Multiple nutrients constrain fine root functioning in a lowland tropical
rainforest: initial responses from a large-scale nutrient manipulation
experiment in Central Amazonia
Abstract
Phosphorus (P) is hypothesised to be the main nutrient limiting forest
productivity in tropical forests, but more recent evidence suggests that
multiple nutrients could regulate forest functioning. Root functional
trait expression represents a trade-off between maximising the
acquisition of limiting resources and minimising root tissue
construction and maintenance. Therefore, if the limiting soil nutrient
supply is increased, plant investment in root biomass and nutrient
uptake strategies should decrease. To test this hypothesis we
investigated how fine root traits associated with nutrient acquisition
responded to large-scale nutrient additions of nitrogen, phosphorus and
cations in a slow-growing mature tropical forest established on low
fertility soils in the Central Amazon. To evaluate short-term responses
to nutrient addition 6 months after fertilisation commenced, we sampled
young fine roots (<2mm diameter), measuring root biomass and
productivity, root morphological traits (root diameter, specific root
length, specific root area and root tissue density) and root phosphatase
enzyme activity. We hypothesised that if tropical forests are P limited,
responses to P addition would be strongest, resulting in i) a decrease
in root production; ii) a shift in root morphology from acquisitive to
more conservative traits by increased root diameter and decreased
specific length and area and iii) decrease in the investment in
phosphatase enzyme. As expected, root phosphatase activity decreased by
~13% with P addition. Among the root morphological
traits, root diameter increased, mainly for the 0-10 cm soil layer, with
the addition of cations and P, but there were no significant effects on
other root morphological traits. Contrary to expectations, root
productivity was >50% higher in plots where cations were
added, with no effects of P addition. Although we found support for the
hypothesis that P limits some aspects of plant functioning in this
Central Amazon forest, the results also suggest that cations could play
an important role in controlling the expression of root traits. We
conclude that multiple nutrients may limit belowground process in
Central Amazon forests and that even slow-growing tropical forest can
respond very rapidly to changes in soil nutrient availability.