『Abstract
　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（eの頭に´）, 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 (CO2ATM-SOIL/HCO3) is less than 50％ (expected value for carbonate basins), and it decreases when the nitrate concentration increase. The 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 Tg(10 ¹²g) 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.』

1. Introduction
2. Study area
　2.1. Geomorphology, bedrock mineralogy and petrology
　2.2. Soil, land use and agricultural practices
　2.3. Hydrology and climatology
3. Materials and methods
　3.1. Sampling sites
　　3.1.1. Network of small catchments: spatial approach
　　3.1.2. Experimental catchments: seasonal variation surveys
　3.2. Sampling and analytical methods
　3.3. Calculation of CO2 uptake by chemical weathering
　3.4. Data in the literature
4. Results
　4.1. Chemical composition of stream waters
　4.2. Atmospheric inputs
　4.3. Relationship between Ca²⁺ + Mg²⁺ and alkalinity in stream waters
5. Discussion
　5.1. Role of vegetation
　5.2. N-fertilizer role
　5.3. Loss of alkalinity
　5.4. Influence of N-fertilizers on atmospheric/soil CO2 uptake by carbonate dissolution
　5.5. Implications for regional and global atmospheric CO2 source/sink
6. Conclusion
Acknowledgments
References