Effect of substrate quality on the N uptake routes of soil microorganisms in different soil depths

• The N uptake route of soil microorganisms was studied using corn residues with different C/N ratios and inorganic N in different soil depths.
• The direct uptake route (direct uptake of simple organic molecules into the cell) was the dominant N uptake route.
• The direct uptake route was more relevant in the surface soil than in the subsoil.
• The direct uptake of organic N was especially relevant for residues with a high C to N ratio.

A few studies have indicated that substrate quality is an important factor affecting the N uptake route of soil microorganisms, but less is known about the effect of soil depth on the N uptake route under different nutrient conditions. Objectives were to investigate (i) the effects of corn residues with different C to N ratios in the presence and absence of mineral N and (ii) the effects of soil depth on the N uptake route of soil microorganisms. An incubation experiment with surface soils (0–5 cm, C/N = 10) and subsoils (30–40 cm, C/N = 9) from three German loess sites was carried out for 21 days at 20 °C and 60% of their water-holding capacity. The following treatments were used: no addition (control), addition of corn residues with a C/N ratio of 20, 40, and 40 + (NH4)2SO4. To distinguish between the N uptake routes, the mineralization rate of amino acids was determined using 15N-labeled amino acids. In the control surface soil and subsoil the direct uptake of organic N was favored with no significant (p < 0.05) differences between depths despite significantly higher microbial activity, protease activity, gross N mineralization rate and availability of inorganic N in the surface soil, suggesting that N availability relative to C was similar at both depths. Substrate additions resulted in significantly increased protease activities at both depths after 3–7 days. Addition of corn residue with a high C/N ratio resulted in an increased direct uptake (97% and 94% in the surface soil and subsoil, respectively), compared with addition of corn residue with a low C/N ratio or addition of corn residue and inorganic N (79 to 91% direct uptake). This suggests that the enzyme system involved in the direct uptake was slightly repressed under conditions of sufficient mineralizable N (C/N of 20) or increased concentrations of NH4+. Substrate additions resulted in an initial significantly higher increase in the direct uptake in the surface soil than in the subsoil.