Satellite inventory of human settlements using nocturnal radiation emissions: a contribution for the global toolchest

Time series data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) have been used to derive georeferenced inventories of human settlements for Europe, North and South America, and Asia. The visible band of the OLS is intensified at night, permitting detection of nocturnal visible-near infrared emissions from cities, towns, and villages. The time series analysis makes it possible to eliminate ephemeral VNIR emission sources such as fire and to normalize for differences in the number of cloud-free observations. An examination of the area lit (km²) for 52 countries indicates the OLS derived products may be used to perform the spatial apportionment of population and energy related greenhouse gas emissions.     

Identifying fern gametophytes using DNA sequences

Identification of fern gametophytes is generally hampered by low morphological complexity. Here we explore an alternative: DNA‐based identification. We obtained a plastid rbcL sequence from a sterile gametophyte of unknown origin (cultivated for more than 30 years) and employed blast to determine its affinities. Using this approach, we identified the gametophyte as Osmunda regalis. To evaluate the robustness of this determination, and the usefulness of rbcL in differentiating among species, we conducted a phylogenetic analysis of osmundaceous fern sequences. Based on our results, it is evident that DNA‐based identification has considerable potential in exploring the ecology of fern gametophytes.     

On the variability of respiration in terrestrial ecosystems: moving beyond Q10

Respiration, which is the second most important carbon flux in ecosystems following gross primary productivity, is typically represented in biogeochemical models by simple temperature dependence equations. These equations were established in the 19th century and have been modified very little since then. Recent applications of these equations to data on soil respiration have produced highly variable apparent temperature sensitivities. This paper searches for reasons for this variability, ranging from biochemical reactions to ecosystem‐scale substrate supply. For a simple membrane‐bound enzymatic system that follows Michaelis–Menten kinetics, the temperature sensitivities of maximum enzyme activity (V_max) and the half‐saturation constant that reflects the affinity of the enzyme for the substrate (K_m) can cancel each other to produce no net temperature dependence of the enzyme. Alternatively, when diffusion of substrates covaries with temperature, then the combined temperature sensitivity can be higher than that of each individual process. We also present examples to show that soluble carbon substrate supply is likely to be important at scales ranging from transport across membranes, diffusion through soil water films, allocation to aboveground and belowground plant tissues, phenological patterns of carbon allocation and growth, and intersite differences in productivity. Robust models of soil respiration will require that the direct effects of substrate supply, temperature, and desiccation stress be separated from the indirect effects of temperature and soil water content on substrate diffusion and availability. We speculate that apparent Q₁₀ values of respiration that are significantly above about 2.5 probably indicate that some unidentified process of substrate supply is confounded with observed temperature variation.     

Debating the greening vs. browning of the North American boreal forest: differences between satellite datasets

A number of remote sensing studies have evaluated the temporal trends of the normalized difference vegetation index (NDVI or vegetation greenness) in the North American boreal forest during the last two decades, often getting quite different results. To examine the effect that the use of different datasets might be having on the estimated trends, we compared the temporal trends of recently burned and unburned sites of boreal forest in central Canada calculated from two datasets: the Global Inventory, Monitoring, and Modeling Studies (GIMMS), which is the most commonly used 8 km dataset, and a new 1 km dataset developed by the Canadian Centre for Remote Sensing (CCRS). We compared the NDVI trends of both datasets along a fire severity gradient in order to evaluate the variance in regeneration rates. Temporal trends were calculated using the seasonal Mann–Kendall trend test, a rank‐based, nonparametric test, which is robust against seasonality, nonnormality, heteroscedasticity, missing values, and serial dependence. The results showed contrasting NDVI trends between the CCRS and the GIMMS datasets. The CCRS dataset showed NDVI increases in all recently burned sites and in 50% of the unburned sites. Surprisingly, the GIMMS dataset did not capture the NDVI recovery in most burned sites and even showed NDVI declines in some burned sites one decade after fire. Between 50% and 75% of GIMMS pixels showed NDVI decreases in the unburned forest compared with <1% of CCRS pixels. Being the most broadly used dataset for monitoring ecosystem and carbon balance changes, the bias towards negative trends in the GIMMS dataset in the North American boreal forest has broad implications for the evaluation of vegetation and carbon dynamics in this region and globally.     

The mechanisms of iron isotope fractionation produced during dissimilatory Fe(III) reduction by Shewanella putrefaciens and Geobacter sulfurreducens

Microbial dissimilatory iron reduction (DIR) is widespread in anaerobic sediments and is a key producer of aqueous Fe(II) in suboxic sediments that contain reactive ferric oxides. Previous studies have shown that DIR produces some of the largest natural fractionations of stable Fe isotopes, although the mechanism of this isotopic fractionation is not yet well understood. Here we compare Fe isotope fractionations produced by similar cultures of Geobacter sulfurreducens strain PCA and Shewanella putrefaciens strain CN32 during reduction of hematite and goethite. Both species produce aqueous Fe(II) that is depleted in the heavy Fe isotopes, as expressed by a decrease in ⁵⁶Fe/⁵⁴Fe ratios or δ⁵⁶Fe values. The low δ⁵⁶Fe values for aqueous Fe(II) produced by DIR reflect isotopic exchange among three Fe inventories: aqueous Fe(II) (Fe(II)_aq), sorbed Fe(II) (Fe(II)_sorb), and a reactive Fe(III) component on the ferric oxide surface (Fe(III)_reac). The fractionation in ⁵⁶Fe/⁵⁴Fe ratios between Fe(II)_aq and Fe(III)_reac was –2.95‰, and this remained constant over the timescales of the experiments (280 d). The Fe(II)_aq – Fe(III)_reac fractionation was independent of the ferric Fe substrate (hematite or goethite) and bacterial species, indicating a common mechanism for Fe isotope fractionation during DIR. Moreover, the Fe(II)_aq – Fe(III)_reac fractionation in ⁵⁶Fe/⁵⁴Fe ratios during DIR is identical within error of the equilibrium Fe(II)_aq – ferric oxide fractionation in abiological systems at room temperatures. This suggests that the role of bacteria in producing Fe isotope fractionations during DIR lies in catalyzing coupled atom and electron exchange between Fe(II)_aq and Fe(III)_reac so that equilibrium Fe isotope partitioning occurs. Although Fe isotope fractionation between Fe(II)_aq and Fe(III)_reac remained constant, the absolute δ⁵⁶Fe values for Fe(II)_aq varied as a function of the relative proportions of Fe(II)_aq, Fe(II)_sorb, and Fe(III)_reac during reduction. The temporal variations in these proportions were unique to hematite or goethite but independent of bacterial species. In the case of hematite reduction, the small measured Fe(II)_aq – Fe(II)_sorb fractionation of −0.30‰ in ⁵⁶Fe/⁵⁴Fe ratios, combined with the small proportion of Fe(II)_sorb, produced insignificant (<0.05‰) isotopic effects due to sorption of Fe(II). Sorption of Fe(II) produced small, but significant effects during reduction of goethite, reflecting the higher proportion of Fe(II)_sorb and larger measured Fe(II)_aq – Fe(II)_sorb fractionation of –0.87‰ in ⁵⁶Fe/⁵⁴Fe ratios for goethite. The isotopic effects of sorption on the δ⁵⁶Fe values for Fe(II)_aq were largest during the initial stages of reduction when Fe(II)_sorb was the major ferrous Fe species during goethite reduction, on the order of 0.3 to 0.4‰. With continued reduction, however, the isotopic effects of sorption decreased to <0.2‰. These results provide insight into the mechanisms that produce Fe isotope fractionation during DIR, and form the basis for interpretation of Fe isotope variations in modern and ancient natural systems where DIR may have driven Fe cycling.