Sorption regulates the fate of the amino acids lysine and leucine in soil aggregates

Amino acids are substrates widely present in soils either in a free or a polymeric state and represent a large input of organic nitrogen into most terrestrial ecosystems. However, their behaviour in soil remains poorly understood. The aim of this study was to assess critically the impact of sorption on the mineralization of two contrastingly charged amino acids, lysine and leucine, in different-sized, real soil aggregates. In a first experiment, we assessed the spatial variability of amino acid mineralization in aggregates of different sizes. After 1 day of incubation coefficients of variation of leucine mineralization were 33% in aggregates of 2-3 mm, 15% in aggregates of 3-5 mm and 14% in aggregates of 5-7 mm diameter. Coefficients of variation of lysine mineralization were, after 1 day of incubation, 24% in aggregates of 2-3 mm, 26% in aggregates of 3-5 mm and 24% in aggregates of 5-7 mm diameter. Compared with other substrates, this variability is small and could be due to the presence of degrading microorganisms in all aggregates. We also studied the activity of such microorganisms by following the evolution with time of 14C-amino acids in different compartments within aggregates 3-5 mm diameter (mineralized 14C, microbial biomass 14C, 14C in soil solution and residual 14C in soil). The microorganisms that degrade amino acids were able to assimilate substrates in soil solution immediately and used them preferentially for their growth and not solely for their energy production. For lysine, intense competition between adsorption and assimilation by microorganisms occurred from the start of the incubation period and persisted with time. Our results clearly show that sorption may retard biodegradation of amino acids in soil.