Soil nitric oxide emissions from two subtropical humid forests in south China

Due to the dense population, rapid industrialization, and intensified agricultural activities, some regions in Asia are hot spots of airborne nitrogen oxides and also areas with increasing nitrogen deposition. Therefore the cycling of nitrogen gases in Asia might be of increasing importance on both a regional and a global scale for atmospheric chemistry and budgets of nitrogen. Yet, to date, knowledge of soil NO emission is quite limited in Asia, particularly in forest ecosystems. In this study, soil NO emissions in two subtropical humid forests, a broadleaf forest in climax successional stage and a pine forest in primary successional stage, were measured throughout the year 2005 in Dinghushan Biosphere Reserve, south China. 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(-2) s(-1)). In both forests, soil water content was the dominant factor controlling the seasonal patterns of NO emissions, and soil NO emission was significantly correlated to percent water filled pore space (% WFPS) in a quadratic manner (p < 0.001). 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.