Effect of substrate quality on the N uptake routes of soil microorganisms in an incubation experiment

Microorganisms may either directly take up organic N or use the mobilization-immobilization-turnover (MIT) route to satisfy their N demand. However, there is only little information available about the factors controlling these two different uptake routes. The objective was to investigate the effects of substrate quality of corn with different C to N ratios in the presence and absence of mineral N on the N uptake route of soil microorganisms. Soil from the Static Fertilization Experiment Bad Lauchstädt was incubated at 20 °C and 60% of its water-holding capacity. The following treatments were used: no addition (control (C to N ratio = 13), treatment I), addition of corn residues with a C to N ratio of 40 (II), 20 (III), and 40 & (NH4)2SO4 (IV). We studied the utilization of added amino acids in order to distinguish between the N uptake routes. The mineralization rate of amino acids increased in the order “addition of corn residues with a wide C to N ratio” (4% of added amino acids) < “corn residues with a small C to N ratio” (27%) < “corn residues with a wide C to N ratio & (NH4)2SO4” and “control treatment” (31% and 32%, respectively) after 21 days of incubation. This means that, under the assumption that all the amino acids were utilized and that no remineralization of consumed amino acids occurred, 96% (treatment II), 73% (III), 69 (IV), and 68% (I) of the added amino acids were taken up directly. These results indicate that the direct route is especially important for substrates with wide C to N ratios in the absence of mineral N (thus for conditions of high C availability). The proportion of added amino acids mineralized increased with time in all treatments, suggesting that the MIT route became more important over time. Overall, the results show that the composition of the residue and the presence or absence of mineral N had a marked effect on the N uptake route of soil microorganisms.