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The microbial communities and nutrient availability in pre and post harvested lodgepole pine stands of West-Central Alberta

Mascarenhas, A.. 2011. University of Saskatchewan M.Sc. thesis. Saskatoon, SK


All organisms within a forested system play a role in the biogeochemical cycle, not only within the forest but also within the global community. Soil microorganisms are a vital part of this cycle, as they sequester or make nutrients available for the development of the forest environment. When a disturbance event occurs, changes to the environment occur; however, it is unclear how these changes affect the soil's microbial community. This 2-year (2007 and 2008) study was carried out to obtain a preliminary assessment of the microbial community structure and nutrient (nitrogen and phosphorus) availability within lodgepole pine stands of the Boreal Plain ecozone in west-central Alberta. Six stands of different ages were selected to determine the differences between pre and post harvest. Nutrient flux measurements were conducted using plant root simulator (PRS) probes to investigate the changes in nutrient availability. The microbial community structures were determined using two biochemical methods. The first one was a community level physiological profile (CLPP), which provides information concerning the functional characteristics of the microbial communities. Phospholipid fatty acid (PLFA) analysis provides information about the physiological characteristics of the microbial community. Analysis of the PRS probes results varied for the two nutrients: phosphorus (P) and nitrogen (N). Nitrogen availability was determined by examining the fluxes of ammonium and nitrate to the PRS probes. These did not show a strong relationship between the different aged stands during 2007 or 2008. In addition, no statistical difference was shown between the 2007 and 2008 data compared to the LFH or the mineral soil of the stands. Phosphorus, however, did show a potential trend where there was an initial increase of available P after harvest and then a gradual decrease, as the forest stands matured. This was strongly observed within the LFH, while there was a slight increase in the mineral layer. These trends remained consistent over the two-year period showing a gradual decrease in P flux to the PRS probes as a stand aged even in just one year. The microbial communities did not show a strong change after a forest-harvesting event. When examining the functional groups, there was a drastic shift in the LFH layer microbial community over the first sampling season. This change remained the same within the beginning of the second sampling year. This shift occurred in all stands due to an environmental factor, which was suspected to be the increase in moisture during the season. The change in the microbial communities was not observed, however, in the mineral layer of the soil when the functional structure was examined. When the physiological composition of the microbial communities was observed, though, using PLFA, it was apparent that the physiological characteristics of the microbial community had changed in the mineral soil. Furthermore, no physiological change was observed in the microbial communities of the LFH, only a functional change.