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Sticky roots - implications of altered rhizodeposition (driven by cryptic, viral infection of plants) for the fate of rhizosphere mineral organic matter associations in natural ecosystems

Active Dates 8/15/2020-8/14/2024
Program Area Environmental Systems Science
Project Description
Sticky roots — implications of altered rhizodeposition (driven by cryptic, viral infection of plants) for the fate of rhizosphere mineral–organic matter associations in natural ecosystems

Zoe Cardon, Marine Biological Laboratory (Principal Investigator)
Marco Keiluweit, University of Massachusetts (Co-Investigator)
Carolyn Malmstrom, Michigan State University (Co-Investigator)
William J. Riley, Lawrence Berkeley National Laboratory (Co-Investigator)

Mineral-associated organic matter (MAOM) is a dominant component of total soil carbon, and once bound to reactive soil minerals, organic matter (OM) can be protected from enzymatic attack for millennia. We are striving to understand mechanisms by which plant roots can dislodge OM from soil minerals, making it available to enzymatic attack and/or biological uptake. Model compounds common in rhizodeposits can destabilize MAOM in laboratory assays, spurring its mineralization by microbes and soil carbon loss, and, potentially, pulling associated nutrients into actively cycling pools. The importance of MAOM in belowground biogeochemistry has therefore spurred ongoing development in DOE’s ELM model (the land model within the new Energy Exascale Earth System Model) . If rhizodeposition has the power to mobilize MAOM, the vast reservoir of OM bound to minerals must be viewed as accessible, at a cost, to plants and the soil microbial community in an abiotic–biotic commodities exchange.

A major challenge in this project is that our target belowground driver–rhizodeposition–is dynamic and largely invisible in soils of natural ecosystems. A novel twist of the planned work therefore lies in the aboveground treatment through which we will perturb belowground function: controlled viral infection. Overall phloem flow is increased by virus infection, potentially spurring increased delivery of phloem contents to growing root tips and the surrounding rhizosphere, making them “sticky.” Virus infection also often decreases root to shoot ratio, meaning that shoot demands for nutrients must be met by more intensive mining of soil per unit root.

We plan an intensive greenhouse experiment explicitly to test whether the presence of plant roots drives mobilization and mineralization of MAOM, and whether the extent of mobilization and mineralization shifts as a function of differential rhizodeposition. We also will delve more deeply into mechanisms underlying how plant roots interact with and mobilize MAOM, at the scale of single roots and whole root systems, using functionally diverse grasses (C3 and C4, annual and perennial, fine-rooted and coarse-rooted, virus-infected and uninfected). Process-level understanding derived from these data will inform development and sensitivity testing of an improved representation of dynamic OM-mineral associations in ELM, incorporating vulnerability to rhizodeposition, potentially with notable implications for long-term soil carbon storage and nutrient availability. 

We will test four hypotheses:
H1. Rhizodeposition leads to mobilization and mineralization of MAOM.
H2. Altered rhizodeposition per unit root, driven by viral infection, increases mobilization of MAOM (per unit root biomass) .
H3. Decreased plant root biomass and root system spatial reach (e.g. as driven by viral infection) reduces whole-soil-column mobilization and mineralization of MAOM. 
H4. In ELM simulations, root-driven mobilization will transiently reduce existing MAOM pools, but ultimately will increase overall soil carbon stocks.

Our linked experimental and modeling efforts are responsive to this FOA in that we are examining a major uncertainty associated with controls over biogeochemical cycling of one of the largest stable terrestrial C pools in temperate and tropical soils – mineral-associated OM. And if viral infection does consistently lead to “sticky roots”, our perception of the potential importance of prevalent virus infection for the terrestrial carbon cycle will be transformed.
Award Recipient(s)
  • Marine Biological Laboratory (PI: Cardon, Zoe)