Microbial degradation and resynthesis of proteins during incubation of beech leaf litter in the presence of mineral phases

Organic N constitutes more than 90% of total N in surface soil horizons. Am ino acids, peptides and proteins represent the most abundant N species. The re are indications that clay minerals influence the degradation of proteins , but little is known about the effect of oxides and hydroxides on protein decay. We therefore conducted an incubation experiment with mixtures of bee ch leaf litter and Fe oxide, Al hydroxide, Mn oxide or quartz sand. The pro tein amounts (quantified as alpha-NH2-N) during the 498-day experiment were recorded. During the first 90-239 days, plant-derived proteins were decomp osed, resulting in a decline of protein amounts to about 60% of the initial value. Later in the experiment, the protein amounts increased again to bet ween 70% and 90% of the initial amount, because microbial resynthesis of pr oteins outweighed decomposition. The change from dominating decomposition t o prevailing microbial resynthesis occurred when the microorganisms had to adapt to less favourable conditions and therefore built new, protein-rich b iomass. Although the mineral phases did not influence protein decomposition initially, Fe oxide and Al hydroxide stabilized plant-derived proteins. Al hydroxide reduced protein resynthesis in the second phase of the experimen t, probably due to a reduction of microbial activity. Mn oxide increased pr otein decomposition and lowered microbial resynthesis due to its oxidative properties. The mineral phases therefore resulted in a shift of the relativ e intensities of protein decomposition and microbial resynthesis.