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.