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

Researchers from FANG Yunting's group at the Institute of Applied Ecology of the Chinese Academy of Sciences recently developed and improved the in situ paired ¹⁵N labeling technique to reveal mature tree nitrogen uptake preference.

The researchers injected highly enriched ¹⁵N substrates to the rooting depth of mature conifer trees in the field, and tracked 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 nitrogen uptake preference. 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 nitrogen 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 nitrogen, 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 nitrogen uptake preference in situ.  

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