Impact of nitrogenous fertilizers on carbonate dissolution in small agricultural catchments: Implications for weathering CO2 uptake at regional and global scales

The goal of this study was to highlight the occurrence of an additional proton-promoted weathering pathway of carbonate rocks in agricultural areas where N-fertilizers are extensively spread, and to estimate its consequences on riverine alkalinity and uptake of CO₂ by weathering. We surveyed 25 small streams in the calcareous molassic Gascogne area located in the Garonne river basin (south-western France) that drain cultivated or forested catchments for their major element compositions during different hydrologic periods. Among these catchments, the Hay and the Montoussé, two experimental catchments, were monitored on a weekly basis. Studies in the literature from other small carbonate catchments in Europe were dissected in the same way. In areas of intensive agriculture, the molar ratio (Ca + Mg)/HCO₃ in surface waters is significantly higher (0.7 on average) than in areas of low anthropogenic pressure (0.5). This corresponds to a decrease in riverine alkalinity, which can reach 80% during storm events. This relative loss of alkalinity correlates well with the content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO₂ to the total riverine alkalinity (CO_{2 ATM-SOIL}/HCO₃) is less than 50% (expected value for carbonate basins), and it decreases when the nitrate concentration increases. This loss of alkalinity can be attributed to the substitution of carbonic acid (natural weathering pathway) by protons produced by nitrification of N-fertilizers (anthropogenic weathering pathway) occurring in soils during carbonate dissolution. As a consequence of these processes, the alkalinity over the last 30 years shows a decreasing trend in the Save river (one of the main Garonne river tributaries, draining an agricultural catchment), while the nitrate and calcium plus magnesium contents are increasing.We estimated that the contribution of atmospheric/soil CO₂ to riverine alkalinity decreased by about 7-17% on average for all the studied catchments. Using these values, the deficit of CO₂ uptake can be estimated as up to 0.22-0.53 and 12-29 Tg¹ yr⁻¹ CO₂ on a country scale (France) and a global scale, respectively. These losses represent up to 5.7-13.4% and only 1.6-3.8% of the total CO₂ flux naturally consumed by carbonate dissolution, for France and on a global scale, respectively. Nevertheless, this loss of alkalinity relative to the Ca + Mg content relates to carbonate weathering by protons from N-fertilizers nitrification, which is a net source of CO₂ for the atmosphere. This anthropogenic CO₂ source is not negligible since it could reach 6-15% of CO₂ uptake by natural silicate weathering and could consequently partly counterbalance this natural CO₂ sink.     

Phytolith assemblages as a promising tool for reconstructing Mediterranean Holocene vegetation

The reliability of phytolith assemblage analysis for characterizing Mediterranean vegetation is investigated in this study. Phytolith assemblages are extracted from modern and buried Holocene soils from the middle Rhône valley (France). The relation between modern phytolith assemblages and the surrounding vegetation, as well as between fossil assemblages and contemporaneous vegetation, already reconstructed through other proxies, is discussed. We demonstrate that the main northwestern Mediterranean biomes are well distinguished by soil phytolith assemblage analysis. In particular, the density of pine and nonconiferous trees (densities expressed relatively to the grass cover) and the overall degree of opening of the vegetation appear well recorded by three phytolith indexes. North Mediterranean vegetation changes during the Holocene period, mainly tree line shifts, pine wood development and deforestation are poorly documented, due to the scarcity of proxy-preserving sites. Phytolith assemblage analysis of soils, buried soils, and sediments appears to be a promising technique to fill this gap.     

Phytoplankton dynamics and cyanobacterial dominance in Murchison Bay of Lake Victoria (Uganda) in relation to environmental conditions

Murchison Bay is a shallow embayment in the north-western part of Lake Victoria, strongly influenced by urban pollution from the Ugandan capital Kampala. Two stations, representing the semi-enclosed innermost part of the bay and the wider outer part of the bay, were sampled in the period from April 2003 to March 2004, in order to assess the phytoplankton community and the nutrient status in the bay. Murchison Bay was highly eutrophic with average concentrations (n=25) of total phosphorous >90 μg L⁻¹ and total nitrogen >1100 μg L⁻¹ in the inner part of the bay. The phytoplankton community was dominated by a variety of cyanobacterial species and diatoms. Cyanobacteria were dominant in the whole bay, whereas diatoms were more abundant in the outer part of the bay. Moreover, the proportion of N-fixing species like Anabaena sp. was higher in the outer part of the bay, whereas species like Microcystis sp. were more abundant in the inner part of the bay. The phytoplankton community, especially in the outer part of the bay, may be influenced by light limitation. Low NO₃-N concentrations in the bay may also indicate a possible N-limitation, thus favouring growth of N-fixing cyanobacteria. The open bay is, however, a complex system, and additional environmental factors and loss processes most likely affect the phytoplankton community.     

