Abstract
We added the stable isotope 15N in the form of (15NH4)2SO4 and K15NO3 to forest ecosystems in eastern China under two different N deposition levels to study the fate of the different forms of deposited N. Prior to the addition of the 15N tracers, the natural 15N abundance ranging from −3.4‰ to +10.9‰ in the forest under heavy N deposition at Dinghushan (DHS), and from −3.92‰ to +7.25‰ in the forest under light N deposition at Daxinganling (DXAL). Four months after the tracer application, the total 15N recovery from the major ecosystem compartments ranged from 55.3% to 90.5%. The total 15N recoveries were similar under the (15NH4)2SO4 tracer treatment in both two forest ecosystems, whereas the total 15N recovery was significantly lower in the subtropical forest ecosystem at DHS than in the boreal forest ecosystem at DXAL under the K15NO3 tracer treatment. The 15N assimilated into the tree biomass represented only 8.8% to 33.7% of the 15N added to the forest ecosystems. In both of the tracer application treatments, more 15N was recovered from the tree biomass in the subtropical forest ecosystem at DHS than the boreal forest ecosystem at DXAL. The amount of 15N assimilated into tree biomass was greater under the K15NO3 tracer treatment than that of the (15NH4)2SO4 treatment in both forest ecosystems. This study suggests that, although less N was immobilized in the forest ecosystems under more intensive N deposition conditions, forest ecosystems in China strongly retain N deposition, even in areas under heavy N deposition intensity or in ecosystems undergoing spring freezing and thawing melts. Compared to ammonium deposition, deposited nitrate is released from the forest ecosystem more easily. However, nitrate deposition could be retained mostly in the plant N pool, which might lead to more C sequestration in these ecosystems.
Highlights
During the last few decades, atmospheric nitrogen (N) deposition has increased sharply as a result of the consumption of fossil fuels, the emission of industrial waste gases, the excessive application of fertilizers, and the rapid development of animal husbandry [1]
The ecosystem N pool of the subtropical forest ecosystem consisted of 5.76 Mg N, which was more than twice the size of the N pool found in the boreal coniferous forest (Table 2)
The N pool of the 0–20 cm soil layer accounted for 54% of the total N in the subtropical forest ecosystem, and this proportion was more than 80% in the boreal forest ecosystem
Summary
During the last few decades, atmospheric nitrogen (N) deposition has increased sharply as a result of the consumption of fossil fuels, the emission of industrial waste gases, the excessive application of fertilizers, and the rapid development of animal husbandry [1]. The anthropogenic reactive N emission has increased from 15 Tg N a21 in 1860 to 165 Tg N a21 in early 1990 s, and more that 70% of the reactive N deposited back to the terrestrial and aquatic ecosystems [2]. N deposition, as well as increasing concentration of carbon dioxide and ongoing land use/land cover change, has been the well documented issues of global change, altering the biogeochemistry of ecosystems [5]. There has been widespread concern about the effect of increasing N deposition on natural forest ecosystems [6], because of the high sensitivity of biodiversity and productivity of these ecosystems to N input [7]. As one of the most important N sources in forest ecosystem, N deposition increasing might lead to remarkable effects on the forest ecosystem N cycling. Chronic atmospheric N deposition can alter N transmission and transformation processes in the forest ecosystem, such as plant absorption [8], microbe immobilization [9], mineralization [10], nitrification [11], as well as volatilization and leaching losses [12,13]
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