Moisture effects on microbial communities in boreal forest floors are stand-dependent
Highlights
► Microbial response to moisture manipulation was related to forest type. ► Moisture manipulation altered aspen microbial communities but not spruce. ► Response to moisture was linked to forest floor physical properties.
Introduction
The organic horizons, or forest floors, of forest soils are often much thinner than the underlying mineral soil horizons. Nevertheless, forest floors are at the forefront of a multitude of biogeochemical cycles and interactions between the biotic and abiotic components of forest ecosystems. Landscape scale factors including forest stand composition (Grayston and Prescott, 2005) along with human (Bååth et al., 1995) and natural disturbances such as fire (D’Ascoli et al., 2005) all play a role in determining the structure and function of the microbial community in the forest floors. Likewise, the community structure of forest floor microbes is the primary driver behind biogenic greenhouse gas release (Schimel and Gulledge, 1998) and nitrogen mineralization (Fraterrigo et al., 2006).
Larger scale factors such as stand composition can have quantifiable effects on forest floor chemical and physical properties (Vesterdal and Raulund-Rasmussen, 1998). However, describing large scale factors alone does not sufficiently capture the micro-environmental gradients present in forest floors. For example, physical attributes such as bulk density dictate the quantity and availability of moisture within the forest floor. Microbial communities exhibit adaptive resilience to wet-dry cycles when such conditions are present in situ (Frier et al., 2003, Griffiths et al., 2003, Lundquist et al., 1999). Conversely, moisture fluctuations can alter soil microbial community structure (Drenovsky et al., 2004), particularly for microbial communities from stable moisture environments (Frier et al., 2003).
The boreal landscape of Northern Alberta is predominantly composed of white spruce (Picea glauca (Moench) Voss) and trembling aspen (Populus tremuloides Michx.) stands. Research in the area has consistently shown that aspen and spruce forest floors have structurally and functionally distinct microbial communities (Hannam et al., 2006, Swallow et al., 2009). Aspen and spruce communities are able to retain their distinctiveness over time even when reciprocally transplanted into the other forest floor environment (Hannam et al., 2007). Past work in this region (Hannam et al., 2006, Hannam et al., 2007) has attributed microbial community differences to chemical properties such as pH. However, other factors influencing community composition have been less studied; in particular, aspen and spruce forest floors provide different physical habitats for microorganisms as aspen forest floors can have higher bulk density but be thinner than spruce forest floors (Redding et al., 2005).
In this study we investigated how the forest floor microbial communities under either aspen or spruce responded when exposed to different physical microenvironments generated by a gradient of soil moisture. Microbial community structural diversity was characterized using phospholipid fatty acid (PLFA) analysis which has successfully been used to fingerprint soil microorganisms across a broad range of ecosystems and is often times more sensitive at detecting community changes than other physiological and molecular methods (Ramsey et al., 2006). We described the functional diversity of the microbial communities using the MicroResp™ system developed by Campbell et al. (2003). This system utilizes the whole soil approach of the original multi-SIR developed by Degens and Harris (1997), but increases the speed and efficiency at which samples can be processed and has been shown to be more sensitive to changes of community function than other physiological based methods (Campbell et al., 2003).
Section snippets
Sample collection and experimental design
Material for the laboratory incubations was collected during the summer of 2008 from representative aspen and white spruce sites located within the boreal mixedwood plains of northwestern Alberta near Fort McMurray, Alberta. The mature aspen site had an overstory of trembling aspen interspersed with the occasional white spruce in the understory, while the mature spruce site was dominated almost entirely by white spruce. Collection of the forest floor material occurred along a 10 m transect at 1 m
Moisture retention
The amount of moisture retained at a specific retention pressure was related to the origin of the litter material. On average, the gravimetric water content for the spruce material was 123% for field capacity (FC), 118% for 60%FC and 111% for the wilting point (WP). Overall, aspen material held less gravimetric moisture at FC (110%), 60%FC (94%) and WP (74%) than spruce. Variation between sample moisture content was low with the coefficient of variation for samples within each moisture
Drivers of microbial community diversity
The chemical environment of the litter generated by the dominant trees and understory plants has often been offered as the primary explanation for the influence that plant communities have on the structure of forest floor microbial communities (Priha et al., 2001). In this study, the origin of the plant litter was a strong determinant of the structure and function of the microbial community. The air-drying of the litter most likely had a strong effect upon the microbial communities.
Acknowledgments
The authors would like to thank the National Science and Engineering Research Council of Canada for its support through a Discovery Grant to SAQ.
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