Testing Mechanisms of How Mycorrhizal Associations Affect Forest Soil Carbon and Nitrogen Cycling

Authors

Caitlin Hicks Pries¹* (caitlin.hicks.pries@dartmouth.edu), Amelia Fitch¹, Siya Shao¹, Nina Wurzburger², Richard Lankau³, Benjamin Sulman⁴, Sarah Goldsmith¹, Cassandra Allsup³

Institutions

¹Dartmouth College, Hanover, NH; ²University of Georgia, Athens, GA; ³University of Wisconsin–Madison, WI; ⁴Oak Ridge National Laboratory, Oak Ridge, TN

Abstract

Mycorrhizal fungi provide plants with nutrients in return for photosynthate, linking above- and belowground processes. Forests dominated by arbuscular (AM) versus ectomycorrhizal (EcM) fungi have well-documented differences in their distributions of soil carbon (C) and nitrogen (N). However, the mechanisms driving these patterns are uncertain.

Potential mechanisms include differences in leaf litter quality, rhizodeposition, fungal nutrient acquisition strategies, and fungal necromass quality. The observations across forests of the Eastern U.S. indicate that fungal nutrient acquisition strategies are likely driving these patterns. Researchers found that as the relative abundance of EcM fungi with the ability to produce peroxidases increased, the proportion of soil organic matter that was mineral-associated decreased. Researchers developed a mycorrhizal fungi-explicit version of the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment model called Myco-CORPSE wherein EcM fungi can mine soil organic matter for N, but AM fungi cannot. Using the model, researchers found that EcM fungi with a strong ability to mine N from the slow and necromass organic matter pools could induce saprotrophic N limitation and thus increase the amount of soil organic matter in the unprotected pool. This effect was strongest in cold ecosystems with more recalcitrant plant litter inputs. To experimentally test how litter quality versus fungal traits affect soil organic matter, researchers are currently incubating six different types of dual ¹³C and ¹⁵N-labeled litter in soil mesocosms at forests in New Hampshire, Illinois, and Georgia where each have six plots differing in the abundance and dominant family of EcM-associated trees. Lastly, to investigate the role of rhizodeposition, the team carried out a greenhouse experiment wherein eight species of seedlings (four AM and four EcM) were grown in a ¹³C-labeled atmosphere under three levels of N fertilization. Over one growing season, rhizodeposition was greater from EcM seedlings than AM seedlings and increased with increasing N availability in EcM but not in AM seedlings. Despite these differences, the net effect of the seedlings on soil carbon was similar across mycorrhizal types implying increases in rhizodeposition were balanced by increases in decomposition. This ongoing research is bringing new insights into the mechanisms driving mycorrhizal differences in soil organic matter cycling that researchers are incorporating into regional simulations of Myco-CORPSE.