Compound driven differences in N2 and N2O emission from soil; the role of substrate use efficiency and the microbial community

Organic C is an important control on the process of denitrification, a process that can result in the production and reduction of the potent greenhouse gas nitrous oxide (N2O). This study identified the influence of different low molecular weight C (LMW-C) compounds on the production of nitrous oxide (N2O) and dinitrogen (N2) and the associated role of the size and structure of the microbial community. We examined this following application of glucose, glutamine or citric acid (250 mg C kg−1 dry soil) and 15N-KNO3 (100 mg N kg−1 dry soil) to a sandy loam soil and measured the production of N2 and N2O by denitrifiers using 15N labeling techniques, changes in the bacterial community as measured by T-RFLP on 16SrDNA fragments and changes in the gene copy number of 16SrDNA, nirK, nirS and nosZ over 144 h. Addition of glucose, citric acid and glutamine all increased emissions of 15N-N2 above that found in the control (P < 0.05) while the addition of glucose and glutamine resulted in higher emissions of 14+15N-N2O (P < 0.001) than the addition of citric acid, resulting in a lower 15N-N2O to 15N-N2 ratio in the citric acid treatment. The 16SrDNA gene copy number increased after addition of citric acid and glutamine, whilst 16SrDNA showed significant shifts in community composition in all C treatments although over different time periods. The gene copy number of nosZ only significantly increased at 120 h in the glutamine treatment (P < 0.05) and nirS at 120 h in the citric acid and glutamine treatments (P < 0.05). This suggests that where C is added as a single input, differences in N2 and N2O emissions between LMW-C compounds were not caused by selection for denitrifiers but likely driven by differences in substrate use efficiency and subsequent differences in C partitioning between growth and respiration. The differing influence of the three selected C compounds on denitrification indicates the potential for lowering net N2O emissions through regulation of C compound availability.