Nitrogen-fixing trees can supply the majority of the nitrogen needed to fuel forest growth. However, nitrogen-fixing trees can also drive significant soil emissions of nitrous oxide, a potent greenhouse gas, via the turnover and decomposition of their nitrogen-rich tissues. Soil nitrous oxide emissions due to nitrogen-fixing trees are poorly understood. A better understanding of the magnitude and controls of soil nitrous oxide emissions due to nitrogen-fixing trees is necessary to predict the offset of global forest carbon storage by soil nitrous oxide emissions.
We measured soil nitrous oxide emissions at the base of non-fixing trees (Betula nigra) and nitrogen-fixing trees (Robinia pseudoacacia) in a field experiment at Black Rock Forest, New York. Non-fixer / nitrogen-fixer pairs were assigned unfertilized or nitrogen fertilized treatments. Biomass and nitrogen fixation rate measurements across these treatments indicate that Robinia pseudoacacia incompletely down-regulates nitrogen fixation rates when growth is not limited by nitrogen. We thus examine the relationship between soil nitrous oxide emissions, nitrogen fixation rate and nitrogen limitation.
Results/Conclusions
In both unfertilized and nitrogen fertilized treatments, soil nitrous oxide emissions under non-fixing trees were higher than soil nitrous oxide emissions under nitrogen-fixing trees. These results contradict those of previous work which suggest higher soil nitrous oxide emissions under nitrogen-fixing trees than under non-fixing trees. Previous work has primarily been conducted in mature forests whereas the trees studied in this field experiment were four years old. As such, enrichment of soil nitrogen due to the turnover and decomposition of nitrogen-fixing tree litter may not have had sufficient time to occur. Low soil nitrous oxide emissions under nitrogen-fixing trees could be a result of the high nitrogen requirements of growing nitrogen-fixing trees.