Summary In the past decades, many studies have shown that atmospheric deposition of nitrogen (N) alters the cycling of this element in forest ecosystems. Nitrogen saturation of forest ecosystems and its consequences have been documented for temperate and boreal systems in Europe and North America. However, little work has been done with respect to the levels of atmospheric N deposition and its ecological effects in sub-tropical forest ecosystems in south China. Sub-tropical south China is strongly affected by air pollution due to SO2, NOx and NH3 and much of this area is included in China’s acid rain control zone. Considerable research has been done in this area focusing on effects of SO2, but serious gaps in our understanding of the relationships between emission, deposition and effects of nitrogen impede the development of an effective N pollution control policy. This thesis focuses on atmospheric N deposition, net N transformations and N dynamics in selected subtropical forested catchments in south China, dominated by Masson pine. The climate at the research sites is monsoonal, with hot and humid summers and cold and dry winters. An abundant ground vegetation layer, including bushes and shrubs, potentially important for interception-evaporation and scavenging of air pollutants are common at all sites. Soils are acidic and have low base saturation. Concentrations of inorganic N (NH4+ and NO3-) in bulk precipitation at the forested sites are at least an order of magnitude greater than those at remote sites in the world, but levels are comparable to those in many of the polluted cities in China. Nitrogen in deposition is dominated by NH4+, except at the LiuXiHe site in southern Guangdong, where NO3 is more prominent. Both the tree canopy and the ground vegetations layer influence throughfall fluxes of N. At the most polluted sites (TieShanPing and CaiJiaTang) tree canopy and ground vegetation layer are net sources of N in throughfall water, probably due to dry deposition. By contrast, at the less N polluted sites (LiuChongGuan, LeiGongShan and LiuXiHe) the canopies are a net sink for atmospheric N, due to net N uptake at leaf surfaces. Minimum estimates of the annual total input of inorganic N at the research sites, based on bulk precipitation and throughfall measurements, ranged from 0.8 to 4.0 g N m-2 yr-1. At the most polluted sites dry deposition may contribute up to about 50% to the total deposition. Nitrogen loads at two of the sites exceed 1.0–2.5 g m-2 yr-1, the deposition level above which NO3- leaching may occur in temperate and boreal forests. In the soil, NH4+, but not NO3-, disappears rapidly from solution. Some net removal of total inorganic N from percolation water occurs in the upper 60 cm of the soil, but this varies per site. Probably, much of the remaining inorganic N in percolation water is removed in the riparian zones or, at the TieShanPing site, in the small reservoir upstream of the V-notch weir. Nitrogen in soil solution and stream water is dominated by NO3--N. At the catchment scale, only a small fraction of the deposited N leaves the catchment with stream water. The contribution of net N transformations to the acid load is 0.16 and 0.10 molc H+ m-2 yr-1 at TieShanPing and CaiJiaTang, respectively, which is significantly greater than the net input of free H+ (0.08 and 0.01 molc H+ m-2 yr-1, respectively). In the upper 15 cm of the soils at TieShanPing and CaiJiaTang, average N pools vary from 1460 to 2040 kg N ha-1, with a contribution of about 25-55% from the O/A horizon (0 to 3 cm). C/N ratios decrease from 18 (O/A horizon) to 11 (B2 horizon). The in situ mineralization rates vary from 63 kg N ha-1 yr-1 at TieShanPing to 53 at CaiJiaTang, with the O/A horizon contributing more than 30%. Due to the lack of a continuous and well developed O horizon, the C/N ratio of this horizon cannot be used as an indicator for N status and N leaching from the system. Nitrogen leaching in the sub-tropical Chinese forest soils is best explained by the sum of the net mineralization rate and the inorganic flux of N entering the O/A horizon (the so-called soil flux density of mineral N). A flux density of about 90 kg N ha-1 yr-1 is required for N leaching to occur at 30cm soil depth. In individual mineral soil horizons the net N mineralization rate per unit organic C (mg N g-1 C yr-1) appears to be well correlated with the C/N ratio. Nitrate concentrations in stream water vary seasonally with highest concentrations in summer, probably due to climatic and hydrological conditions rather than N saturation. Thus, the elevated NO3 concentrations in summer cannot be used as an indicator for N saturation of sub-tropical forested catchments with a monsoonal climate. However, the average concentrations of inorganic N in stream water may be used to compare the relative N status between the sites. The seasonal variation of in situ net N mineralization rate is related to the seasonality of soil water potential and soil temperature, with warm and wet months in early summer having greatest rates. The spatial variation of in situ net N mineralization rates correlates significantly with substrate factors. Availability of NH4+-N in soils, rather than soil pH, is a limiting factor for the average annual nitrification rate. Principal component analysis (PCA) integrates the variables of climate and soil properties into a substrate index and a microenvironment index. The substrate index integrates the soil’s content of organic C, total N, NH4+-N and NO3--N. The microenvironment index includes soil pH, soil temperature and soil water potential. Regression analysis suggests that the substrate index is primarily responsible for the spatial variation of N mineralization. PCA is a useful tool to integrate the multi-variant measurements from the field, and may serve as a data fusion technique for the manipulation of field observations prior to upscaling.
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