Comparison of D- and L-amino acid metabolism in soils with differing microbial biomass and activity

The rates of CO₂ production from soils over 6 h in response to addition of either the L- or the D-isomers of alanine, glutamine and glutamic acid have been determined for soils from 13 experimental grassland plots. These plots were established in the 1896-97 winter and have received regular additions of farm yard manure (FYM) either with or without inorganic N, P and K applied, or factorial combinations of inorganic N (as (NH₄)₂SO₄), inorganic P (as basic slag and triple superphosphate) and K (as KCl), or no nutrient addition (control). As a result of these treatments, the soils have contrasting chemical and biological properties. Biomass C, determined from glucose-induced respiration, was well correlated with the L-amino acid- and D-alanine-induced respiration rates. The rates of D-amine acid-induced respiration were consistently lower than those of the corresponding L-isomers, except for alanine in the more acidic soils which had small microbial biomass contents. The D-amino acid-induced respiration accounted for smaller fractions of the variation in microbial biomass than that induced by the corresponding L-isomers. Relative to the control soil, L-amino acid-induced respiration rates were increased by FYM treatment, decreased slightly by the NPK treatment and decreased to a larger extent by the acidifying (NH₄)₂SO₄ treatments. In contrast, the rates of D-glutamine- and D-glutamic acid-induced respiration were reduced by addition of FYM, NPK and (NH₄)₂SO₄. The ratio of L-amino acid-induced respiration to D-amino acid-induced respiration (L-to-D ratio) ranged between 6.3 and 1.4 for glutamine and 8.7 and 2.6 for glutamic acid. The L-to-D ratios for glutamine and glutamic acid increased with increasing soil microbial biomass and declined abruptly, but not linearly, with increasing rate of CO₂ evolution per unit of biomass (qCO₂). The declines in L-to-D ratios may have been related to increased physiological stress in the soil microbial community, since they occurred along a gradient of increasing soil acidity.