Simulated nitrogen inputs influence methane and nitrous oxide fluxes from a young larch plantation in northern Japan

Future increases in nitrogen (N) deposition have the potential to change underground nutrient dynamics in forest ecosystems and alter the soil–atmosphere exchange of greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O). To clarify changes in soil CH4 and N2O fluxes following N application, N input was initiated at the onset of our experiment and included two treatments (0 and 50 kg N m−2 yr−1) with four replicates in a young larch plantation in northern Japan. The N was added as an ammonium nitrate (NH4NO3) solution and distributed four times per year during two growing seasons. Soil GHG fluxes and soil variables were determined on ten occasions during a period that spanned the two growing seasons. Before the N application, no significant differences were observed in any of the soil variables between the two treatments. Ammonium–N (NH4–N) and nitrate–N (NO3–N) concentrations increased immediately after N applications in the 50 kg-N addition plot. With an increasing pool of inorganic-N, we found that soil CH4 uptake in the 50 kg-N addition plot decreased by 48% when compared to the Zero-N control plot. In particular, reduced CH4 uptake or CH4 emissions were observed in 50 kg-N addition soils immediately after N input events. Moreover, N inputs increased soil N2O emissions by an average of 69% in the 50 kg-N addition plot compared to the control during two growing seasons. At this experimental site, a negative spatial relationship was observed between soil CH4 uptake and the NH4–N concentration, and a positive spatial relationship was seen between soil N2O emissions and the NO3–N concentration, despite high inherent spatial variability in the soil CH4 flux. Overall, these results suggest that N deposition-induced increases in the availability of inorganic-N in the soil may affect processes linked to CH4 and N2O dynamics in cool-temperate forest soils, at least over the short term.