Mineralization of carbon from D- and L-amino acids and D-glucose in two contrasting soils

Following addition of either the D- or the L-isomers of alanine, glutamine or glutamic acid or D-glucose, the CO₂ production from an arable and a forest soil was measured until the pulses of CO₂ production associated with substrate addition subsided. The maximum rate of additional CO₂ production from the D-glucose amended soils occurred within the first 48 h for both soils. The greatest rates of additional CO₂ production from L-amino acid amended soils occurred within 108 h for the forest soil and 60 h for the arable soil. Following addition of D-amino acids to the forest soil, the maximum rate of additional CO₂ production was less than that following addition of the corresponding L-amino acid addition. However, for this soil the pulse of additional CO₂ production following D-amino acid amendment lasted longer and by the time it had subsided (360 h), the total additional CO₂ production did not differ between isomeric forms of the same amino acid. Following D-amino acid addition to the arable soil there were delays of between about 24 and 48 h before the onset of rapid additional CO₂ production and the CO₂ pulse subsided relatively rapidly. The total additional CO₂ produced from the arable soil was significantly less for the D-amino acid than for the corresponding L-amino acid treatments. Successive additions of D-glucose led to significant increases in the subsequent rates of additional CO₂ production from the forest soil, but not from the arable soil. Each successive L-amino acid amendment led to increases in the rate of additional CO₂ production from both soils, as did successive additions of the D-amino acids to the forest soil. However, successive additions of the D-amino acids to the arable soil did not lead to consistent responses in the additional rate of CO₂ production.