Root and mycorrhizal strategies for nutrient acquisition in forests under nitrogen deposition: A meta-analysis

Global increase in nitrogen (N) deposition influences the belowground allocation of plant photosynthates and the formation of roots and rhizosphere-associated symbionts as well as soil nutrient availability, thereby affecting the nutrient acquisition by trees. Trees obtain nutrients primarily through fine root growth or mycorrhizal symbioses. These two mechanisms have an antagonistic relationship, but how they are modified by N deposition remains unknown. Arbuscular mycorrhizae (AM) and ectomycorrhizae (EM) are the two dominant types of mycorrhizal fungi that form symbioses with the roots of most trees. However, the divergent adaptive mechanisms and nutrient acquisition strategies of trees with a symbiotic relationship with AM (AM trees) or with EM (EM trees) in response to N deposition are unclear. To clarify these points, we conducted a meta-analysis of 116 studies on global forest ecosystems. Following prolonged (>2 years) or high-load (>140 kg ha−1 y−1) N addition, both AM and EM trees decrease the amount of energy allocated to mycorrhizal symbioses and increasingly rely on fine roots to obtain nutrients. The faster growth of AM trees than of EM trees under N deposition is partly attributed to their divergent nutrient acquisition strategies: the fine root biomass and length decreased significantly in EM trees (−13% and −17%, respectively), but not in AM trees. Furthermore, the acid phosphatase activity increased more for the AM trees than for the EM trees (28% and 4%, respectively), indicative of a greater abundance of available P for the AM trees than for the EM trees after N addition. Consequently, AM trees have advantages in terms of root morphology and phosphatase activity over EM trees, suggesting they are better adapted to high N deposition.