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

• Main Authors: H. Reuter, J. Gensel, M. Elvert, D. Zak
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-01-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/499/2020/bg-17-499-2020.pdf

<p><i>Phragmites australis</i> litters were incubated in three waterlogged anoxic wetland soils of different nutrient status for 75&thinsp;<span class="inline-formula">d</span>, 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&thinsp;% to 42&thinsp;% 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 (<span class="inline-formula"><i>α</i></span>), 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 <i>Sphagnum</i> peat, the most nutrient-poor substrate in this experiment. Preferential protein depolymerization from litter decomposing in <i>Sphagnum</i> 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 <i>Sphagnum</i> peatlands.</p>