Long-term Impacts of Silvicultural Treatments on Soil Microbial Biomass, Community Composition, and N Mineralization
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Date
2005-06-02
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Abstract
This study was conducted in a late-rotation loblolly pine (Pinus taeda L.) plantation to assess the effects of silvicultural treatments (site preparation and vegetation control) on (1) microbial biomass, (2) microbial community composition, and (3) nitrogen (N) mineralization in the humus layer (Oa horizon) and upper mineral soil (0-5 cm). An additional objective was to determine relationships among stand characteristics, substrate properties, and the microbial community. The study was established in 1981 in a 22-year-old loblolly pine plantation. Site preparation treatments were chopping and burning and shearing, piling and disking and the vegetation control treatments were no control and complete control with herbicide for the first five years. For this experiment, humus layer and mineral soil samples were collected three times in 2004. To determine microbial biomass, samples were analyzed using the chloroform fumigation extraction method, microbial community composition was evaluated using phospholipid fatty acid (PLFA) analysis and an aerobic incubation was conducted to determine net N mineralization. After 22 years, site preparation and vegetation control still had an effect on microbial biomass, microbial community composition and net N mineralization. Both microbial biomass and microbial community composition were affected by the different plant communities that formed over the life of the rotation and resulted from the interaction of site preparation and vegetation control. The addition of hardwood basal area in plots with no vegetation control was strongly correlated with higher pH and lower C:N ratios which affected the substrate environment and therefore the soil microbial community. Microbial biomass in the mineral soil was affected by the vegetation control treatment; plots with no vegetation control had higher MBC than plots with no vegetation control (0.63 mg g-1 and 0.56 mg g-1, respectively). In the humus layer, bacteria and arbuscular mycorrhizae PLFAs were greatest in plots with no vegetation control, and in the mineral soil, fungi were most abundant in the chop and burn plots with no vegetation control. Microbial biomass carbon and bacteria and arbuscular mycorrhizae PLFA mole percentages were positively correlated with pH and negatively correlated with the C:N ratio of plots with no vegetation control and greater hardwood biomass. In both the mineral soil and the humus layer, the concentration of net mineralized N was affected by site preparation, with the highest rates occurring on the chop and burn plots. The effects of site preparation on net N mineralization may be attributed to the lasting effects of the removal of around 600 kg N ha-1 at study establishment from the shear, pile and disked plots. Site preparation and vegetation control influence directly or indirectly, through plant community composition, microbial biomass, the composition of the microbial community, and net N mineralization. These results suggest that the impacts of intensive silvicultural practices do have an impact on the complex interrelationship of plant communities and substrate properties with microbial community characteristics and function.
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nitrogen mineralization, PLFA, loblolly pine, microbial biomass, silviculture
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Degree
MS
Discipline
Forestry
