Nitrogen and carbon mineralization in soil amended with d- and l-leucine

The rates of gross C mineralization over short-term (0–6 h) and longer-term (0–6 d) incubation of soil amended with the l- and d-enantiomers of leucine have been determined from the CO2 production. The rates of N mineralization over 6 d incubations of similarly amended soils were determined by 15N isotope pool dilution for gross N mineralization and both α-amino N disappearance and NH4+ accumulation for net N mineralization. The rates of net and gross N mineralization and of C mineralization from d-leucine were all significantly less than those from l-leucine and we present the first data for mineralization of N from a d-amino acid in soil. The hypothesis that the onset of rapid utilization of d-leucine by microorganisms was delayed whilst deamination occurred to yield an achiral keto-acid was not supported because the pulse in gross N mineralization following d-leucine addition to the soil was coincident with that of C mineralization and occurred later than the pulse of gross N mineralization in soil amended with l-leucine. By the time all the added amino acid had disappeared from the extractable pool, 26% of the N added as l-leucine had been taken up and possibly assimilated by the soil microorganisms compared with 47% for d-leucine. Based on these estimates of N assimilation, the C:N ratio of the microorganisms assimilating l-leucine could have been nearly twice that of the microorganisms assimilating d-leucine.