In this treatment, the addition of corn residue resulted in a
strong net N immobilization due to the lack of N relative to C
(threshold C/N z 25 to 30 for N mineralization [23,24]) (Fig. 2a, b).
The depletion of the mineral soil N pool may result in the derepression
of enzyme systems used for the acquisition of alternative
N sources, such as amino acids, and therefore the direct route
may be favored over the MIT route [2]. The NHþ
4 content in the soil
solution increased marginally with time (Fig. 2b). The NHþ
4 may
have either been produced in the MIT route or may have been
excreted by microorganisms after direct uptake of amino acids. The
latter, however, is unlikely since the corn residue added in treatment
II had a wide C to N ratio. Thus, both parameters e mineralization
of amino acids added and the NHþ
4 data in this treatment
with a substrate with a wide C to N ratio e point to only a small
contribution of the MIT route. The simultaneous uptake of added
organic N (99e96% direct uptake in treatment II) and mineral N
(1e4% in treatment II) may be due to the coexistence of different
microbial communities and soil heterogeneity [4]. The overall
dominance of the direct route (being equal to or greater than 96%)
in treatment II is greater than in other studies where a contribution
of the direct uptake ranging from 55 to 70% was estimated [1,11,12]
and very close to the finding of Barraclough [8], who reported for
addition of leucine and glycine to soil that the MIT route was not
operative.