Investigation of Biogeochemical Activities in the Soil and Unsaturated Zone of Weathered Granite

International audienceThis study, based on field and laboratory work, investigates the biogeochemical activity below the organic top soil horizons, particularly the potential for nitrate removal processes in the deep vadose zone (1–2.5 m depth) of a weathered granite. An experimental site located in the Kerbernez agricultural catchment (Brittany) has been equipped with ceramic cups from 0.5 to 2.5 m depth since November 2001. This arrangement allowed collection of water samples from the soil profile and the upper part of the unsaturated weathered granite. Samples were analysed twice a month for chloride, nitrate and sulphate concentrations over a period of 2.5 years. Laboratory measurements were carried out on three soil horizons and four weathered granite facies sampled in October 2003 for hydrolasic activity, potential nitrification, potential denitrification and batch experiments to study nutrient dynamics. Anion analyses in the field show that the nitrate and chloride concentrations were linearly correlated at each depth. The nitrate/chloride ratio decreased with depth in the upper part of the weathered granite from 4.93 to 2.82. This suggests that nitrate was removed during its vertical transport in the unsaturated zone. The laboratory experiments show that the bacterial activity decreased with depth. However, a significant potential for biogeochemical reactions exists below the organic soil layers. The denitrification rates obtained in the laboratory were significant, up to 800 ng N h−1 g−1 after about 100 h of incubation for the most reactive weathered granite facies. These rates agree with effective rates usually measured in riparian zones, but they were 50 times higher than those observed in the field. This difference suggests that the denitrification processes occurring in the field were spatially limited to localised anaerobic microsites, where the bacterial activities are controlled by the availability of N and C substrate. Finally, the laboratory measurements lead us to assume that heterotrophic denitrification was clearly the predominant process occurring in the field because of the good correlation between nitrate concentration variation and carbon content (r = −0.94). Moreover, the slight increase in sulphate concentrations observed in the field and in the laboratory was insufficient to explain the complete removal of nitrate