Estimation of nitrate use cost by woody plant

氮沉降的增加使植物有效氮的主要形态由铵态氮（NH4+）转变为硝态氮（NO3-）。与使用NH4+相比，使用NO3-的过程较为复杂。NO3-的使用成本较高。为了估算氮素利用的成本，我们测量了不同有效氮形式下以14C标记的CO2通量。结果表明，常绿栎树和落叶栎树利用NO3-和NH4+的CO2通量占光合作用总量的比例分别为8%和4%。在各速效氮源下栽培3个月后，两种植物根系结构在不同氮源下无显著差异。这些结果表明，木本树种利用NO3的成本是利用NH4+的2倍。而成本差异对根系结构的影响不显著。

Increase of N deposition has changed the major form of available N for plant from ammonium (NH4+) to nitrate (NO3-). There are the complicate processes to use NO3- comparing with the use of NH4+. It results in higher cost for the plant to use NO3-. To estimate the cost of N use we measured CO2 flux labeled by 14C under the different forms of available N. The ratio of CO2 fluxes to total photosynthesis were 8% for NO3- use vs 4% for NH4+ use by both of evergreen Quercus glauca and deciduous Quercus serrate. After three months cultivation under each form of available N source the root structures were not significantly different between N forms in both species. These results suggested that the cost of NO3 use by woody tree species was two times larger than NH4+ use. However, the difference in cost was not significant influences on the root structure.