Plastic responses of fine root morphology and architecture traits to nitrogen addition in ectomycorrhizal and arbuscular mycorrhizal tree species in an evergreen broadleaved forest.

We measured the morphology traits (specific root length, specific root surface area, root tissue density, average root diameter) and architecture traits (root fork, root fork ratio, increase rate of root length, root tip density, root fork density) of fine roots in two mycorrhiza tree species, Castanopsis faberi (ectomycorrhizal) and Schima superba (arbuscular mycorrhizal), in an evergreen broadleaved forest in the middle subtropical zone. Root bags method was used in an in situ nitrogen deposition experiment. The aim of this study was to reveal the differences in the plastic responses of fine root morphology and architecture traits to nitrogen deposition between the different mycorrhizal trees. The plastic responses of specific root length, specific root surface area and root fork to nitrogen addition decreased from the first-order root to the fourth-order root, while root tissue density showed an opposite pattern. Such a result indicated a trade-off between nutrient acquisition and resource maintenance of different fine root orders. Different mycorrhizal tree species adopted diffe-rent adaptation strategies to the variations of soil nitrogen availability. C. faberi adopted an opportuni-stic strategy, which relied on fine root to improve nutrient absorption efficiency, enhanced the capacity of space expansion and in-situ nutrient absorption to focus on rapid nutrient absorption strategy. S. superba did not change fine root morphological traits through the trade-off between nutrient absorption efficiency and root construction cost, but relied more on the complementarity between mycorrhizal fungi and fine root architecture traits for nutrient acquisition. The differences in the cost of maintaining and constructing fine root C between different mycorrhizal trees led to fine root adopting the most suitable nutrient capture strategy.

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Nitrogen deposition will affect the morphology of fine roots and its absorption of nutrien-ts, resulting in changes nutrient cycling in terrestrial ecosystems. In order to understand the effects of nitrogen deposition on fine root morphological traits of Castanopsis fabri and C. carlesii, two ectomycorrhizas tree species, we carried out in situ experiment using the root bags method in an evergreen broadleaved forest in the subtropical zone. The results showed that the plastic responses of specific root length and specific root surface area of low-order roots (first to third order) to nitrogen addition was higher than that of high-order roots (fourth order). The plastic responses of root tissue density to nitrogen addition increased from the first-order to the fourth-order, while the average root diameter of each order had no significant plastic responses to nitrogen addition. There was a certain synergistic change between the plastic response of specific root length and specific surface area in the low order fine root and the plastic response of tissue density in the high order fine root. The specific root length, specific root surface area and root tissue density of the two species showed opposite plastic responses to nitrogen addition, indicating that different ectomycorrhizal tree species had different nutrient foraging strategies. C. fabri adopted rapid absorption strategy by increasing specific root length, specific root surface area, and proliferation rate of fine root length, while C. carlesii adopted a relatively conservative resource absorption strategy by increasing tissue density of fine roots.