Evidence for preferential protein depolymerization in wetland soils in response to external nitrogen availability provided by a novel FTIR routine

Reuter, Hendrik; Gensel, Julia; Elvert, Marcus; Zak, Dominik

Phragmites australis litters were incubated in three waterlogged anoxic wetland soils of different nutrient status for 75 inline-formulad, 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 (inline-formulaα), 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.



Reuter, Hendrik / Gensel, Julia / Elvert, Marcus / et al: Evidence for preferential protein depolymerization in wetland soils in response to external nitrogen availability provided by a novel FTIR routine. 2020. Copernicus Publications.


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