Nitrogen dynamics in Turbic Cryosols from Siberia and Greenland

Turbic Cryosols (permafrost soils characterized by cryoturbation, i.e., by mixing of soil layers due to freezing and thawing) are widespread across the Arctic, and contain large amounts of poorly decomposed organic material buried in the subsoil. This cryoturbated organic matter exhibits retarded decomposition compared to organic material in the topsoil. Since soil organic matter (SOM) decomposition is known to be tightly linked to N availability, we investigated N transformation rates in different soil horizons of three tundra sites in north-eastern Siberia and Greenland. We measured gross rates of protein depolymerization, N mineralization (ammonification) and nitrification, as well as microbial uptake of amino acids and NH4+ using an array of 15N pool dilution approaches. We found that all sites and horizons were characterized by low N availability, as indicated by low N mineralization compared to protein depolymerization rates (with gross N mineralization accounting on average for 14% of gross protein depolymerization). The proportion of organic N mineralized was significantly higher at the Greenland than at the Siberian sites, suggesting differences in N limitation. The proportion of organic N mineralized, however, did not differ significantly between soil horizons, pointing to a similar N demand of the microbial community of each horizon. In contrast, absolute N transformation rates were significantly lower in cryoturbated than in organic horizons, with cryoturbated horizons reaching not more than 32% of the transformation rates in organic horizons. Our results thus indicate a deceleration of the entire N cycle in cryoturbated soil horizons, especially strongly reduced rates of protein depolymerization (16% of organic horizons) which is considered the rate-limiting step in soil N cycling.publishedVersio

Simulations des relations populations - ressources naturelles : prototype de modèle pour un terroir du Nord-Cameroun

Ce document présente un modèle de simulation qui permet de comparer l'impact de la croissance démographique sur la gestion des ressources naturelles dans un système agraire des zones de savanes africaines. Le modèle montre comment une population en croissance consomme de plus en plus de ressources jusqu'à un certain seuil en dessous duquel la productivité de ces ressources est affectée. Le modèle réagit en investissant dans les techniques de régénération des ressources. L'objectif principal est de comparer plusieurs technique de restauration de la fertilité comme l'intégration de l'agriculture-élevage, l'agroforesterie ou le semi sous couverture végétale. (Résumé d'auteur