| 摘 要： |
The 15N:14N ratio (815N) is often used as an integrated indicator of nitrogen (N) status in natural terrestrial ecosystems. However, the mechanisms by which long-term N addition affects N cycling and status in temperate plantations with different ages remain poorly understood. Here, we used an approach of leaf-soil continuum of 815N with the combination of incubation experiments to evaluate the effects of three rates of decadal N addition (urea) on N status in young, intermediate, and mature larch (Larix principis-rupprechtii) plantations in northern China. We found that leaf 815N decreased significantly from 1.6%o in young trees to -0.5%o and -1.0%o in intermediate and mature trees, respectively, due to increases in the use of NO- 3-N relative to NH+ 4-N with stand age, and probably a shift in N acquisition from mineral soil in the young plantation to organic and/or surface mineral soil layers in the intermediate and mature plantations. Decadal N addition decreased leaf 815N, which was likely to be due to the direct imprints of 15N -depleted urea input, the inhibition of soil net mineralization, and nitrification, and greater dependence on N absorption by ectomycorrhizal fungi after N addition. Soil 815N decreased with stand age, which was likely to be due to reduced soil N transformation (e.g., mineralization) and/ or relative abundance of ectomycorrhizal fungi along the chronosequence; however, soil 815N was not affected by long-term N inputs. Combined with the decreased enrichment factor (epsilon = 815Nleaf - 815Nsoil), these results suggest a shift from more open N cycling (potentially greater ecosystem N losses) in young plantation to more closed N cycling (facilitation of ecosystem N retention) in mature plantation, which showed negative response to decadal N addition. These findings provide new insights into N acquisition strategies and cycling in the typically Nlimited temperate plantations under heightened atmospheric N deposition. |
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