Abstract
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 (N2O) emissions from forest ecosystems, its impact on N2O flux is poorly known. In this study, 3-years monitoring of N2O 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 N2O 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 N2O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N2O flux varied across nine forest sites almost in proportional to the stream water NO3− concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N2O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation.
Highlights
Nitrous oxide (N2O), the third most influential greenhouse gas in global warming, is estimated to account for approximately 6% of the radiative forcing of the global climate (IPCC 2007)
The average annual N2O emissions from the N-saturated forest (Tama) were estimated to be 0.88 kg N ha-1 year-1; instantaneous flux rate ranged from - 0.03 to 90 lg N m-2 h-1
The N2O flux varied across nine forest sites almost in proportional to the stream water NO3- concentration in the watershed that ranged from 0.14 mg N/L to 1.64 mg N/L (Fig. 8)
Summary
Nitrous oxide (N2O), the third most influential greenhouse gas in global warming, is estimated to account for approximately 6% of the radiative forcing of the global climate (IPCC 2007). Current estimates of N2O emissions from temperate forests vary markedly. In several temperate forests in Europe receiving elevated levels of atmospheric nitrogen deposition, high annual N2O flux has been reported; e.g., 0.67–0.92 kgN2O-N ha-1 year -1 (Schmidt et al 1988), 1.15–2.29 kgN2O-N ha-1 year -1 (Pitcairn et al 2002), and 2.6 kgN2O-N ha-1 year-1 (Eickenscheidt et al 2011). The highest N2O flux was observed in an N-saturated forest site in Germany (‘Solling’), which has a low pH of 3.5 (Eickenscheidt and Brumme 2013). Due to the adoption of such variable fluxes in different forests to evaluate global emissions, current estimates of N2O emissions from temperate forests vary widely from 0.1 to 2.0 Tg N2O-N year-1 (Brumme et al 2005; IPCC 2001; Kroeze et al 1999), implying significant uncertainty. It is important to understand the key factor associated with this gap in N2O emissions from forests
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