Forest succession drives systematic change of root-mycorrhizal foraging strategies

Plant nutrient foraging depends on roots and mycorrhizal fungi, which are affected by plant carbon (C) investment and soil nutrient availability. The C supply for root metabolism and associated fungi might be diminished as the host plant size increases, while the quality and quantity of soil nitrogen (N) change with forest succession. There is still no holistic understanding of how the organization of belowground mycorrhizal root structure and fungi in the nutrient acquisition continuum shifts with forest age and soil resources, which restrains our understanding of the functional relations among roots, fungi, and soil. Here we examined the shifts in the absorptive root and mycorrhizal strategies, and changes in soil-associated fungal community compositions along a temperate larch forest chronosequence nested with a long-term N fertilization gradient. We found that the effect of forest age outweighed soil N addition in our forest. As tree age increased, root respiration and specific root length decreased, but protective investments such as tissue density and phenolics decreased. Meanwhile, the proportion of ectomycorrhizal fungi with a short-distance exploration type increased, but those with a long-distance exploration type decreased. The shifts in root and mycorrhizal fungal traits demonstrate a nutrient acquisition continuum from "young explorative roots with long mycorrhizas" to "mature conservative roots with short mycorrhizas". A trade-off between the root architecture and root segment metabolism, and a complementarity between the size of the root system and mycorrhizal exploration types functionally constrains this nutrient acquisition continuum. Our results thus suggested forest succession drives the covariations among root system size, root metabolic rate, mycorrhizal fungal exploration type, and soil-associated fungal functional groups.