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 Montousse´, 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 NO3 content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO2 to the total riverine alkalinity (CO2 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 Nfertilizers (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 yr1 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

Silicate rock weathering and atmospheric/soil CO2 uptake in the Amazon basin estimated from river water geochemistry: seasonal and spatial variations

Using the data of the CAMREX project (1982–1984) on the water geochemistry of the Amazon river and its main tributaries, it was possible to assess the silicate rock weathering processes and the associated consumption of atmospheric/soil CO2, taking into account seasonal and spatial variations. This study confirms the important role of the Andes in the fluvial transport of dissolved and particulate material by the Amazon, and it shows for the first time that the silicate weathering rate and atmospheric/soil CO2 consumption are higher in the Andes than in the rest of the Amazon basin. The seasonal variations exhibit the significant role of runoff as a major factor controlling silicate weathering processes and show that the chemical erosion rates vary greatly from low discharge to high discharge. The average weathering rate estimated for the whole Amazon basin (15 m/My) is comparable to other estimations made for other tropical–equatorial environments. A comparison between physical and chemical weathering rates of silicate rocks for the Amazon basin and for each tributary basin show that in the Andes and in the Amazon trough, the soil thicknesses are decreasing whereas in the Shield the soil profiles are deepening

Organic carbon transport and C/N ratio variations in a large tropical river: Godavari as a case study, India

This study gives an insight into the source of organic carbon and nitrogen in the Godavari river and its tributaries, the yield of organic carbon from the catchment, seasonal variability in their concentration and the ultimate flux of organic and inorganic carbon into the Bay of Bengal. Particulate organic carbon/particulate organic nitrogen (POC/PON or C/N) ratios revealed that the dominant source of organic matter in the high season is from the soil (C/N = 8–14), while in the rest of the seasons, the river-derived (in situ) phytoplankton is the major source (C/N = l–8). Amount of organic materials carried from the lower catchment and flood plains to the oceans during the high season are 3 to 91 times higher than in the moderate and low seasons. Large-scale erosion and deforestation in the catchment has led to higher net yield of organic carbon in the Godavari catchment when compared to other major world rivers. The total flux of POC, and dissolved inorganic carbon (DIC) from the Godavari river to the Bay of Bengal is estimated as 756 · 109 and 2520 · 109 g yr1, respectively. About 22% of POC is lost in the main channel because of oxidation of labile organic matter, entrapment of organic material behind dams/sedimentation along flood plains and river channel; the DIC fluxes as a function of alkalinity are conservative throughout the river channel. Finally, the C/N ratios (12) of the ultimate fluxes of particulate organic carbon suggest the dominance of refractory/ stable soil organic matter that could eventually get buried in the coastal sediments on a geological time scale

Characteristics of heavy metals and their evaluation in suspended sediments from Piracicaba river basin (São Paulo, Brazil)

Fluxes of the particulate heavy metals like chromium (Cr), copper (Cu), cobalt (Co), nickel (Ni), zinc (Zn) and lead (Pb) were determined along the Piracicaba River basin. Suspended sediments from the main channel and tributaries were sampled during January 2005 (high water period) and analyzed for total concentrations after extraction by alkaline fusion procedure. The average concentrations showed a significant increase pattern from upstream to downstream for Zn, Cr, Ni, Cu and Pb, except for Co, in which only a small variation was noted. The specific fluxes of Zn, Cr and Ni were slightly higher (79.9, 33.3 and 20 kg km-2 yr-1, respectively) in comparison to other industrialized regions in Asia, Europe, North and South America. Significative correlations between organic matter and Cr, Cu and Co, observed in the suspended sediments along the Piracicaba River basin, revealed that heavy metals were adsorbed or complexed by the organic matter, while Ni and Pb were significatively correlated with Fe oxide. The geo-accumulation index (Igeo) used to evaluate the pollution degree of the Piracicaba River sediments, revealed a medium to strong pollution for Zn in the entire basin and moderate pollution for Cr and Ni at downstream direction around an important area covered by sugar cane plantations and urban areas influenced by domestic sewage

Impact of mercury atmospheric deposition on soils and streams in a mountainous catchment (Vosges, France) polluted by chlor-alkali industrial activity: The important trapping role of the organic matter

Total atmospheric Hg contamination in a French mountainous catchment upstream from a chlor-alkali industrial site was assessed using Hg concentrations in the deepest soil horizon, in the stream bottom sediments, in river waters and in bryophytes. The natural background level of Hg content deriving from rock weathering was estimated to 32 ng g1 in the deepest soil layers. The soils appear to be Hg contaminated in two stages: atmospheric deposition and leaching through the soil profiles of Hg-organic matter complexes. The Hg enrichment factor (EFHgSc) which could be calculated by normalization to a conservative element like Sc, allows to estimate the major contribution (63% to 95%) of the atmospheric inputs, even in the upper part of the basin. This contribution may be attributed to diffuse regional atmospheric deposition of Hg and is mainly due to the geographic location of the chlor-alkali plant. This study shows for the first time that the mercury enrichment is proportional to the carbon content indicating that most of the atmospheric mercury deposition is trapped by the organic matter contained in the soils and in the stream sediments. The Hg stock in the soils of the upper catchment and the soil erosion contribution to the riverine Hg fluxes are estimated for the first time and allow to assess the Hg residence time. It indicates that Hg is trapped in the soils of such a polluted catchment for probably several thousand years

