ANAEROBIC DECOMPOSITION AND DENITRIFICATION DURING PLANT DECOMPOSITION IN AN ORGANIC SOIL

Nitrate concentrations in groundwater have been shown to be reduced du ring passage through riparian soils and a possible mechanism for this reduction is bacterial denitrification. For denitrification to occur t here must be sufficient available C as an energy source. We examined t he competition for organic substrate between microbial processes durin g the anaerobic decomposition of plant matter in a laboratory study. F resh and senescent pine needles (Pinus radiata D. Don) and watercress leaves (Rorippa nasturtium-aquaticum L.Hayek) were added to an organic riparian soil, incubated anaerobically for 90 d and production of CO2 and CH4 measured. At 9-d intervals NO3 and acetylene were added to a replicate and production of CO2, CH4, and N2O was followed. In the abs ence of NO3, watercress produced the most CO2 and CH4 (21% of added C) , followed by fresh pine needles (10%), and senescent pine needles (6% ). First-order rate constants calculated for gaseous C production were 0.033 d-1, 0.0088 d-1, and 0.0071 d-1 for watercress, fresh, and sene scent pine needles, respectively. As plant tissue become increasingly decomposed via fermentation, less N2O and CO2 was produced following N O3 addition, presumably because the remaining plant matter was more re sistant to further degradation. Denitrification and CO2 production in the watercress and fresh pine needle treatments were up to 5 times hig her than that measured in the senescent pine needle treatment. As the same amount of C was added to all treatments, these results suggested that the lability of added C was of greater importance than the quanti ty of C added in regulating microbial response. The response of denitr ifying bacteria to the addition of NO3 was rapid, even after 99 d of i ncubation in the absence of either NO3 or oxygen as an electron accept or. This suggested that denitrifying bacteria could survive and compet e for C in riparian soils where NO3 concentrations fluctuate.