Hobbie E A, Macko S A, Williams M
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA, , , , , , US.
Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02536, USA, , , , , , US.
Oecologia. 2000 Feb;122(2):273-283. doi: 10.1007/PL00008856.
Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δN signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δN of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δN varied widely, between 0 and -2‰ for nitrogen-fixing species, between 0 and -7‰ for deciduous non-fixing species, and between 0 and -11‰ for coniferous species. Relatively constant δN values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δN differences among species at individual sites. In fact, the largest variation among plant δN values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δN values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δN signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T ). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δN in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δN and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T .
氮同位素测量可以为了解植物、菌根真菌和土壤过程在环境梯度上不断变化的相互作用提供见解。在此，我们报告了由于物种组成和氮（N）动态变化导致的δN特征变化。通过测量阿拉斯加冰川湾六个代表不同冰后期年龄的地点的细根生物量、净氮矿化以及叶片、细根、土壤和矿质氮的氮浓度和δN来评估这些变化。叶片δN变化很大，固氮物种在0至 -2‰之间，落叶非固氮物种在0至 -7‰之间，针叶树种在0至 -11‰之间。铵的δN值相对恒定，且土壤硝酸盐含量普遍较低，这表明铵或硝酸盐利用的差异对各个地点不同物种间植物δN差异的影响并不重要。事实上，在最年轻和最古老的地点观察到植物δN值的最大变化幅度，而这些地点的土壤硝酸盐浓度较低。这些地点较低的矿质氮浓度和较低的氮矿化表明氮的有效性较低。解释植物叶片中低δN值最合理的机制是氮从菌根真菌转移到植物过程中存在较大的同位素分馏。除固氮植物外，在氮有效性低的地点，单个物种的叶片δN特征通常较低，这表明要么从菌根吸收获得的氮比例（f）增加，要么转移到植被中的菌根氮比例（T）降低。这些地点的叶片和细根氮浓度也较低。杨树（Populus）、柳树（Salix）、云杉（Picea）和异叶铁杉（Tsuga heterophylla）的叶片中，叶片氮浓度与δN显著相关，细根中也是如此。δN与氮浓度之间的相关性可能反映了氮有效性、碳向菌根真菌的相对分配以及f或T的变化之间强大的潜在关系。
