Abstract
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.
Highlights
Soil organic nitrogen (SON) mineralization is essentially a sequence of depolymerization of polymeric organic compounds followed by the mineralization of the liberated monomers (Schimel and Bennett, 2004)
Amino acid mineralization is important for Inorganic nitrogen (IN) availability; our research question is whether the peptide depolymerization and free amino acids (FAAs) mineralization rates are two important steps co-limiting for N availability
Prior to the 15N addition, the Umbrisol had a 6 times higher FAA content compared to the Podzol, and the relative abundance of individual FAAs differed between the two soils (Fig. 4)
Summary
Soil organic nitrogen (SON) mineralization is essentially a sequence of depolymerization of polymeric organic compounds followed by the mineralization of the liberated monomers (Schimel and Bennett, 2004). Wanek et al (2010) provided a methodological development of 15N pool dilution assays to determine gross peptide depolymerization rates, and by combining this with 15N tracing, the quantification of gross FAA mineralization can be achieved (Andresen et al, 2015) These approaches apply analytical calculations (Kirkham and Bartholomew, 1954; Watkins and Barraclough, 1996), handling one flux at a time, which has some limitations: (1) the analytical solutions only provide total consumption and production rates and not the specific processes, (2) analytical solutions only consider zero-order kinetics, (3) the possibility of remineralization or remobilization limits the experimental work to short time steps, and (4) with the analytical approach gross rates are sequentially quantified, which does not take into consideration possible interactions; the numerical modelling provides a more coherent framework as the process rates are quantified simultaneously (Rütting et al, 2011). For our selected mineral soils from Swedish spruce forest, our hypotheses are (1) that FAA mineralization is a major, important part of gross N mineralization and that 2) due to year-long forestry in this area, we expect the soil to be carbon limited rather than N limited
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