Objective Nutrient is an important limiting factor for tree growth in drought and barren sites, and the way trees absorbing and using nutrients in drought and barren sites determines their ecological adaptation strategies. In this paper, the kinetics of root nitrogen uptake and the coupling relationship between root morphological traits were measured in situ in the field, which laid a foundation for revealing the physiological function of trees in drought and barren environments.

Method We took Prunus davidiana, Acer truncatum and Quercus variabilis in Baiwangshan Forest Park of Beijing as the research objectives. We used modified Hogland nutrient solution with \rmNO_3^ - concentration gradients to carry out in-situ measurement of root \rmNO_3^ - uptake kinetics in generally and extremely drought and barren site conditions, respectively. The relationship between root \rmNO_3^ - uptake rate and root functional traits was analyzed by Pearson correlation and path analysis.

Result Tree species, site conditions and the interaction of above two factors all had an significant or extremely significant effect on root \rmNO_3^ - uptake rate and kinetic parameters. Three tree species all had high nitrogen affinity. The uptake rate of \rmNO_3^ - in the root of A. truncatum was lower, and it was significantly lower than that of P. davidiana and Q. variabilis under the above two site conditions. Under the site conditions of more drought and barren, fast growing tree species had compensatory absorption of \rmNO_3^ - . Root functional traits and the uptake rate of \rmNO_3^ - had a good coupling relationship. The result showed that the morphological traits of specific root length (SRL) and specific root surface area (SRA) had significantly positive effects on \rmNO_3^ - uptake rate of roots, while root diameter (RD) and root tissue density (RTD) had negative effects. In terms of branching structure traits, branching intensity and number of links had weak effects on \rmNO_3^ - uptake rate.

Conclusion The \rmNO_3^ - uptake rate of P. davidiana and Q. variabilis with a faster growth rate decreases significantly under the extremely drought and barren site stress, while A. truncatum is the opposite. The “speed strategy” of increasing the maximum absorption rate and reducing nitrogen affinity ensures the compensatory absorption of \rmNO_3^ - by the roots of fast-growing tree species. The combination of morphological traits with higher SRL, higher SRA, lower RD and lower RTD can effectively improve the uptake rate of \rmNO_3^ - by roots in drought and barren sites.