Analyse du fonctionnement biologique du sol par l'étude de la nématofaune : semis direct versus labour sur les hautes terres près d'Antsirabé (Madagascar)

International audienceLes nématodes du sol possèdent des qualités multiples (abondance, diversité taxonomique et fonctionnelle, existence d'indices liés à l'état du sol) qui en font des indicateurs pertinents du fonctionnement du sol. Ils intègrent un grand nombre d'informations sur l'état de la micro-chaîne trophique du sol (compartiment microbien, microfaune et mésofaune) qui est responsable de la décomposition et de la minéralisation des nutriments. La nématofaune a été analysée dans un essai comparant le semis direct avec restitution des résidus de récolte, au labour avec exportation des résidus de récolte dans une rotation soja/riz sur les hautes terres de Madagascar. Les résultats obtenus durant les deux années d'étude (année en soja & année en riz) ne sont pas significativement différents pour une grande majorité de paramètres nématologiques ce qui signifie qu'à moyen terme de cette expérience (essai mis en place depuis 10 ans), les différences interannuelles liées à la culture sont très inférieures aux différences liées au travail du sol et à la fertilisation. L'absence de travail du sol simultané à la restitution des résidus de récolte, ainsi que la fertilisation minérale et organique, induit une augmentation de l'abondance de la nématofaune tellurique. L'analyse de la composition de la nématofaune indique que les densités de nématodes phytophages d'une part et de nématodes omnivores et prédateurs d'autre part, sont significativement augmentées dans les systèmes en semis direct, alors que les fertilisations organiques et minérales induisent une augmentation de la plupart des groupes trophiques incluant également les microbivores (bactérivores et fongivores). L'analyse des indices nématofauniques montre que le travail du sol ainsi que l'absence de fertilisation sont aussi responsables de la simplification qualitative de la micro-chaîne trophique du sol. Les systèmes en semis direct présentent une nématofaune plus complexe et moins opportuniste incluant des taxons sensibles aux perturbations contrairement au système labouré

Activity, size and structure of the nitrate reducer community in relation to land-use and fertilization regime in Madagascar

Communication orale, résuméabsen

Differential Responses of Nitrate Reducer Community Size, Structure, and Activity to Tillage Systems▿ †

The main objective of this study was to determine how the size, structure, and activity of the nitrate reducer community were affected by adoption of a conservative tillage system as an alternative to conventional tillage. The experimental field, established in Madagascar in 1991, consists of plots subjected to conventional tillage or direct-seeding mulch-based cropping systems (DM), both amended with three different fertilization regimes. Comparisons of size, structure, and activity of the nitrate reducer community in samples collected from the top layer in 2005 and 2006 revealed that all characteristics of this functional community were affected by the tillage system, with increased nitrate reduction activity and numbers of nitrate reducers under DM. Nitrate reduction activity was also stimulated by combined organic and mineral fertilization but not by organic fertilization alone. In contrast, both negative and positive effects of combined organic and mineral fertilization on the size of the nitrate reducer community were observed. The size of the nitrate reducer community was a significant predictor of the nitrate reduction rates except in one treatment, which highlighted the inherent complexities in understanding the relationships the between size, diversity, and structure of functional microbial communities along environmental gradients

Near infrared reflectance spectroscopy (NIRS) could be used for characterization of soil nematode community

