我国西南地区氮沉降量不同的森林小流域中土壤自然^(15)N丰度的分布特征

Nitrogen-15 natural abundance in the soils of forested small catchments

稳定性同位素^(15)N自然丰度法近年来在生态系统氮素循环研究中发挥了和正在发挥着极为重要的作用。在森林生态系统中,自然^(15)N丰度值是氮循环转换的综合结果,被有的研究者用作生态系统氮饱和状态的指示指标。在本研究中,我们测定了我国西南三个典型森林小流域的植物和土壤的自然^(15)N丰度值,并进行了比较。植物的自然^(15)N丰度值的变化范围为-5.0‰～2.7‰,土壤自然^(15)N丰度值的变化范围为从有机层的-6.9‰增加到矿质土壤的6.9‰,这些值的变化范围和温带森林生态系统的自然^(15)N值的变化范围没有明显的区别。在土壤表层,鹿冲关和铁山坪的自然^(15)N丰度值是明显低于雷公山流域的。在缺氮的鹿冲关流域,土壤表层低的^(15)N丰度值表明了该流域比较封闭的氮循环状态,而在氮相对饱和的铁山坪流域表层土壤并没有出现^(15)N富化,这可能是由于大量^(15)N贫化的氮沉降输入的结果。这些结果表明,土壤的^(15)N丰度值在我国西南部并不一定能单独作为森林生态系统氮饱和程度的指示指标,但土壤和植被的δ^(15)N值对于理解当地森林生态系统的氮循环具有一定的意义。

Recent studies have shown that natural abundance of stable nitrogen (N) isotopes can be used for interpreting N cycles and N saturation in forest ecosystems. This paper examined the stable isotopic composition of soils in selected subtropical forested catchments. The delta(15)N values ranged from -6.9 parts per thousand to 6.9 parts per thousand for vegetation and soil. The soil profiles showed continuous decrease in total N (TN) concentration from 0.64% to < 0.10% with soil depth. In the upper soil horizons, soil delta(15)N values in LCG and TSP were significantly lower than those in LGS. The lower delta(15)N values the N-limited site (LCG) indicated tightly closed N cycles. The (15)N-depletion the N-enriched site (TSP) was due probably to high inputs of (15)N-depleted atmospheric N. Our results indicated that natural abundance (15)N in soil alone may be not a good indicator of N saturation in subtropical forest ecosystems in southern China.