Abstract
Abstract. Phragmites australis litters were incubated in three waterlogged anoxic wetland soils of different nutrient status for 75 d, and litter nitrogen (N) dynamics were analyzed by elemental analyses and Fourier transform infrared spectroscopy (FTIR). At the end of the incubation time, the N content in the remaining litter tissue had increased in most samples. Yet, the increase in N content was less pronounced when litters had been decomposed in a more-N-poor environment. FTIR was used to quantify the relative content of proteins in litter tissue and revealed a highly linear relationship between bulk N content and protein content. Changes in bulk N content thus paralleled and probably were governed by changes in litter protein content. Such changes are the result of two competing processes within decomposing litter: enzymatic protein depolymerization as a part of the litter breakdown process and microbial protein synthesis as a part of microbial biomass growth within the litter. Assuming microbial homeostasis, DNA signals in FTIR spectra were used to calculate the amount of microbial N in decomposed litter which ranged from 14 % to 42 % of the total litter N for all leaf samples. Microbial carbon (C) content and resultant calculated carbon use efficiencies (CUEs) indicate that microbial N in litter accumulated according to predictions of the stoichiometric decomposition theory. Subtracting microbial C and N contributions from litter, however, revealed site-dependent variations in the percentual amount of the remaining still-unprocessed plant N in litter compared to remaining plant C, an indicator for preferential protein depolymerization. For all leaf litters, the coefficient of preferential protein depolymerization (α), which relates N-compound depolymerization to C-compound depolymerization, ranged from 0.74–0.88 in a nutrient-rich detritus mud to 1.38–1.82 in Sphagnum peat, the most nutrient-poor substrate in this experiment. Preferential protein depolymerization from litter decomposing in Sphagnum peat leads to a gradual N depletion in the early phase of litter decomposition, which we propose as a preservation mechanism for vascular litter in Sphagnum peatlands.
Highlights
Accounting for 2 %–15 % of tissue dry mass, proteins are a minor biopolymeric constituent of plant litter (KögelKnabner, 2002)
To elucidate the reason for these patterns, we developed a new method to disentangle plant and microbial N in decomposed litter using Fourier transform infrared spectroscopy (FTIR)
Our results indicate that site-dependent deviations of N dynamics were not due to variation in the accumulation of microbial N within litter but due to variations in the remaining amount of plant N, an indicator of preferential protein depolymerization
Summary
Accounting for 2 %–15 % of tissue dry mass, proteins are a minor biopolymeric constituent of plant litter (KögelKnabner, 2002). Protein depolymerization is the process by which proteins in dead organic matter are broken down into smaller, N-containing fragments (amino acids) which are readily absorbable by microbial decomposers. Protein depolymerization is often described as a central driver of the terrestrial N cycle as it is the dominant process by which chemically bonded N, inaccessibly stored in litter or soil, becomes ac-. H. Reuter et al.: Preferential protein depolymerization cessible for microorganisms or higher plants (Mooshammer et al, 2014a; Schimel and Bennett, 2004; Wild et al, 2015)
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