CARBON AND NITROGEN ACCUMULATION AND MICROBIAL ACTIVITY IN MOUNT ST. HELENS PYROCLASTIC SUBSTRATES AFTER 25 YEARS

Authors: Halvorson, Jonathan; Smith, Jeffrey

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2008
Publication Date: 2/20/2009
Citation: Halvorson, J.J., Smith, J.L. 2009. CARBON AND NITROGEN ACCUMULATION AND MICROBIAL ACTIVITY IN MOUNT ST. HELENS PYROCLASTIC SUBSTRATES AFTER 25 YEARS. Plant and Soil Journal. 315(1)(2):211.228.

Interpretive Summary: Legumes, like lupines, increase soil organic matter and microbial activity and influence other complex processes but more basic information needed to understand their role on soil development in disturbed infertile sites. We measured various soil properties in Mount St. Helens volcanic substrates to determine lupine effects, 25 years after the eruption, and to test the hypothesis that organic matter was accumulating at an increasing rate. While lupines continue to be associated with higher soil fertility our data also indicate no recent changes in concentration of soil organic matter in volcanic sites. This work is important because it implies that inputs of carbon and nitrogen from lupines are in balance with losses. Understanding mechanisms of organic matter accumulation in sites will benefit researchers and land managers seeking to improve soil conditions or sequester carbon in soils.

Technical Abstract: Lupines are important ecosystem engineers, linking above and belowground recovery of Mount St. Helens pyroclastic substrates by increasing soil organic matter and microbial activity and by influencing other biotic processes. Various soil properties were measured in samples collected from locations without lupines or other plants, under living, or non-living L. lepidus to add to the body of basic information needed to understand soil development during primary succession and to corroborate evidence for increasing rates of organic matter accumulation suggested by earlier work. In 2005, substrates generally had low cation exchange capacity but contained an appreciable amount of Mehlich III-P. Higher concentrations of soluble and total C and N more microbial biomass-C, Bradford reactive soil protein (BRSP), higher metabolic quotients, more dehydrogenase and phosphatase activity, greater rates of respiration, evidence for higher N mineralization rates, and higher concentrations of readily mineralizable C were found under lupines than in bare soil. Conversely, lupine soil was less dense and had lower carbon to nitrogen ratios and respiration rate constants. Lupines did not affect pH, inorganic nitrogen, or stable isotope composition. Soil microbial biomass-C, determined by substrate-induced respiration (SIR-C) was unchanged under lupines since 1990 and indistinguishable from hot water soluble-C suggesting soil microorganisms represent a predominant pool of labile-C in soil. However, due to increased soil C and N between 1990 and 2005, the ratio of SIR-C to soil organic matter decreased. Detection of meaningful quantities of BRSP suggests glomalin is present in pyroclastic substrates where it may represent an important pool of recalcitrant organic matter forming relatively early during soil development. Concentrations of total carbon and nitrogen under lupines have risen to nearly 4 and 0.4 g kg-1 respectively since the 1980 eruption. However, unlike earlier trends our most recent data indicate no increase in concentration of soil C and N in pyroclastic sites since 2000. This lack of change implies that inputs of C and N from lupines and other sources balance losses from biotic and abiotic processes.