Nitrogen (N) is often the limiting nutrient for plant photosynthesis and productivity in temperate forests. Variation in N uptake among different plant species is important to reduce competition and sustain biodiversity. N uptake preference refers to plants’ unequal uptake of different N forms in soils, i.e., the two dominant inorganic N forms of ammonium and nitrate. However currently we lack standard method to determine plant N uptake preference, especially for mature trees growing in the field.   

Researchers from the Yunting Fang’s group in Institute of Applied Ecology, Chinese Academy of Sciences, recently developed and improved the in situ paired ¹⁵N labeling technique to reveal mature tree N uptake preference. They injected highly enriched ¹⁵N substrates to the rooting depth of mature conifer trees in the field, and track ¹⁵N uptake and transport by different tree organs to calculate uptake rates and preference. After comparison of different sampling organs and sampling time points, it was suggested that sampling tree leaves after 4 days of ¹⁵N addition would be adequate to determine mature tree N uptake preference. These results were published in New Phytologist and Tree Physiology 

In addition to method development, the researchers also found that mature larch of different ages can uptake and transport nitrate efficiently (nitrate contribution to tree N use of 50~78%). This is different from traditional view that conifer species commonly used ammonium. This could be related with method difference. Previous studies often adopted hydroponic experiments, avoiding microbial competition and soil interception of N, which may result in overestimation of ammonium uptake.  

This method has the potential to be applied to other conifer and broad-leaf species in more forest types to reveal mature tree N uptake preference in situ.  

Fig.1 Schema of the in situ paired ¹⁵N labeling technique to study mature tree N uptake preference (Image by ZHU Feifei)