International audienceStudying soil nematofauna provides useful information on soil status and functioning but requires high taxonomic expertise. Near infrared reflectance (NIR) spectroscopy (NIRS) has been reported to allow fast and inexpensive determination of numerous soil attributes. Thus the present study aimed at assessing the potential of NIRS for determining the abundance and diversity of soil nematodes in a set of 103 clayey topsoil samples collected in 2005 and 2006 from agricultural soils in the highlands of Madagascar. The morphological characterization of soil nematofauna involved extraction through elutriation then counting under binoculars and identification at family or genus level using microscopy, on ca. 150-g fresh soil samples. Taxa were assigned to five trophic groups, namely bacterial feeders, fungal feeders, obligate plant feeders, facultative plant feeders, and omnivores and predators (together). In addition, four ecological indexes were calculated: the Enrichment index, Structure index, Maturity index, and Plant parasitic index. Oven-dried (40 degrees C) = 2) or acceptable (RPD >= 1.6). Predictions were accurate, acceptable, or quasi-acceptable (RPD >= 1.4) for several of the six most abundant taxa, and to a larger extent, for most trophic groups (except facultative plant feeders); but they could not be made for taxa present in a small number of samples or at low abundance. By contrast, NIRS prediction of relative abundances (in proportion of total abundance), was poor in general, as was also the prediction of ecological indexes (except for the 2006 set). On the whole, these results were less accurate than NIRS predictions of soil attributes often reported in the literature. However, though not very accurate, NIRS predictions were worthwhile considering the labor-intensity of the morphological characterization. Most of all, NIRS analyses were carried out on subsamples that were probably too small (5 g) to allow representative sampling of nematofauna. Using larger samples for NIRS (e.g. 100 g) would likely result in more accurate predictions, and is therefore recommended. Scanning un-dried samples could also help improve prediction accuracy, as morphological characterization was carried out on samples not dried after sampling. Examining wavelengths that contributed most to NIRS predictions, and chemical groups they have been assigned to, suggested that NIRS predictions regarding nematofauna depended on constituents of both nematodes and preys' food. Predictions were thus based on both nematofauna and soil organic properties reflected by nematofauna

Impact of fertilization, tillage and direct seeding on nitrate reducers in Madagascar

National audienc

Near infrared reflectance spectroscopy (NIRS) could be used for characterization of soil nematode community

Studying soil nematofauna provides useful information on soil status and functioning but requires high taxonomic expertise. Near infrared reflectance (NIR) spectroscopy (NIRS) has been reported to allow fast and inexpensive determination of numerous soil attributes. Thus the present study aimed at assessing the potential of NIRS for determining the abundance and diversity of soil nematodes in a set of 103 clayey topsoil samples collected in 2005 and 2006 from agricultural soils in the highlands of Madagascar. The morphological characterization of soil nematofauna involved extraction through elutriation then counting under binoculars and identification at family or genus level using microscopy, on ca. 150-g fresh soil samples. Taxa were assigned to five trophic groups, namely bacterial feeders, fungal feeders, obligate plant feeders, facultative plant feeders, and omnivores and predators (together). In addition, four ecological indexes were calculated: the Enrichment index, Structure index, Maturity index, and Plant parasitic index. Oven-dried (40 degrees C) = 2) or acceptable (RPD >= 1.6). Predictions were accurate, acceptable, or quasi-acceptable (RPD >= 1.4) for several of the six most abundant taxa, and to a larger extent, for most trophic groups (except facultative plant feeders); but they could not be made for taxa present in a small number of samples or at low abundance. By contrast, NIRS prediction of relative abundances (in proportion of total abundance), was poor in general, as was also the prediction of ecological indexes (except for the 2006 set). On the whole, these results were less accurate than NIRS predictions of soil attributes often reported in the literature. However, though not very accurate, NIRS predictions were worthwhile considering the labor-intensity of the morphological characterization. Most of all, NIRS analyses were carried out on subsamples that were probably too small (5 g) to allow representative sampling of nematofauna. Using larger samples for NIRS (e.g. 100 g) would likely result in more accurate predictions, and is therefore recommended. Scanning un-dried samples could also help improve prediction accuracy, as morphological characterization was carried out on samples not dried after sampling. Examining wavelengths that contributed most to NIRS predictions, and chemical groups they have been assigned to, suggested that NIRS predictions regarding nematofauna depended on constituents of both nematodes and preys' food. Predictions were thus based on both nematofauna and soil organic properties reflected by nematofauna