Ecophysiological response of aspen (Populus tremuloides) and jack pine (Pinus banksiana) to atmospheric nitrogen deposition on reconstructed boreal forest soils in the Athabasca oil sands region.

Oil and gas extraction in the Athabasca Oil Sands Region of northeastern Alberta, Canada has increased anthropogenic nitrous oxide (NOx) and ammonia (NH3) emissions over the past three decades, leading to a potential increase in N deposition. Deposition on reclaimed sites was hypothesized to be higher than in surrounding boreal forests, but had not been quantified. The objective of this study was to assess the implications of this potentially increased deposition on reclaimed aspen (Populus tremuloides Michx.) and pine (Pinus banksiana Lamb.) ecosystems through the use of several N status indicators, including N deposition, total and available concentrations in plants and soils, and δ15N values in deposition and plants and soils. Atmospheric N deposition, which was dominated by ammonium (NH4+), averaged 24 kg N ha-1 year-1 as bulk precipitation and 6 kg N ha-1 year-1 as throughfall. Increased N deposition influenced the N cycle in both aspen and pine stands. Aspen appeared to be actively biocycling N as indicated by a closed N cycle, resulting in minimal N losses. Whereas the N cycle in pine may be more open as indicated by the dominance of soil nitrate (NO3-), and enrichment of 15N in available soil NH4+, root and foliar N. Therefore, we suggest that pine stands on reclamation sites may be at kinetic N saturation where the rate of N inputs exceeds vegetation and soil N net sinks, and do not require additional N fertilizer inputs.

Flow effects in the laser-induced thermal loading of optical traps and optofluidic devices.

Flow effects on the thermal loading in different optofluidic systems (optical trap and various microfluidic channels) have been systematically explored by using dye-based ratiometric luminescence thermometry. Thermal images obtained by fluorescence microscopy demonstrate that the flow rate plays a key role in determining both the magnitude of the laser-induced temperature increment and its spatial distribution. Numerical simulations were performed in the case of the optical trap. A good agreement between the experimental results and those predicted by mathematical modelling was observed. It has also been found that the dynamics of thermal loading is strongly influenced by the presence of fluid flow.

Wildfire and charcoal enhance nitrification and ammonium-oxidizing bacterial abundance in dry montane forest soils.

All forest fire events generate some quantity of charcoal, which may persist in soils for hundreds to thousands of years. However, few studies have effectively evaluated the potential for charcoal to influence specific microbial communities or processes. To our knowledge, no studies have specifically addressed the effect of charcoal on ammonia-oxidizing bacteria (AOB) in forest soils. Controlled experiments have shown that charcoal amendment of fire-excluded temperate and boreal coniferous forest soil increases net nitrification, suggesting that charcoal plays a major role in maintaining nitrification for extended periods postfire. In this study, we examined the influence of fire history on gross nitrification, nitrification potential, and the nature and abundance of AOB. Soil cores were collected from sites in the Selway-Bitterroot wilderness area in northern Idaho that had been exposed twice (in 1910, 1934) or three times (1910, 1934, and 1992) in the last 94 yr, allowing us to contrast soils recently exposed to fire to those that experienced no recent fire (control). Charcoal content was determined in the O horizon by hand-separation and in the mineral soil by a chemical digestion procedure. Gross and net nitrification, and potential rates of nitrification were measured in mineral soil. Analysis of the AOB community was conducted using primer sets specific for the ammonia mono-oxygenase gene (amoA) or the 16S rRNA gene of AOB. Denaturing gradient gel electrophoresis was used to analyze the AOB community structure, while AOB abundance was determined by quantitative polymerase chain reaction. Recent (12-yr-old) wildfire resulted in greater charcoal contents and nitrification rates compared with sites without fire for 75 yr, and the more recent fire appeared to have directly influenced AOB abundance and community structure. We predicted and observed greater abundance of AOB in soils recently exposed to fire compared with control soils. Interestingly, sequence data revealed that Clusters 3 and 4, and not Cluster 2, of genus Nitrosospira dominated these forest soils, with a shift toward Cluster 3 in recently burned sites.

Nitrogen mineralization and microbial activity in oil sands reclaimed boreal forest soils.

Organic materials including a peat-mineral mix (PM), a forest floor-mineral mix (L/S), and a combination of the two (L/PM) were used to cap mineral soil materials at surface mine reclamation sites in the Athabasca oil sands region of northeastern Alberta, Canada. The objective of this study was to test whether LFH provided an advantage over peat by stimulating microbial activity and providing more available nitrogen for plant growth. Net nitrification, ammonification, and N mineralization rates were estimated from field incubations using buried bags. In situ gross nitrification and ammonification rates were determined using the 15N isotope pool dilution technique, and microbial biomass C (MBC) and N (MBN) were measured by the chloroform fumigation-extraction method. All reclaimed sites had lower MBC and MBN, and lower net ammonification and net mineralization rates than a natural forest site (NLFH) used as a control, but the reclamation treatment using LFH material by itself had higher gross and net nitrification rates. A positive correlation between in situ moisture content, dissolved organic N, MBC, and MBN was observed, which led us to conduct a moisture manipulation experiment in the laboratory. With the exception of the MBN for the L/S treatment, none of the reclamation treatments ever reached the levels of the natural site during this experiment. However, materials from reclamation treatments that incorporated LFH showed higher respiration rates, MBC, and MBN than the PM treatment, indicating that the addition of LFH as an organic amendment may stimulate microbial activity as compared to the use of peat alone.