Submitted to: Springer Verlag
Publication Type: Book / chapter
Publication Acceptance Date: 9/5/2004
Publication Date: 5/20/2005
Citation: Halvorson, J.J., Smith, J.L., Kennedy, A.C. 2005. Lupine effects on soil quality and function during primary succession at Mount St. Helens. IN Dale, V.H., Swanson, F.J., and Crasafulli, C.M. (editors). Ecology Responses to the 1980 Eruptions of Mount St. Helens. New York: Springer Verlag, pp. 243-254. Interpretive Summary: Increasing plant productivity, soil fertility, biological diversity and ecosystem stability are desirable management goals for pasture grazing systems, also associated with recovery of natural ecosystems following disturbance. Legumes, important components of high quality forage mixtures, also mediate ecosystem development of low fertility, disturbed sites where the carbon from photosynthesis and nitrogen from symbiotic dinitrogen fixation accumulate in soil. The abundance and stoichiometry of these key elements influence soil quality, plant growth and soil microbial communities and ecosystem properties such as stability and efficiency. However, more information is needed about how legumes, such as lupines, control soil development and functioning during early primary succession in order to devise management systems that amplify natural patterns of ecosystem reorganization and development. This chapter presents and synthesizes results of research on lupine effects on early successional, soil ecosystems at Mount St. Helens. Besides information about patterns and rates of C and N accumulation, organic matter formation, and changes in other soil quality indicators, these studies link natural ecosystem development to changes in soil substrate quantity and complexity, microbial community structure and metabolic potential and suggest soil quality is an important driver of aboveground succession. The desired outcome of such studies is the development of agricultural management systems that produce sustainable, stable and harmonious living environments that enhance human and non-human well being.
Technical Abstract: Lupines, early legume colonists of Mount St. Helens pyroclastic flows, are important mediators of above and belowground succession because they are sources of C and N that impact soil genesis, establishment of other plant species and soil microbial communities. Rates of N2 fixation by lupines can vary daily and seasonally and occur in lupine seedlings soon after germination. Seedlings can occur in high population densities but have low probability of survivorship, and thus may be a significant source of C and N into pyroclastic soils. Lupines affect spatial patterns of soil variables near the soil surface, creating resource islands in pyroclastic soil that diminish with depth. The retention of C and N fixed by lupines in pyroclastic soil is correlated to increasing soil microbial biomass and activity but under limiting conditions, heterotrophic microorganisms may compete with plants for N. Now, more than 20 years since deposition, significantly higher concentrations of C and N are observed than in 1990, with evidence of increasing rates of accumulation. Developing soil under lupines, has significantly greater concentrations of C, N, enzymes, microbial-C and evidence of higher microbial activity than bare uncolonized bare soil but there is a significant, non-linear relationship between microbial-C and soil C and N. Numbers of fatty acids in soil, indicative of C quality, are also significantly but nonlinearly correlated with total soil C; most numerous and variable under lupines suggesting a more complex microbial community. Microorganisms in soils under lupines respond to a more diverse array of C substrates more quickly than uncolonized soils. Our understanding of the effects of lupines and other legumes on soil ecosystem development and functioning is extended by linking above-ground primary succession to changes in soil substrate quantity and complexity, microbial community structure and metabolic potential. Subsequent ecosystem development will likely be driven by changes in pyroclastic soil quality as indicated by increasing C and N, microbial activity and respiration efficiencies.