Rhizodeposition and the Fate of Mineral-Associated Soil Carbon

Authors

Zoe Cardon¹* (zcardon@mbl.edu), Marco Keiluweit², Carolyn Malmstrom³, William Riley⁴, Mariela Garcia Arredondo⁵, Sherlynette Pérez Castro¹, Zelalem Mekonnen⁴, Kota Nakasato³, Suzanne Thomas¹, Jacob Gantz³, Malak Tfaily⁶, Matthew Winnick⁵, Glyn Mardis⁵, Charlotte Koch⁵

Institutions

¹Marine Biological Laboratory, Woods Hole, MA; ²University of Lausanne, Lausanne, Switzerland; ³Michigan State University, East Lansing, MI; ⁴Lawrence Berkeley National Laboratory, Berkeley, CA; ⁵University of Massachusetts, Amherst, MA; ⁶University of Arizona, Tucson, AZ

Abstract

Mineral-associated organic matter (MAOM) is a dominant component of total soil carbon. Once bound to reactive soil minerals, that organic matter can be protected for millennia. Individual compounds commonly released by roots can mobilize MAOM off minerals, making the OM vulnerable to microbial attack. Carbon dioxide (CO₂) can then be released, and nutrients can be moved back into rapidly cycling pools. Because rhizodeposits from living roots are complex mixes of compounds, and their quality and quantity are known to vary with environmental conditions, this study conducted two greenhouse and growth chamber experiments exploring whether live root systems mobilize MAOM off minerals as a function of (1) altered morphology and physiology triggered by Barley Yellow Dwarf Virus (BYDV) and (2) low soil nutrient and water availability. Also being examined are the interactive effects of mycorrhizal infection and BYDV infection on plant physiology, soil solution DOM, and association of OM with ferrihydrite.

BYDV attacks many grasses, often reducing root:shoot and making roots “sticky” with organic compounds. Researchers added labeled MAOM (¹³C-glucose bound to ferrihydrite) to soil prior to planting Avena sativa (oats) either infected or uninfected with BYDV. Among other results, respiration from soil under infected plants had a higher percent ¹³C-MAOM-derived CO₂ (p = 0.001), but a similar total amount of MAOM-derived CO₂ was respired from whole pots planted with infected and uninfected plants.

To investigate nutrient and water limitations, researchers tested the individual and interactive effects of nitrogen (N), phosphorus (P), and water limitations on the fate of ¹³C-labeled MAOM added to soil then planted with oats. Overall, total MAOM mineralization was strongly correlated with root biomass, and drought-reduced MAOM mineralization. P limitation intensified MAOM mineralization most during initial plant growth stages; N limitations spurred enhanced MAOM mineralization during later growth stages.

To examine mycorrhizal infection and BYDV infection, researchers grew oats in sand with mycorrhizal symbiont inoculum (Rhizofagus irregularis, formerly Glomus intraradices) or with R. irregularis and BYDV infection. Analyses are ongoing. To date, results indicate shoot biomass predicts the number of mycorrhizal vesicles per cm of root, and the extent of extra-radical hyphae is best predicted by root branching frequency and crown biomass. Analysis is underway of mycorrhizosphere contributions of OM that can become associated with ferrihydrite in soil.

Using the ecosys modeling program, researchers are exploring the system-scale implications of altered kinetics of binding of OM to minerals and altered capacity for OM binding in 16 grasslands and 7 forests.