Nitrogen Addition Decreases Root Exudation of Four Temperate Tree Species Seedlings, Independent of the Applied Nitrogen Form

Root exudations play a crucial role in plant nutrient uptake; however, studies on their response to different forms of nitrogen (N) are lacking. To bridge these gaps, we examined the effect of different N forms on the root exudation rates among tree species. NO3−-N, NH4+-N, and organic N were added to the soil at identical concentrations for three months. Root exudate, fine root, and soil samples were collected from the seedlings of four temperate tree species to determine the root exudation rate, aboveground biomass, root characteristics (morphological, physiological, and nutrient traits), and soil characteristics (soil physicochemical properties and enzymatic activity). The differences in all variables among different N forms and tree species were investigated to explore the causative factors for the variations in root exudation. All three forms of N addition reduced the root exudation rate (up to 43.8