Nitrous oxide emissions in proportion to nitrification in moist temperate forests

Chronic elevated nitrogen deposition has increased nitrogen availability in many forest ecosystems globally, and this phenomenon has been suggested to increase soil nitrification. Although it is believed that increased nitrogen availability would also increase nitrous oxide (N 2 O) emissions from forest ecosystems, its impact on N 2 O flux is poorly known. In this study, 3-years monitoring of N 2 O emissions was performed in a forested watershed receiving elevated nitrogen deposition and located in the suburbs of Tokyo, Japan. In addition, a comparative field survey was carried out in nine temperate forest sites with varying nitrogen availabilities. In the intensively studied forest site showing typical nitrogen saturation, the average annual N 2 O emissions from the whole watershed were estimated to be 0.88 kg N ha −1 year −1 , comparable to the highest observed levels for temperate forests except for some very high emission sites in Europe. Although no correlation was found for humid spots with WFPS > 60%, a clear positive correlation was noted between N 2 O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N 2 O flux varied across nine forest sites almost in proportional to the stream water NO 3 − concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N 2 O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation.