Abstract
Abstract. Natural abundance of 15N (δ15N) in plants and soils can provide time-integrated information related to nitrogen (N) cycling within ecosystems, but it has not been well tested in warm and humid subtropical forests. In this study, we used ecosystem δ15N to assess effects of increased N deposition on N cycling in an old-growth broad-leaved forest and a secondary pine forest in a high-N-deposition area in southern China. We measured δ15N of inorganic N in input and output fluxes under ambient N deposition, and we measured N concentration (%N) and δ15N of major ecosystem compartments under ambient deposition and after decadal N addition at 50 kg N ha−1yr−1, which has a δ15N of −0.7 ‰. Our results showed that the total inorganic N in deposition was 15N-depleted (−10 ‰) mainly due to high input of strongly 15N-depleted NH4+-N. Plant leaves in both forests were also 15N-depleted (−4 to −6 ‰). The broad-leaved forest had higher plant and soil %N and was more 15N-enriched in most ecosystem compartments relative to the pine forest. Nitrogen addition did not significantly affect %N in the broad-leaved forest, indicating that the ecosystem pools are already N-rich. However, %N was marginally increased in pine leaves and significantly increased in understory vegetation in the pine forest. Soil δ15N was not changed significantly by the N addition in either forest. However, the N addition significantly increased the δ15N of plants toward the 15N signature of the added N, indicating incorporation of added N into plants. Thus, plant δ15N was more sensitive to ecosystem N input manipulation than %N in these subtropical forests. We interpret the depleted δ15N of plants as an imprint from the high and 15N-depleted N deposition that may dominate the effects of fractionation that are observed in most warm and humid forests. Fractionation during the steps of N cycling could explain the difference between negative δ15N in plants and positive δ15N in soils, and the increase in soil δ15N with depths. Nevertheless, interpretation of ecosystem δ15N from high-N-deposition regions needs to include data on the deposition 15N signal.
Highlights
Nitrogen (N) deposition onto terrestrial ecosystems has dramatically increased due to anthropogenic activities (Galloway, 2005) and since the 1980s the increase has been strong in China, including in the warm and humid regions (Liu et al, 2011)
We evaluated δ15N values of subtropical forests, and their responses to increased N deposition using long-term Naddition experimental plots established in 2003 in an oldgrowth broad-leaved forest and a pine plantation forest in the Dinghushan Biosphere Reserve in southern China (Mo et al, 2006)
The study was conducted in the Dinghushan Biosphere Reserve (DHSBR) in Guangdong Province, southern China (112◦33 E, 23◦10 N), with typical subtropical monsoon climate
Summary
Nitrogen (N) deposition onto terrestrial ecosystems has dramatically increased due to anthropogenic activities (Galloway, 2005) and since the 1980s the increase has been strong in China, including in the warm and humid regions (Liu et al, 2011). Nitrogen deposition that exceeds plant and microbial demand may increase nutrient leaching and soil acidification (Lu et al, 2014), which potentially causes nutritional imbalances in vegetation (Schulze, 1989). Studies of fates and process responses to increased N de-. Gurmesa et al.: Nitrogen input 15N signatures are reflected in plant 15N natural abundances position using coordinated N-addition experiments in temperate and boreal forests show that the effects of increased
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