Pedogenetic and microbial contributions to selective accumulation of labile proteins in soil during primary succession
Proteinaceous compounds including amino acids, peptides, and proteins contribute to large portion of soil organic matter (~30%) and play an important role in two main ecosystem functions in soil: (1) regulating the availability of nitrogen which is most limiting nutrient for plant growth and (2) sequestrating carbon. The emerging evidences of preferential accumulation and long residence time of proteinaceous compounds in soil are counter to the traditional view that their structure is readily broken down by soil microbial activities. The shift in thinking of their residence time represents an important change for understanding global biogeochemical carbon and nitrogen cycling, yet it is not known whether there are their compositional changes during soil development. The objective was to determine the compositional pattern of amino acids –structure unit of peptides and proteins- in soils across a sand dune chronosequence (~4010 years). We analyzed amino acid composition of labile proteinaceous compounds released by chemical hydrolysis in mineral soils. To determine mineral and microbial contributions, we compared the amino acid profiling on organo-mineral associations and microbial biomass and investigate the relationship of amino acids profiling with edaphic properties and microbial community compositions.
In support of the main hypothesis, relative abundance of proteinaceous amino acids changed across the chronosequence; and the changes were associated with both biotic and abiotic shifts during primary succession. Positively-charged amino acids (arginine, lysine, histidine) showed a clear pattern of accumulation (~130%) during the 4010years of development and ~50% more enriched on the organo-mineral associations. On the other hand, negatively changed amino acids (glutamic acid, aspartic acid) decreased (~15%) with time and ~30% less associated with mineral. Shifts in the amino acid profile also showed patterns that resemble the proteinogenic amino acid composition of microbial groups based on RNA-codons deposited in Genbank. For examples, actinobacteria (high CG content) which positively select alanine and glycine were a dominant phylum across the sites and their relative abundance decreased with time; this correlated with the decline of alanine (R2=0.82, p=0.0019) and glycine (R2 = 0.41, p=0.1670) from the amino acid profiling. Evidence supports the current working hypothesis: that both the source and sink of proteinaceous compounds originated from microbes and plants (source) combine with the accretion of mineral cation exchange sites during pedogensis (sink) and foster the accumulation of soil organic matter.