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Anaerobic Decomposition and Denitrification during Plant Decomposition in an Organic Soil

106 Newell Hall, Dep. of Soil and Water Science, Univ. of Florida, Gainesville, FL 32608 (currently LandCare Research New Zealand Ltd., Private Bag 3127, Hamilton, New Zealand);

C. G. Harfoot

Biological Sciences, Univ. of Waikato, Hamilton, New Zealand;

P. N. McFarlane

New Zealand Forest Research Inst., Private Bag 3020, Rotorua, New Zealand;

A. B. Cooper

Ecosystems Division, NIWAR, P.O. Box 11115, Hamilton, New Zealand.

^* Corresponding author (schipperl{at}landcare.cri.nz).

ABSTRACT

Nitrate concentrations in groundwater have been shown to be reduced during passage through riparian soils and a possible mechanism for this reduction is bacterial denitrifieation. For denitrification to occur there must be sufficient available C as an energy source. We examined the competition for organic substrate between microbial processes during the anaerobic decomposition of plant matter in a laboratory study. Fresh 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 absence of NO₃, watercress produced the most CO₂ and CH₄ (21% of added C), followed by fresh pine needes (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 senescent pine needles, respectively. As plant tissue became increasingly decomposed via fermentation, less N₂O and CO₂ was produced following NO₃ addition, presumably because the remaining plant matter was more resistant to further degradation. Denitrification and CO₂ production in the watercress and fresh pine needle treatments were up to 5 times higher 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 quantity of C added in regulating microbial response. The response of denitrifying bacteria to the addition of NO₃ was rapid, even after 99 d of incubation in the absence of either NO₃ or oxygen as an electron acceptor. This suggested that denitrifying bacteria could survive and compete for C in riparian soils where NO₃ concentrations fluctuate.

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