Bigger may be better in soil N cycling: Does rapid acquisition of small l-peptides by soil microbes dominate fluxes of protein-derived N in soil?

In natural plant–soil systems and following incorporation into plants in agricultural systems, most nitrogen enters soil as protein or other biological polymers. Before this nitrogen can be taken up by soil microorganisms or plants, it must be cleaved to a form which can be transported into the organism. It is well known that soil microorganisms and many plants can utilise l-amino acids and a growing body of evidence shows that short l-peptides can be taken up without further modification. However, there has been very little empirical investigation of rates of uptake in soil. d-amino acids and their peptides are less abundant in soil than their l-enantiomeric counterparts, but do occur in significant quantities. To date their potential rate of cycling in the soil solution remains virtually unknown. We directly measured rates of uptake of 14C-labelled l- and d-alanine and their di- and tripeptides in a temperate agricultural soil. All were taken up extremely rapidly by soil microorganisms. Half-times for l-enantiomers and the d-amino acid monomer were all less than 1 min, with l-peptides accounting for the highest potential fluxes (9 nmol N g−1 DW soil min−1), more than double that of the amino acid monomer. Uptake of d-peptides was slowest with half-times in soil solution of 10 and 36 min (di- and tripeptide, respectively). Availability of l-peptides had a much stronger effect on the rate of uptake of the amino acid monomer than the amino acid monomer had on the uptake of the peptides, suggesting a microbial preference for peptides. We suggest that very high potential N fluxes and an apparent microbial preference for l-peptides indicates that most N is cycled through the soil solution as l-peptides early in protein degradation, and that this is the site of fiercest competition for soil N.