Estimation of nitrate use cost by woody plant

氮素沉降量的增加使植物速效氮的主要形态由铵(NH4+)变为硝酸盐(NO3-)。与使用NH4+相比，使用NO3-的过程比较复杂。这会导致工厂使用NO3-的成本更高。为了估算氮素利用的成本，我们测量了不同有效氮形态下14C标记的CO2通量。常绿栎类和落叶锯齿栎类的CO2通量占总光合作用的比例，常绿栎类和落叶栎类均为8%，NH4+类为4%。在每种有效氮源下培养3个月后，两种植物在不同氮源形态下的根系结构差异不显着。这些结果表明，木本植物使用NO3的成本是使用NH4+的两倍。但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.