Plasticity in nitrogen form uptake and preference in response to long-term nitrogen addition of the main tree species in a temperate old growth forest.

We conducted a short-term field hydroponic ¹⁵N labeling experiment to study the effects of different nitrogen (N) addition treatment on the N uptake preference and rate of plant fine roots in a temperate old growth forest in 2018. The results indicated that the N preference of dominant tree species for different N chemical forms was not influenced by the N addition, with the N acquisition pattern being ammonium > nitrate > glycine. However, N addition altered the plant's N uptake rates. Under low N treatment, the uptake rates of glycine for three target tree species significantly increased. Both low and medium N additions significantly enhanced the NH₄–N uptake rate, but had little impact on the NO₃–N uptake rate. Conversely, high N addition exerted negative effects on NO₃–N and NH₄–N uptake rate. Different tree species may flexibly adapt to new environmental conditions by adapting to their root morphology and the relationship between plant growth and N uptake. N addition has pronounced effects on N uptake rate and root morphology traits of dominant tree species. Overall, this study contributes to a deeper understanding of the response of N uptake patterns in temperate forests to long-term N addition. [ABSTRACT FROM AUTHOR]