Contrasting responses of fine root biomass and traits to large-scale nitrogen and phosphorus addition in tropical forests in the Guiana shield

Fine roots mediate plant nutrient acquisition and growth. Depending on
soil nutrient availability, plants can regulate fine root biomass and
morphological traits to optimise nutrient acquisition. Little is known,
however, about the importance of these parameters influencing forest
functioning. In this study, we measured root responses to nutrient
additions to gain a mechanistic understanding of plant adaptations to
nutrient limitation in two tropical forests in French Guiana, differing
two-fold in their soil nutrient statuses. We analysed the responses of
root biomass, mean root diameter (RD), specific root length (SRL),
specific root area (SRA), root tissue density (RTD) and carbon (C),
nitrogen (N) and phosphorus (P) concentrations in roots down to 15 cm soil
depth after three years of N and P additions. At the lower-fertility site
Paracou, no changes in root biomass or morphological traits were detected
with either N or P addition, although P concentrations in roots increased
with P addition. In the higher fertility site, Nouragues, root biomass and
P concentrations in roots increased with P addition, with no changes in
morphological traits. In contrast, N addition shifted root traits from
acquisitive to more conservative by increasing RTD. A significant
interaction between N and P in Nouragues pointed to stronger responses to
P addition in the absence of N. Our results suggest that the magnitude and
direction of root biomass and trait expression were regulated by soil
fertility, corroborated by the response to N or P additions. At low
fertility sites, we found lower plasticity in root trait expression
compared to more fertile conditions, where N and P additions caused
stronger and antagonistic responses. Identifying the exact role of
mechanisms affecting root nutrient uptake in Amazon forests growing in
different soils will be crucial to foresee if and how rapid global changes
can affect their carbon allocation.