Graduate Student Researcher University of Oregon Eugene, Oregon, United States
Soil aggregation is an essential process contributing to below-ground carbon sequestration in which carbon in soil organic matter becomes physically protected from decomposition and stored within stable aggregates. Mycorrhizal fungi play an important role in the formation and stabilization of aggregates through several processes including hyphal branching and production of extracellular enzymes, organic acids, and other binding particles. It is unclear how arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi differ in their contributions to aggregate formation. The goal of this study is to identify the effects of these two types of plant-fungal symbioses on the relative stabilities and carbon storage capabilities of soil aggregates. We analyzed soil organic carbon (SOC) and stability of aggregates from the rhizospheres of four tree species (two AM and two EM tree species) planted three years ago within an experimental reforestation site in Oregon’s Willamette Valley. In addition to the carbon content of bulk soil samples, we analyzed SOC within each of the three aggregate size fractions after separation by wet sieving. These fractions included macro ( > 2 mm), meso (2 > 0.25 mm), and micro (0.25 > 0.053 mm) aggregate sizes. Our preliminary results revealed similar values for the total SOC on a mass basis within EM and AM rhizosphere bulk soils. However, aggregates from AM rhizospheres had higher stabilities after undergoing wet sieving, with the mean weight diameter of AM aggregates 10% higher than those of EM rhizospheres. Furthermore, each size fraction of AM aggregates had higher SOC than EM aggregates. AM macro, meso, and micro aggregates contained 13%, 22%, and 15% higher SOC than each corresponding EM fraction, respectively. These preliminary results suggest that, while the total bulk SOC of rhizospheres is similar between the two types of plant-fungal symbioses, AM fungi may be contributing to more long-term protection and sequestration of carbon within stable aggregates. These influences on below-ground carbon storage indicate that reforestation efforts should consider AM and EM plant-fungal symbioses when assessing SOC values and carbon residence times. Future work in this study will consider the potential impacts on carbon storage in stable aggregates when AM and EM plant communities are in combination, as these results could further serve to guide future plant community decisions in reforestation.
