Phase 3: Belowground Drivers of Aboveground Nutrient Cycling and Productivity in Growing Forests


National Science Foundation Awards: DEB 1119223, 1120015, and 1119169


This is a collaboration among:

  • Iowa State University, Ames, IA

    Department of Natural Resource Ecology and Management
  • University of Texas, Austin, TX

    Section of Integrative Biology
  • Organization for Tropical Studies, Durham, NC
  • USDA-ARS National Laboratory for Agriculture and the Environment


To gain a better understanding of belowground drivers in order to develop the capacity to predict and model the coupling of carbon and nutrient cycling during forest growth, and their feedbacks to the atmosphere.

Main Questions:

 1. How does C allocation to different belowground pathways and processes differ among forest types that vary in belowground C allocation (BCA) under conditions of similar soils and climate?
 2. What are the consequences of these differences for nutrient cycling?


 We address these questions from the following conceptual framework (Fig. 1). Forests allocate a substantial proportion of their photosynthate belowground, referred to here as BCA. Once C is transported from leaves to roots, it has at least three alternative anabolic pathways: a) allocation to fine root (FR) growth and, consequently, to FR detrital production, which also supports free-living microbes; b) direct transfer to arbuscular mycorrhizal fungi (AMF), which have a symbiotic relationship with fine roots; and c) root exudates that support rhizosphere heterotrophs. The quantity and partitioning of the anabolic components among these pathways influence plant nutrient uptake via at least four distinct mechanisms (Fig. 1): 1) mineralization of soil organic matter (SOM) in bulk soil; 2) exploration of soil and scavenging of nutrients; 3) rhizosphere priming; and 4) support of asymbiotic N-fixing microbes. Aboveground net primary productivity (ANPP) also contributes to detrital production (not depicted in Fig. 1), as does FR production; however, rhizosphere priming and AMF relationships are unique to fine roots. It is important to note that there are many other C fluxes also NOT shown in Fig. 1; in particular, all pf these anabolic components of BCA are accompanied by catabolic or respiratory fluxes.


1. To quantify total BCA and the proportional patterns of allocation to fine roots and AMF and to assay exudate production in forest types that differ in their dominant species and in C-cycling and N-cycling characteristics.
2. To test the consequences of increased BCA for aboveground nutrient cycling and productivity.


  • Determine total BCA in four of the tree species where BCA is calculated as:

BCA = Rsoil – Litterfall + net root biomass increment + DSOM, where Rsoil (soil respiration) and root growth are measured in situ.  

  • Determine Arbuscular Mycorrhizal Fungi (AMF) biomass, colonization in fine roots, and community composition.  
  • Evaluate microbial processes, such as enzyme production and decay rates, and functional genes.
  • Quantify soil carbon and nutrient dynamics.
  • Quantify carbon and nutrient uptake by the vegetation.