Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14850, USA.
Plant Cell Environ. 2015 Jan;38(1):89-100. doi: 10.1111/pce.12373. Epub 2014 Jun 24.
Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ(15) N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ(15) N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ(15) N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ(15) Nxylem sap and δ(15) Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ(15) N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ(15) Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ(15) N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ(15) N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone.
生物固氮（BNF）在木本植物中经常通过叶片 δ（15）N 的测量来研究，在木本植物入侵导致 BNF 增加的生态系统中，这尤其有趣。我们沿着整个氮吸收途径包括土壤溶液、木质部汁液和叶片取样，以（1）检验使用叶片 δ（15）N 作为 BNF 替代物的固有假设；（2）确定 δ（15）N 木质部汁液和 δ（15）N 土壤无机氮之间是否存在季节性差异，这些差异可用于推断 BNF 的变化；（3）评估 δ（15）N 与树木年龄的关系，作为 BNF 或氮循环变化的指标。对三个季节时间点的木质固氮 Prosopis glandulosa 和配对的非固氮参考 Zanthoxylum fagara 的氮测量表明，δ（15）N 土壤无机氮在物种之间随时间和空间变化。木质部和叶片 δ（15）N 之间的分馏在物种之间的方向始终相反，平均差异为 2.4‰。考虑到这些变异源，Prosopis 的固氮百分比值可能会变化高达约 70%。土壤-木质部 δ（15）N 分离随时间变化而增加，并随 Prosopis 年龄的增加而增加，这表明氮循环和 BNF 的季节性变化，以及通过叶片采样单独观察不到的长期 BNF 增加的潜在可能性。