Characterisation of Soot Emitted by Domestic Heating,Aircraft and Cars Using Diesel or Biodiesel

The characterisation of aggregates, like soot, firstly requires the determination of the size distribution of the primary particles. The primary particle size of combustion generated aggregates depends upon the combustion environment and the formation conditions, such as temperature, pressure and fuel-to-air ratio, among others. Since the combustion characteristics are different in the different types of burners, the characterisation of primary particles may offer the possibility to distinguish soot from different sources. In this paper, we present the signature of the primary particles and the aggregates of soot emitted by cars using diesel or biodiesel, by domestic heating, and by aircraft exhausts, which can be considered as the major sources as derived from measurements on transmission electron micrographs. The size distributions of all aggregates types with different aerodynamic diameter were log-normal and quasi-monodisperse. The size distribution of the primary particles for soot emitted by different sources showed minor differences. However, a comparison between the diameter of the primary particles and those obtained using a standard method for carbon black revealed discrepancies. The median diameter of the primary particles was combined with the median number of primary particles in an aggregate to calculate the relative particle surface area available for adsorption. In a similar way, the relative specific surface area was determined. The surface area was measured using the Brunauer-Emmett-Teller (B.E.T.) nitrogen adsorption method and the relative surface area available for adsorption was calculated.     

From economic integration to socioeconomic inclusion: day labor worker centers as social intermediaries

Day labor worker centers have emerged as an important mode of regulatory action in the informal economy of major US cities. Research suggests that these organizations are beneficial in improving employment outcomes experienced by migrant workers engaged in this labor market sector. Yet, the extent to which these organizations impact the social integration of this working population remains relatively undeveloped in the literature. Using data from the National Day Labor Survey, we examine the impact of day labor worker centers on the level of social inclusion experienced by migrant day laborers. We find that worker centers have a modest, but statistically significant, impact on the levels of social integration experienced by this working population and that this varies from city to city. Ultimately we argue that the social intermediary role of these organizations may offer a type of counter mobilization necessary to promote the socioeconomic integration of this working population, but that issues of capacity remain.     

Characterization of Water Quality in Xiao Xingkai Lake: Implications for Trophic Status and Management

Increasing cases of lake eutrophication globally have raised concerns among stakeholders, and particularly in China. Evaluating the causes of eutrophication in waterways is essential for effective pollution prevention and control. Xiao Xingkai Lake is part of and connected to Xingkai(Khanka) Lake, a boundary lake between China and Russia. In this study, we investigated the spatio-temporal variabilities in water quality(i.e., dissolved oxygen(DO), total nitrogen(TN), total phosphorus(TP), chemica...     

Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola

We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a^?1 has remained stable during the entire Quaternary.     

Assessing the effects of catchment‐scale urban green infrastructure retrofits on hydrograph characteristics

Run‐off from impervious surfaces has pervasive and serious consequences for urban streams, but the detrimental effects of urban stormwater can be lessened by disconnecting impervious surfaces and redirecting run‐off to decentralized green infrastructure. This study used a before–after‐control‐impact design, in which streets served as subcatchments, to quantify hydrologic effectiveness of street‐scale investments in green infrastructure, such as street‐connected bioretention cells, rain gardens and rain barrels. On the two residential treatment streets, voluntary participation resulted in 32.2% and 13.5% of parcels having green infrastructure installed over a 2‐year period. Storm sewer discharge was measured before and after green infrastructure implementation, and peak discharge, total run‐off volume and hydrograph lags were analysed. On the street with smaller lots and lower participation, green infrastructure installation succeeded in reducing peak discharge by up to 33% and total storm run‐off by up to 40%. On the street with larger lots and higher participation, there was no significant reduction in peak or total stormflows, but on this street, contemporaneous street repairs may have offset improvements. On the street with smaller lots, lag times increased following the first phase of green infrastructure construction, in which streetside bioretention cells were built with underdrains. In the second phase, lag times did not change further, because bioretention cells were built without underdrains and water was removed from the system, rather than just delayed. We conclude that voluntary green infrastructure retrofits that include treatment of street run‐off can be effective for substantially reducing stormwater but that small differences in design and construction can be important for determining the level of the benefit. Copyright © 2015 John Wiley & Sons, Ltd.