Nitrogen deposition and decreased precipitation does not change total nitrogen uptake in a temperate forest

Decreased precipitation and increased anthropogenical by derived nitrogen (N) are important climate change factors that alter the availability of soil water and N which are crucial to root function and morphological traits. However, these factors are seldom explored in forests. To clarify how altered precipitation and N addition affect the uptake of organic and inorganic N by fine roots, a field hydroponic experiment using brief 15N exposures was conducted in a temperate forest in northern China. The root traits related to nutrient foraging (root morphology and mycorrhizal colonization) were measured simultaneously. Our results showed that all three tree species preferred ammonium (NH4+) over glycine and nitrate (NO3-), and NH4+ contributed 73% to the total N uptake from the soil. Uptake of glycine was higher than that of NO3-. Decreased precipitation, N addition, and their interaction increased NH4+ uptake rate compared with the control. Decreased precipitation decreased the glycine and NO3- uptake rate. Moreover, N addition, decreased precipitation and their interaction changed root morphological traits and significantly decreased mycorrhizal colonization. Although our treatments resulted in changes to the root traits and the forms of N uptake by plants, the total amount of N uptake did not change among all treatments. We conclude that although fine root traits of dominant tree species in temperate forests have high plasticity in response to climate change, nutrient balance in plants causes the total amount of N uptake to remain unchanged.