respectively, suggesting that extracellular deamination and
direct uptake of amino acids may have occurred simultaneously during
the later stage of the incubation. The simultaneous uptake of organic
and mineral N may be due to the coexistence of different microbial communities
(Pinggera et al., 2015). Geisseler et al. (2012) also reported the
simultaneous activity of both routes in soils. Soil is a very heterogeneous
environment containing microsites with contrasting C and N availabilities
(Garnier et al., 2003; Gibbs and Barraclough, 1998; Manzoni et al.,
2008). The conditions prevalent in the different microsites likely determined
which N utilization pathway was favorable. Our results indicated
the relative importance of the different uptake routes depends on incubation
time and substrate qualities.