Forest Investment Account (FIA) - Forest Science Program
FIA Project Y103136

    Developing a new indicator of soil functioning for use in designing variable-retention harvesting
Project lead: Sue Grayston (University of British Columbia)
Contributing Authors: Grayston, Susan J.; Churchland, Carolyn; Weatherall, Andrew; Prescott, Cindy E.; Addison, J.A.; Basiliko, Nathan; Berch, Shannon M.; Berg, N.; Daradick, Shannon; Del Bel, Kate; Durall, Daniel M.; Jones, Melanie D.; Mohn, William W. (Bill); de Montigny, Louise E.; Srivastava, Diane S.; Twieg, Brendan D.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Although this project is not one of the eligible research topic areas for 2007/08, it does address high-priority topics of the Sustainability Strategy 2006-16 under Theme 3 - Indicators, thresholds and monitoring systems; Topic 3.1 - Development of indicators and monitoring systems; Priority a: Indicators and monitoring systems for soils.
Soils contain an immense population and diversity of fauna and microorganisms and it remains an enigma how such a collection of organisms co-exist and together create the nutritional environment that determines forest composition and function. Although the importance of these organisms in nutrient cycling is well-recognized the structure of the belowground food web and the interactions between these organisms are poorly understood. As a consequence we do not know which of these species are “keystone” and essential for carrying out particular ecosystem processes and what the repercussions of a loss of any of these organisms may have on ecosystem function and resilience. This is largely because until recently we lacked methodologies to study them (22). Natural abundance stable-isotope analyses (13C/12C; 15N/14N) have recently emerged as powerful techniques with which to address some of these mysteries and are increasingly being used in studies of food web structure and trophic connections (12). Most elements of biological interest (e.g. C, H, O, N) have two or more stable isotopes, with the lightest of these present in much greater abundance (e.g. natural abundance of heavy isotopes 13C and 15N is <1%). Natural variations in the ratios of stable isotopes of 2H/1H, 13C/12C, 15N/14N, and 18O/16O can enable discovery of the origin and flow of these elements in the environment (16). The technique is based on the fact that consumer’s tissues are enriched in 15N by 3.4‰, C by 1‰ relative to its food, in addition many herbivorous food sources have different 13C and 15N content (e.g. leaves, wood, SOM) (17). Therefore, the 13C/12C and 15N/14N ratios can be used to help identify the consumers’ diet and ultimately trace the initial food source (7,10,16,20). For example, Chahartagi et al (2) used 15N/14N ratios to study the feeding guilds of 20 Collembola (springtail) taxa in three deciduous forest stands and showed that these soil fauna spanned three trophic levels (herbivores which feed on lichens and plant tissues; primary decomposers which feed on litter and adhering microbes; secondary decomposers which feed on microbes) and this likely contributes to Collembola species diversity. Schneidera et al (20) showed mites occupied four trophic levels in a beech forest. However, the technique has limitations because food sources with similar isotopic ratios cannot be distinguished e.g. different microorganisms. Analysis of biomarker molecules, such as lipids, does not suffer from the same limitations; phospholipid fatty acid (PLFA) and phospholipid ether lipid (PLEL) analysis has been used to assess microbial community structure in soil as different microbial groups possess different signature PLFA (bacteria and fungi) and PLEL (archaea) in their cell membranes (5,6). For example, branched, saturated fatty acids are characteristic of Gram positive bacteria and monoenoic and cyclopropane unsaturated fatty acids are characteristic of Gram negative bacteria (24). Recently Ruess et al. (18) analyzed neutral lipid fatty acids (NLFA) together with PLFA to determine the food sources and feeding strategies of collembola in forest soil (PLFA of food sources are incorporated into NLFA without conversion in the consumers). They were able to distinguish between collembolan herbivores, bacterivores and fungivores and predators. A combination of lipid analysis with stable-isotope probing (analysis of the 13C/12C; 15N/14N ratios in the lipids) would offer a tremendous methodological advance with which to study food-web interactions and nutrient fluxes in soil and identify the keystone organisms in terrestrial ecosystems, which has never been done before.
In addition to using natural abundance ratios of stable isotopes to study terrestrial trophic structures and food webs, labeled compounds are now available with the heavy isotope making up 99% of the tracer element (e.g.13CO2; 13C15N glutamic acid) which enables biological tracing of elements. For example, pulse and continuous labeling of plants can be undertaken in the field using 13CO2 (13). Pulse labeling allows the fate of recently fixed carbon (exudates) to be traced in the microbial community. Continuous labeling enables 13C enrichment of structural plant components and identification of microorganisms using root turnover for growth. Turnover times of microorganisms can be estimated by taking samples over time. 13C incorporation into shoots, roots, soil organic matter and microbial biomass is quantified using EA-IRMS (23). 13C signatures within microbial lipids and DNA/RNA is analyzed by GC-IRMS (23) and density centrifugation (11,14) followed by molecular fingerprinting, respectively.
As part of a current FSP proposal “Green Tree Retention as a tool to sustain soil function after harvest” (Y073049) and its successor (new app. Y081005) “Green tree retention as a tool to provide carbon substrates for the living soil”, we are quantifying changes in the diversity and functioning of soil microbial and faunal communities in response to variable-retention harvesting in the STEMS experiment. In the new Green Tree proposal we are planning to use stem-injection stable-isotope labeling techniques as a tracer to determine whether retention trees “feed” the soil microbial community with C and N and if C and N are fed directly to young seedlings through common mycorrhizal networks. STEMS is a large, multi-disciplinary field experiment that compares the ecological, biological and socio-economic effects of seven silvicultural systems including clear-cut, uncut, group selection, patch cuts, dispersed retention and aggregated retention. We are determining (i) if green-tree preservation is a suitable management option for maintaining ‘healthy’ soil (in terms of maintenance of soil organisms and their associated functions), and (ii) what size and density of green tree retention patch is required for this purpose, we are comparing aggregated retention patches of 4 different sizes and the dispersed retention treatment in our study. We have found a tremendous diversity of soil organisms at the site, mites, collembola and nematodes are the most abundant soil fauna at the site. 10,000 collembola (42 species) and 32,000 mites (92 species) have been collected, identified and stored in ethanol. However, the role of each of these species in nutrient cycling processes is presently unknown, as is the knowledge of whether any are “keystone” species, whose loss from an ecosystem as a result of management or environmental change may be a key indicator of disruption of soil health and ecosystem services.
Related projects:  FSP_Y081136FSP_Y092136


Executive Summary (26Kb)
ISME Abstract 1: A novel stable isotope labelling and probing technique successfully traces carbon belowground in 22-year old Sitka spruce (Picea sitchensis) (14Kb)
ISME abstract 2: Response of soil organisms to varying sizes of aggregated green-tree retention (13Kb)

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Updated August 19, 2010 

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