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Developing a new indicator of soil functioning for use in designing variable-retention harvesting
Grayston, Susan J.
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 ecosyste ...
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