A stable isotopic study to determine carbon and nitrogen cycling in a disturbed southern Californian forest ecosystem

This study utilized isotope analyses to contrast nitrogen and carbon dynamics at four sites located along an air pollution gradient in the San Bernardino National Forest in southern California. Natural N-15 and C-13 abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen delta(15)N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7 parts per thousand more enriched than the other, less polluted sites. Mean soil delta(15)NH(4)(+) was also significantly enriched in N-15 at CP compared to Barton Flats (BF), the least polluted site, by 3.8 parts per thousand. Soil delta(15)NO(3)(-) signatures were not statistically different among sites. The litter delta(15)NH(4)(+) values followed a trend similar to that of the soil. Furthermore, the litter delta(15)NO(3)(-) at CP was significantly depleted in N-15 compared to the other sites. The isotopic discrimination for the eventual production of nitrate from organic nitrogen in soil and litter was maximized at CP and minimized at BF. A stable carbon isotopic gradient of decreasing soil, litter, and foliar delta(13)C was also observed with increasing site pollution level. These results support the hypothesis that chronic atmospheric deposition has enhanced nitrogen cycling processes and has affected carbon metabolism at CP

Seasonal Variation and Ecosystem Dependence of Emission Factors for Selected Trace Gases and PM2.5 for Southern African Savanna Fires

[1] In this paper we present the first early dry season (early June-early August) emission factor measurements for carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nonmethane hydrocarbons (NMHC), and particulates with a diameter less than 2.5 μm (PM2.5) for southern African grassland and woodland fires. Seasonal emission factors for grassland fires correlate linearly with the proportion of green grass, used as a surrogate for the fuel moisture content, and are higher for products of incomplete combustion in the early part of the dry season compared with later in the dry season. Models of emission factors for NMHC and PM2.5 versus modified combustion efficiency (MCE) are statistically different in grassland compared with woodland ecosystems. We compare predictions based on the integration of emissions factors from this study, from the Southern African Fire-Atmosphere Research Initiative 1992 (SAFARI-92), and from SAFARI-2000 with those based on the smaller set of ecosystem-specific emission factors to estimate the effects of using regional-average rather than ecosystem-specific emission factors. We also test the validity of using the SAFARI-92 models for emission factors versus MCE to predict the early dry season emission factors measured in this study. The comparison indicates that the largest discrepancies occur at the low end (0.907) and high end (0.972) of MCE values measured in this study. Finally, we combine our models of MCE versus proportion of green grass for grassland fires with emission factors versus MCE for selected oxygenated volatile organic compounds measured in the SAFARI-2000 campaign to derive the first seasonal emission factors for these compounds. The results of this study demonstrate that seasonal variations in savanna fire emissions are important and should be considered in modeling emissions at regional to continental scales

Improving global estimates of atmospheric emissions from biomass burning

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94770/1/jgrd11567.pd

Foliar δ15N values characterize soil N cycling and reflect nitrate or ammonium preference of plants along a temperate grassland gradient

The natural abundance of stable 15N isotopes in soils and plants is potentially a simple tool to assess ecosystem N dynamics. Several open questions remain, however, in particular regarding the mechanisms driving the variability of foliar δ15N values of non-N2 fixing plants within and across ecosystems. The goal of the work presented here was therefore to: (1) characterize the relationship between soil net mineralization and variability of foliar Δδ15N (δ15Nleaf − δ15Nsoil) values from 20 different plant species within and across 18 grassland sites; (2) to determine in situ if a plant’s preference for NO3− or NH4+ uptake explains variability in foliar Δδ15N among different plant species within an ecosystem; and (3) test if variability in foliar Δδ15N among species or functional group is consistent across 18 grassland sites. Δδ15N values of the 20 different plant species were positively related to soil net mineralization rates across the 18 sites. We found that within a site, foliar Δδ15N values increased with the species’ NO3− to NH4+ uptake ratios. Interestingly, the slope of this relationship differed in direction from previously published studies. Finally, the variability in foliar Δδ15N values among species was not consistent across 18 grassland sites but was significantly influenced by N mineralization rates and the abundance of a particular species in a site. Our findings improve the mechanistic understanding of the commonly observed variability in foliar Δδ15N among different plant species. In particular we were able to show that within a site, foliar δ15N values nicely reflect a plant’s N source but that the direction of the relationship between NO3− to NH4+ uptake and foliar Δδ15N values is not universal. Using a large set of data, our study highlights that foliar Δδ15N values are valuable tools to assess plant N uptake patterns and to characterize the soil N cycle across different ecosystems

Using MODIS to evaluate heterogeneity of biomass burning in southern African savannahs: a case study in Etosha

International audienc

Regional fuel load for two climatically contrasting years in southern Africa

International audienc

The MODIS fire products

International audienc

Overview of Sun photometer measurements of aerosol properties in Scandinavia and Svalbard

An overview on the data of columnar aerosol properties measured in Northern Europe is provided. Apart from the necessary data gathered in the Arctic, the knowledge of the aerosol loading in nearby areas (e.g. sub-Arctic) is of maximum interest to achieve a correct analysis of the Arctic aerosols and transport patterns. This work evaluates data from operational sites with sun photometer measurements belonging either to national or international networks (AERONET, GAW-PFR) and programs conducted in Scandi- navia and Svalbard. We enumerate a list of sites, measurement type and periods together with observed aerosol properties. An evaluation and analysis of aerosol data was carried out with a review of previous results as well. Aerosol optical depth (AOD) and Ångström exponent (AE) are the current parameters with suf␣cient long-term records for a ␣rst evaluation of aerosol properties. AOD (500 nm) ranges from 0.08 to 0.10 in Arctic and sub-Arctic sites (Ny-Ålesund: 0.09; Andenes: 0.10; Sodankylä: 0.08), and it is somewhat higher in more populated areas in Southern Scandinavia (AOD about 0.10e0.12 at 500 nm). On the Norwegian coast, aerosols show larger mean size (AE � 1.2 at Andenes) than in Finland, with continental climate (AE � 1.5 at Sodankylä). Columnar particle size distributions and related parameters derived from inversion of sun/sky radiances were also investigated. This work makes special emphasis in the joint and collaborative effort of the various groups from different countries involved in this study. Part of the measurements presented here were involved in the IPY projects Polar-AOD and POLARCAT