Soil Nitric Oxide Emissions From Forests And Vegetable Fields In The Pearl River Delta

Nitrogen oxides (NO_x) play crucial roles in troposphereic photochemical smog, formation of secondary aerosols and acid rain. Ozone, fine particles and acid rain are the right air quality issues facing China's highly industrialized and densely populated Pearl River Delta (PRD). The understanding of NO_x sources and their fluxes is of top priority not only for the control of the regional air pollution and the improvement of the regional air quality, but also for the regional nitrogen biogeochemical cycling. However, the sources of NO_x in PRD have not been well documented except that from fossil fuel combustion.Soils are an important source for atmospheric NO_x, especially for the China's agricultural lands experiencing extensive management, high nitrogen fertilizer application and high nitrogen deposition. In the present study, soil NO emissions from forests (a broadleaf forest and a pine forest) and vegetable fields were measured and their regulating factors were investigated; the nitrogen isotope signature of soil-derived NO was also characterized as well. The main results were as the following:1. In the broadleaf forest, mean NO emission in wet season (14.9 ng N m^-2 s^-1) was lower than in dry season (23.8 ng N m^-2 s^-1). In the pine forest, however, mean NO emission in wet season (17.1 ng N m^-2 s^-1) was higher than in dry season (7.9 ng N m^-1 s^-1). Field measurements showed that soil water content was the dominant controlling factor determining the seasonal patterns of NO emissions in both forests. Annual NO emissions in the broadleaf forest and the pine forest were preliminarily estimated to be 6.1-6.9 and 4.0-4.3 kg N ha^-1 yr^-1, respectively, by using three upscaling methods. 2. In the broadleaf forest, both NO₃^- and NH₄⁺ addition enhanced NO emissions greatly, but in the pine forest, NH₄⁺ addition had much greater enhancement than NO₃^-to soil NO emissions. N addition alone caused more N loss as NO than simultaneous addition of N and P did in both forests. In the pine forest, nitrification was the prevailing process responsible for NO production, while in the broadleaf forest both nitrification and denitrification played important roles in soil NO production. NO emissions will likely increase by 12.8%-23.0% in the broadleaf forest and by 12.0%-33.1% in the pine forest after N deposition doubles in the future.3. Based on the results of the water addition experiment in the dry season, we estimated that total NO emissions from pulses in the broadleaf forest and pine forest were 29.4 mg N m^-2 and 22.2 mg N m^-2, respectively, which accounted for 4.6% and 5.3% of the annual emission, respectively.4. NO fluxes measured in a field of Flowering Chinese cabbage drastically increased after nitrogen fertilizer application, and other practices involving loosening the soil also enhanced NO emission. Mean NO emission flux was 47.5 ng N m^-1 s^-1 over a complete growth cycle. Annual NO emission from the vegetable field was about 10.1 kg N ha^-1 yr^-1. Fertilizer-induced NO emission factor was estimated to be 2.4%. Total NO emission from vegetable fields in Guangdong province was roughly estimated to be 11.7 Gg N yr^-1 based on the vegetable field area and annual NO emission rate, and to be 13.3 Gg N yr^-1 based on fertilizer-induced NO emission factor and background NO emission.5. We measured NO and NH₃ emissions from a typical vegetable-land soil in south China following the applications of urea, ammonium nitrate and ammonium bicarbonate, respectively. Over a 72-day sampling period, averaged fertilizer-induced NO emission factors from urea, ammonium bicarbonate (ABC) and ammonium nitrate treated soils amounted to 2.6%, 2.3% and 2.2%, respectively. Averaged fertilizer-induced NH3 emission factors were 10.9%, 15.2% and 3.1%, respectively, over a 39-day sampling period. Annual above-soil NO emission from vegetable lands in Guangdong province was estimated to be 12.9 Gg N yr^-1, but reduced to 10.2 Gg N yr^-1 when the canopy reduction effect was considered. 6.δ¹⁵N values of soil-derived NO after urea and ammonium bicarbonate application were about -49‰(day 2), but increased to -28‰(urea application) and -19.8‰(ammonium bicarbonate application), respectively, after 13 days of fertilization. The case was similar for NH₃.δ¹⁵N values of NH₃ volatized from urea treated soil increased from -42.6‰(day 1) to -17.1‰(day 7), and those from the ABC treated soil increased from -39.4‰(day 0) to -21.5‰(day 6). Theδ¹⁵N values of NO emitted by fertilized soil could be distinguished from those of NO emitted by fossil fuel combustion, but theδ¹⁵N values of NH₃ volatilized from fertilized soil overlapped with those from other sources.