Root functional traits reveal complexity in soil nutrient limitation in a lowland tropical forest

Background/Question/Methods

The potential for soil nutrients to limit the productivity of lowland tropical forests is poorly understood, despite the significance of this biome in regulating the global climate system. Fine roots and their symbionts are the primary gatekeepers of plant nutrient acquisition. The morphology of fine roots and the abundance of mycorrhizal symbionts depict strategies that plants employ to acquire the nutrients that limit their growth. These root functional traits can be highly responsive to changes in nutrient availability and can offer insight into which nutrients drive belowground allocation in terrestrial ecosystems. To understand how nitrogen, phosphorus and potassium regulate the expression of various plant strategies for nutrient acquisition in a lowland tropical forest, we quantified a suite of root functional traits after 13 years of maintaining a fully-factorial nitrogen, phosphorus and potassium fertilization experiment. We sampled five soil cores from each of 32 experimental plots and quantified the biomass, length, tissue density and arbuscular mycorrhizal abundance of fine roots.

Results/Conclusions

Nitrogen, phosphorus and potassium, either alone (potassium), or in combination (nitrogen and phosphorus), reduced the standing biomass of fine roots (< 2 mm in diameter) in this tropical forest. Additions of all three nutrients combined resulted in a ~50 % reduction in standing biomass relative to control plots. This strong adjustment in the partitioning of resources below ground indicates that all three elements limit plant growth. Furthermore, each nutrient altered the expression of root function traits revealing adjustments in plant strategies for nutrient acquisition. Nitrogen alone (< 1 mm roots), or in combination with P (1-2 mm roots), reduced root length. Root tissue density declined with additions of potassium (1-2 mm) and additions of nitrogen and phosphorus combined (<1 and 1-2 mm). The abundance of arbuscular mycorrhizal fungi responded negatively to additions of nitrogen and positively to additions of phosphorus. Our results indicate that multiple soil nutrients regulate the partitioning of resources in this lowland tropical forest.  In contrast to the traditional view of phosphorus as the sole limiting element in tropical forest ecosystems, our results offer support for the idea that these systems are regulated by complex interactions among multiple soil nutrients.