Biomasse microbienne et "statut organique" des sols tropicaux : exemple d'un sol vénézuélien des Llanos sous différents systèmes de culture

National audienc

Les sols de savanes des llanos vénézuéliens et le sol ferrugineux tropical de Barinas = Los suelos de sabanas de los llanos venezolanos y el alfisol de Barinas

Les savanes constituent une des dernières ressources en terres dont la mise en culture est relativement facile malgré de faibles réserves organiques et minérales qui ne leur assurent qu'un équilibre assez fragile. La station expérimentale du Jardin botanique de l'université expérimentale des Llanos à Barinas sert de cadre depuis 1977 à des expériences effectuées sur un sol ferrugineux tropical lessivé. Le présent travail est consacré à la description : du contexte régional de la station; du profil du sol utilisé en comparaison avec d'autres sols de savanes; de quelques résultats obtenus dans la station et dans la région sur des types de sol voisins. Avant de conclure est discutée la représentativité de la station de Barinas au niveau des savanes en général en vue d'établir la portée des expériences qui y sont ou y seront effectuées. (Résumé d'auteur

80-річчя академіка НАН України В.І. Старостенка

Qualitative and quantitative changes in soil and fertilizer-derived organic N fractions were assessed during a cropping season in an intertropical Alfisol, under maize and pasture, fertilized with15 N-urea. Before the sowing, after fertilizing and after the harvest, the organic N of top soil samples was fractionated by a two-step acid hydrolysis under reflux (H1 = 1 M HCl for 3 h; H2 = 3 M HCl for 3 h). The total hydrolysable N (HN) from H1 decreased significantly during the cropping season in both maize and pasture soils. Contrastingly, the content of HN from H2 and that of non-hydrolysable N did not vary significantly during the cropping season. The easily hydrolysable fractions, especially amino acid N, amino sugar N and amide N, were the most active N pools and the major source of N potentially available for plants. The urea-derived N that remained in the soil was mainly in organic forms at both 7 and 108 d after fertilizing (70–82% and 93–98%, respectively), higher figures being found in pasture than in maize soil. The total amount of urea-derived HN decreased significantly during the crop period in both maize and pasture soils. This decrease was largely due to the decline in HN from H1. The amount of non-hydrolysable urea-derived N was significantly higher in pasture than in maize soil and it decreases in the former and increases in the latter, during the cropping season. During the crop period, the decrease of urea-derived organic N was 4.6 to 9.1 times higher than that of native organic N. Shortly after fertilizing, the proportion of urea-derived N in the easily hydrolysable (H1) organic fractions was higher than that of soil N, whereas the reverse was true for the slowly hydrolysable (H2) or insoluble fractions. These differences were less marked, but still significant, at the end of cropping. The easily hydrolysable organic N fractions were more sensitive than total N to the impact of land use intensification and are, therefore, a more useful index for early detection of soil biological degradation.Peer reviewe

Une vie d’avatar, ou les rebonds virtuels de la présence

International audienc