Multiple nutrients constrain fine root functioning in a lowland tropical rainforest: initial responses from a large-scale nutrient manipulation experiment in Central Amazonia

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.