Contents of ninhydrin-reactive N components were 0 in treatment
I and approximately 0.2 mg kg1 oven dry soil (corresponding
to 2% of the amino acids added) in treatments II, III and IV (data not
shown). That less than 2% of the added amino acids were still in
solution after 48 h suggests that at least 98% of the added amino
acids were either mineralized, taken up by microorganisms as
intact molecules or immobilized by soil colloids. The latter, however,
is small for the following reasons: the amino acids used in our
study, glycine and L-leucine, have no net charge at the pH of the soil
used, thus they are only weakly adsorbed to cation exchange sites
[20]. Rousk and Jones [21] reported for similar experimental conditions
that about 80% of the added amino acids were recovered
with a 0.5 M K2SO4 solution after shaking the samples for 1 h.
However, addition of HgCl2 as a microbial inhibitor resulted in an
extraction of 100% of the amino acids, indicating that the reduced
extraction efficiency could be attributed to microbial activity and
not to sorption by soil colloids [21]. In the following discussion, we
assume that the proportion of amino acids abiotically immobilized
is insignificant, which may result in a slight overestimation of the
direct uptake of amino acid utilization.