerical model gives robust and simultaneous quantification.