Respiration costs associated with nitrate reduction as estimated 14CO2 pulse labeling and nitrate redactase activity of evergreen oak

Background/Question/Methods

Increasing nitrogen (N) deposition changes the biogeochemical cycling in forest ecosystems where plant growth is limited by N availability. N cycling in soil may also change by an increase in N deposition, i.e., available N form for plants in soil will be dominated by nitrate (NO₃^-) with increasing N deposition, because the availability of ammonium N (NH₄⁺) increases and nitrification is enhanced. Recent studies have shown the difference of metabolism and energy budget of plant use between NO₃^- and NH₄⁺. Especially root respiration called the researchers’ attention, because NO₃^- reduction generally occurs in roots under the dark condition. To evaluate the influences of the increase in deposited N on net productivity, it is important to quantify the cost of N use, namely, NO₃^- reduction. However, in contrast to herbaceous species, energy budget of NO₃^- use has never been quantified for woody species. We carried out experiments of pulse labeling of ¹⁴CO₂ on an evergreen oak species, Quercus glauca, to evaluate the effect of N form on root respiration. We also measured nitrate reductase activity (NRA) of leaves and roots to estimate the potential NO₃^- use by plant.

Results/Conclusions

Pulse labeling of ¹⁴CO₂ showed that the rates of root respiration were significantly higher for plants under NO₃^- fertilization than those under NH₄⁺ fertilization in Quercus glauca. There was no significant difference of the rates of root respiration between the light and dark conditions. Leaf NRA was significantly higher than root NRA irrespective of light condition. Root NRA tended to be higher in the dark condition than in the light condition, while leaf NRA tended to be higher in the light than in the dark condition. This discrepancy in the effects of light condition on NO₃^- use between root respiration and root NRA suggests that there is a higher potential of NO₃^- reduction in roots than estimated from root respiration by measuring ¹⁴CO₂. Overall, these results strongly suggest that, as similar to herbaceous species, woody plant species also change the metabolism and energy budget of plant N use between NO₃^- and NH₄⁺. This study further suggests that woody species need higher expense for NO₃^- reduction in roots than herbaceous species, though further studies including the different tree species are needed to conclude.