COS 176-1 - Historical patterns and current influences on soil carbon and nitrogen dynamics on Mount St. Helens

Friday, August 11, 2017: 8:00 AM
B112, Oregon Convention Center
Rebecca Evans, School of Biological Sciences Washington State University, Pullman, WA, R. Dave Evans, School of Biological Sciences, Washington State University, Pullman, WA, John Bishop, Biology, Washington State University, Vancouver, Vancouver, WA and Jonathan J. Halvorson, USDA-ARS, Mandan, ND

Primary successional landscapes offer an opportunity to investigate the abiotic and biotic drivers behind the fundamental mechanisms and interconnectedness of communities. Succession is often characterized by the change in plant species composition but may be more dependent on the below ground nutrient cycles and soil microbial communities (SMC). Few studies have investigated the identity and role of SMC in the development of soil and the feedbacks between successional forces, including insect herbivory and plant inputs. Here we address C and N accumulation through time on Mount St. Helens (MSH) and the interactions between plants; specifically the N2-fixing Lupine lepidus and insect herbivores on soil development and SMC. We investigated the C and N sources and transformations using an archived chronosequence of soils, from post eruption in 1980 to 2010 via stable isotope analysis. We also chronicled the soil SMC diversity content using phospholipid fatty acid analysis on a 2007 debris flow on MSH, which re-initiated primary succession. We measured SMC and soil C and N content and stable isotope ratios under three treatments: with insect herbivores, without insect herbivores, and without L. lepidus.


Soil C increased from 1981 (0.31 mgC/g soil) to 1988 (7.6 mgC/g soil) in soils originating from pyroclastic flows. Soil N also increased from 1981(0.002mgN/g soil) to 1988 (0.023mgN/g soil). Results from experiments indicate that soils have a mean C:N ratio of 12.20, even where L. lepidus is present. Under L. lepidus mean soil C and N were greater compared to plant interspaces or plots where L. lepidus was removed. At the soil surface under L. lepidus the mean C content was 2.25 mgC/g soil and 1.90 mgC/g soil at the rhizosphere compared to 0.19 mg C/g soil in interspaces and 0.63 mgC/g soil where L. lepidus was removed. Mean soil N content in interspaces was 0.02 mgN/g soil compared to 0.17 and 0.14 mg N/g soil at the soil surface under L. lepidus and in the rhizosphere, respectively. Soil microbial diversity is generally low across all treatments but favors fungal operational taxonomic units. Results from the archived chronsequence and experiments demonstrate that soil N remains low and SMC development is slow under the harsh conditions at MSH.