Quaternary global change: review and issues(Special issue in memory of Hugues Faure)

The French National Committee of INQUA, the IGCP Project n° 459 (Carbon Cycle and Hydrology in the Paleo Terrestrial Environments),the CNRS (Centre National de la Recherche Scientifique), the CCGM (Commission de la Carte Géologique du Monde), the IRD (Institut de Recherche pour le Développement), and the BRGM (Bureau de Recherches Géologiques et Minières) have organized in June 2004 a special International Colloquium dedicated to the memory of Hugues Faure who passed away in May 2003. It was the occasion to make a review and to emphasize new results and issues on the different topics initiated by Hugues and his collaborators. It was also a chance for all his colleagues and friends to remember a rare human being and a great scientist, passionately involved in the observation and understanding of the planet, which he called “the real world”, and assuming his destiny up to the end. The Scientific committee of this colloquium has decided to publish a special issue of Global and Planetary Change with some selected papers presented during this colloquium. This GPC special issue includes some of the papers presented at this colloquium and some invited papers from scientific personalities who wished to contribute to this special volume. The general theme of the special publication in honour to Hugues Faure is the Quaternary and the Global Changes. It will focus on the global cycles and Quaternary climate (3 papers), sea-level fluctuations, tectonics and climate variations (3 papers), climate changes in terrestrial records (9 papers), and man, environment and global change (4 papers). A total of 20 contributions, including a foreword on Hugues Faure, are proposed by the participants of this colloquium

Heavy metals partioning in three French forest soils by sequential extraction procedure

To know the concentration of heavy metal associated to chemical compounds is important to evaluate the environmental risks on soils, particularly regarding ion bioavailability. The relative mobility and strength of binding of heavy metals to the soil components can be studied using a sequential extraction procedure which provides a meaningful comparison between different soil profiles. The heavy metals partitioning bas been identified in three different french forest soils : one cambic podzol, one calcaric cambisol and one mollic andosol, using a new sequential extraction method. Results show that metal fractionation is metal and site specific. The water leaching phase does not contain any metals. The proportion of heavy metal leached in the exchangeable and the acid — soplhuabslee is significant for Cd. The residual phase is important for Cr, Pb, and to a lesser extend for Ni. The organic matter fraction is dominant for Zn and Cu. Thus, the considered heavy metals are mainly bounded to iron oxides, to the organic matter and to the mineral residue. The order of heavy metals availability in the three soils would be : Zn>Cd>Cu>Pb>NiL&egatd;C r.i sotopes in some extracts of the extraction procedure corroborate the anthropogenic inputs for two soils. These both infomations allow to trace the origin, the mobility and the distribution of Pb in the soil

Dissolved organic matter contribution to rain water, throughfall and soil solution chemistry

A method is proposed to determine the acidbase properties of natural water samples containing relatively high amounts of dissolved organic matter. The electroneutrality principle as well as titration data are used to estimate the organic anion concentration in open field precipitation, throughfall and soil solutions, and to develop empirical models based on pH and dissolved organic carbon content. The organic acids dissolved in throughfall have a similar acidic site density but are weaker than those dissolved in soil solution, stream and lake waters. This method is usefull to determine the contribution of organic anions to the charge balance and to the buffering capacity of dissolved organic rich waters with low acid neutralizing capacity. It can be used also to determine the respective contribution of natural organics and anthropogenic minerals to the total acidity of throughfall and rain waters

Variability of the chemical index of alteration (CIA) in the Paraná River suspended load

Variability of the chemical index of alteration (CIA) in the Paraná River suspended loa

Composition chimique des eaux et variabilité naturelle

La composition chimique d’une eau, avant toute perturbation anthropique,est contrôlée par son milieu environnemental (atmosphère, biosphère, sols et roches). En suivant le cycle de l’eau, depuis l’atmosphère vers les eaux continentales puis les océans,la concentration des eaux en éléments majeurs et traces et leur composition chimique changent grandement. Les précipitations atmosphériques sont faiblement concentrées en éléments majeurs (du μg/L au mg/L) et majoritairement composées de sodium (Na), de chlorure (Cl) et de sulfate (SO4), alors que les rivières et les nappes sont plus concentrées (10 à 102 mg/L) et composées surtout de calcium (Ca),de magnésium (Mg) et de bicarbonate(HCO3) (figure 1)