Atmospheric Deposition, Mineralization and Leaching of Nitrogen in Subtropical Forested Catchments, South China

Abstract

In recent years, China has conducted considerable research focusing on the emission and effects of sulphur (S) on human health and ecosystems. By contrast, there has been little emphasis on anthropogenic nitrogen (N) so far, even though studies conducted abroad indicate that long-range atmospheric transport of N and ecological effects (e.g. acidification of soil and water) may be significant. The Sino-Norwegian project IMPACTS, launched in 1999, has established monitoring sites at five forest ecosystems in the southern part of PR China to collect comprehensive data on air quality, acidification status and ecological effects. Here we present initial results about N dynamics at two of the IMPACTS sites located near Chongqing and Changsha, including estimation of atmospheric deposition fluxes of NO_x and NH_x and soil N transformations. Nitrogen deposition is high at both sites when compared with values from Europe and North America (25–38 kg ha⁻¹ yr⁻¹). About 70% of the deposited N comes as NH₄, probably derived from agriculture. Leaching of N from soils is high and nearly all as NO₃ ⁻¹. Transformation of N to NO₃ ⁻¹ in soils results in acidification rates that are high compared to rates found elsewhere. Despite considerable leaching of NO₃ ⁻¹ from the root zone of the soils, little NO₃ ⁻¹ appears in streamwater. This indicates that N retention or denitrification, both causing acid neutralization, may be important and probably occur in the groundwater and groundwater discharge zones. The soil flux density of mineral N, which is the sum of N deposition and N mineralization, and which is dominated by the N mineralization flux, may be a good indicator for leaching of NO₃ ⁻¹ in soils. However, this indicator seems site specific probably due to differences in land-use history and current N requirement.