Indicators for nitrogen status and leaching in subtropical forest ecosystems, South China

The deposition of nitrogen (N) is high in subtropical forest in South China and it is expected to increase further in the coming decades. To assess effects of increasing deposition on N cycling, we investigated the current N status of two selected 40-45-year-old masson pine-dominated Chinese subtropical forest stands at Tieshanping (TSP, near Chongqing City) and Caijiatang (CJT in Shaoshan, Hunan province), and explored the applicability of several indicators for N status and leaching, suggested for temperate and boreal forest ecosystems. Current atmospheric N deposition to the systems is from 25 to 49 kg ha-1 year-1. The concentration of total N in the upper 15 cm of the soil is from as low as 0.05% in the B2 horizon to as high as 0.53% in the O/A horizon. The concentration of organic carbon (C) varies from 0.74 (B2) to 9.54% (O/A). Pools of N in the upper 15 cm of the soils range from 1460 to 2290 kg N ha-1, where 25-55% of the N pool is in the O/A horizon (upper 3 cm of the soil). Due to a lack of a well-developed continuous O horizon (forest floor), the C/N ratio of this layer cannot be used as an indicator for the N status, as is commonly done in temperate and boreal forests. The net N mineralization rate (mg N g-1 C year-1) in individual horizons correlates significantly with the C/N ratio, which is from as high as 18.2 in the O/A horizon to as low as 11.2 in the B2 horizon. The N2O emission flux from soil is significantly correlated with the KCl extractable NH+4-N in the O/A horizon and with the net nitrification in the upper 15 cm of the soil. However, the spatial and temporal variation of the N2O emission rate is high and rates are small and often difficult to detect in the field. The soil flux density of mineral N, defined as the sum of the throughfall N input rate and the rate of in situ net N mineralization in the upper 15 cm of the soil, i.e., the combination of deposition input and the N status of the system, explains the NO-3 leaching potential at 30 cm soil depth best. The seasonality of stream water N concentration at TSP and CJT is climatic and hydrologically controlled, with highest values commonly occurring in the wet growing season and lowest in the dry dormant season. This is different from temperate forest ecosystems, where N saturation is indicated by elevated NO-3 leaching in stream water during summer.