WARMING AND CHRONIC HIGH NUTRIENT MANIPULATIONS YIELD DIFFERING LEGACY EFECTS ON THE SOIL MICROBIAL COMMUNITY AND NUTRIENT POOLS IN THE LOW ARCTIC

Climate warming is predicted to increase summer air temperatures in the Arctic, warming soils and enhancing microbial decomposition of soil organic matter. Given the size of the soil carbon stores in the Arctic, even a fraction of its release as CO2 to the atmosphere could result in a positive feedback to climate warming. Fertilizers have been used in the past to quickly increase soil solution nutrients pools to mimic predicted concentrations under climate warming. However, because it may have inadvertent affects on the soil microbial community, fertilizer-induced patterns in microbial decomposition may be unrealistic. This study aimed to better understand the proposed mechanism of enhanced microbial decomposition under nutrient addition and warming treatments to discern whether warming alone is enough to stimulate enhanced microbial decomposition, or if nutrients in excess (i.e. chronic high nutrient additions) are necessary to yield such a response. I investigated the impacts of 10 years of greenhouse summer warming, chronic low nutrient factorial addition (5 g N and 1g P m-2 year-1, respectively), and chronic high nutrient factorial addition (10 g N and 5g P m-2 year-1, respectively) treatments on a mesic birch hummock tundra ecosystem near Daring Lake, NWT, Canada. Soil microbial nutrient pools, soil solution nutrient pools, and microbial community structure were measured in the upper organic, lower organic, and uppermost mineral soil depth intervals of all treatment plots in Spring 2014. Interestingly, the low nutrient additions did not yield any significant trends, yet the warming treatment increased soil bacterial richness suggesting a legacy effect of warming from the previous summers. Enhanced microbial nutrient uptake occurred only in the high nutrient addition treatments, and did not significantly alter soil carbon at least within the ten year period of this experiment. Together, these results and the absence of significant impacts of the low nutrient and greenhouse warming treatments suggests that nutrient and carbon cycling in these low arctic soils may be resilient against climate warming, at least over the initial decades.

STRUCTURAL AND FUNCTIONAL CORRELATES OF EXERCISE VENTILATORY INEFFICIENCY IN CHRONIC OBSTRUCTIVE PULMONARY DISEASE PATIENTS WITH MILD-TO-MODERATE SPIROMETRIC ABNORMALITIES

Background: Individuals with chronic obstructive pulmonary disease (COPD) have higher than normal ventilatory equivalents for carbon dioxide (VE/VCO2) during exercise. There is growing evidence that emphysema on thoracic computed tomography (CT) scans is associated with poor exercise capacity in COPD patients with only mild-to-moderate airflow obstruction. We hypothesized that emphysema is an underlying cause of microvascular dysfunction and ventilatory inefficiency, which in turn contributes to reduced exercise capacity. We expected ventilatory inefficiency to be associated with a) the extent of emphysema; b) lower diffusing capacity for carbon monoxide; c) a reduced pulmonary blood flow response to exercise; and d) reduced exercise capacity. Methods: In a cross-sectional study, 19 subjects with mild-to-moderate COPD (mean ± SD FEV1= 82 ± 13% predicted, 12 GOLD grade 1) and 26 age-, sex-, and activity-matched controls underwent a ramp-incremental symptom-limited exercise test on a cycle ergometer. Ventilatory inefficiency was assessed by the minimum VE/VCO2 value (nadir). A subset of subjects also completed repeated constant work rate exercise bouts with non-invasive measurements of pulmonary blood flow. Emphysema was quantified as the percentage of attenuation areas below -950 Housefield Units on CT scans. An electronic scoresheet was used to keep track of emphysema sub-types. Results: COPD subjects typically had centrilobular emphysema (76.8 ± 10.1% of total emphysema) in the upper lobes (upper/lower lobe ratio= 0.82 ± 0.04). They had lower peak oxygen uptake (VO2), higher VE/VCO2 nadir and greater dyspnea scores than controls (p<0.05). Lower peak O2 and worse dyspnea were found in COPD subjects with VE/VCO2 nadirs ≥ 30. COPD subjects had blunted increases in pulmonary blood flow from rest to iso-VO2 exercise (p<0.05). Higher VE/VCO2 nadir in COPD subjects correlated with emphysema severity (r= 0.63), which in turn correlated with reduced lung diffusing capacity (r= -0.72) and blunted changes in pulmonary blood flow from rest to exercise (r= -0.69) (p<0.01). Conclusions: Ventilation “wasted” in emphysematous areas is associated with reduced exercise ventilatory efficiency in mild-to-moderate COPD. Exercise ventilatory inefficiency links structure (emphysema) and function (gas transfer) to a key clinical outcome (reduced exercise capacity) in COPD patients with modest spirometric abnormalities.