Long-term nitrogen addition alters nutrient foraging strategies of Populus davidiana and Betula platyphylla in a temperate natural secondary forest

The continuous increase in global atmospheric nitrogen deposition may change the stoichiometric balance of temperate forest plants and affect the nutrient absorption of plants and the carbon and nitrogen cycle of forests. Leaf, fine root and other ephemeral tissues are very sensitive to nutrient changes. However, it has been not clear whether nitrogen addition will affect the nutrient uptake strategies of plant leaves and fine roots. In order to explore the effects of long-term nitrogen addition on nutrient absorption and distribution of main dominant tree species in temperate secondary forests and to cope with the sustainable development of forests under the background of increasing atmospheric nitrogen deposition in the future, we conducted a 12-year continuous nitrogen addition experiment in the natural secondary forest in Changbai Mountains of Northeast China. Responses of fine root morphology, leaf nutrient resorption, fine root exudates and stoichiometry of Populus davidiana and Betula platyphylla to nitrogen addition were studied. Our study found that (1) nitrogen addition could lead to phosphorus limitation, and high nitrogen addition treatment significantly reduced the nitrogen resorption efficiency of B. platyphylla and P. davidiana and then increased the phosphorus resorption efficiency. (2) Nitrogen addition promoted the metabolism and activity of B. platyphylla fine root, which was more inclined to transport and store, while P. davidiana fine root was more inclined to capture and absorb nutrients. (3) Nitrogen addition increased the secretion rate of fine root exudates carbon and decreased that of fine root exudates nitrogen. The release of root exudates from roots to soil may stimulate soil microbial activity and nutrient availability. These different results indicate that both P. davidiana and B. platyphylla are attempting to improve resource acquisition under nitrogen deposition change, which will effectively change the soil carbon and nitrogen cycle of temperate forests.