Effects of livestock on nitrogen and carbon cycling in a savanna in Burkina Faso

International audienceThe nitrogen and carbon cycles are fundamental ecosystem processes influenced by several factors including soil type and other abiotic factors, plant species, grazing and soil organisms. Herbivores profoundly influence the functioning of ecosystems and the recycling of nutrients in interaction with plants in natural ecosystems. This study focuses on the effects of livestock on carbon and nitrogen cycling in a grazed savanna in Burkina Faso. Dominant grass species (aerial and root parts) and soil samples were collected under grasses and bare soil patches in 48 plots (24 protected and 24 unprotected plots), 18 months after setting up herbivores exclosures. Soil and grass 13C and 15N were used as integrative indicators of carbon and nitrogen cycles. The results revealed no significant effect of livestock on soil total carbon and nitrogen and on soil δ13C and δ15N values. Moreover, grazing had no significant effect on grass carbon and δ13C, while it significantly increased grass total nitrogen and δ15N. Therefore, our hypothesis that grazing would increase soil 13C and 15N values and plant biomass was only verified for grass 15N. Grass δ15N results suggest that grazing improves the immediate availability of nitrogen but could also increase nitrogen losses

Effets Du Pâturage Sur La Biomasse Herbacée et Sur Des Paramètres Chimiques et Biologiques Des Sols Dans Une Savane Arbustive Au Burkina Faso

Cette étude a porté sur les sols et la végétation herbacée d’une savane pâturée au Burkina Faso. Elle décrit les effets du pâturage sur la biomasse herbacée et sur les paramètres chimiques et biologiques des sols. Pour ce faire, des couples de parcelles (protégées et non protégées du pâturage) ont été installés dans les faciès des quatre espèces de graminées dominantes : deux pérennes (A. ascinodis, A. gayanus) et deux annuelles (A. pseudapricus et L. togoensis) afin de tester les effets du pâturage. En plus de la mesure de la biomasse végétale, une évaluation des caractéristiques chimiques et biologiques des sols a aussi été effectuée. Les pHeau et pHKCl, l’ammonium, le nitrate, la respiration basale, la bêta-glucosidase et la fluorescéine di-acétate des sols sont plus élevées sous les pérennes (A. ascinodis et A. gayanus), que sous les annuelles (A. pseudapricus et L. togoensis) et le sol nu. Le carbone total, l’azote total, le phosphore assimilable, le C/N, la biomasse microbienne et la phosphatase acide sont plus élevés sous la pérenne A. ascinodis que sous les autres espèces et le sol nu. Les résultats indiquent que le pâturage diminue significativement la biomasse aérienne sans affecter la biomasseracinaire. Par ailleurs, il n’y a pas d’effet significatif sur les paramètres chimiques et biologiques, excepté pour la respiration basale du sol qui augmente significativement en situation de non pâturage. On a donc obtenu en 18 mois d’exclos peu d’effets sur le fonctionnement du sol mais d’importants effets sur les biomasses herbacées.© 2016 International Formulae Group. All rights reserved.Mots clés: Graminées, herbivore, pâturage, caractéristiques chimiques et biologiques, savaneEnglish Title: Effects of grazing on herbaceous biomass and chemical and biological parameters of soils in a shrub savanna in Burkina FasoEnglish AbstractThis study focuses on the soils and herbaceous vegetation of a grazed savanna in Burkina Faso. It describes the effects of grazing on herbaceous biomass and soil chemical and biological parameters. Pairs of plots (protected from grazing and unprotected) were installed under the four dominant grass species (A. ascinodis, A. gayanus, A. pseudapricus and L. togoensis). In addition to the measure of plant biomass, an assessment of soil chemical and biological characteristics was also performed. pHwater and pHKCl, ammonium, nitrate, basal respiration, β-glucosidase and fluorescein diacetate of soils are higher under perennial grasses (A. ascinodis and A. gayanus), than under annual species (A. pseudapricus and L. togoensis) and bare soil. Total carbon, total nitrogen, available phosphorus, C/N, microbial biomass, acid phosphatase of soil are higher under the perennial grass A. ascinodis than under the other species of grasses and bare soil. Results indicate that grazing decreases significantly herbaceous aboveground biomass (but not root biomass). However, grazing does not impact soil chemical and biological parameters, except for soil basal respiration that increases significantly in grazing situation. Thus, after 18 months of exclosure, we detect very little effect of cattle on soil functioning but important effects on the herbaceous biomass.© 2016 International Formulae Group. All rights reserved.Keywords: Grasses, herbivore, grazing, chemical and biological characteristics, Savann

Contrasted effects of annual and perennial grasses on soil chemical and biological characteristics of a grazed Sudanian savanna

Growth and composition of plant communities depends on physico-chemical and biological characteristics of soils. Conversely, plants influence nutrient cycling and soil characteristics. Thus, they affect the long-term availability of nutrients, which feedbacks on their own growth and the primary productivity of ecosystems. This study focuses on the fertility and functioning of soils of a grazed savanna in Burkina Faso. It describes the effects of annual and perennial grasses involved in fallow succession, on chemical and biological (microbial biomass, mineralization and enzymatic activities) characteristics of soils. To do this, soil samples were collected under the four dominant species of grasses (A. ascinodis, A. gayanus, A. pseudapricus and L. togoensis) and under bare areas within 48 plots, as well as above- and belowground grass biomass. Results show that root biomass, pHKCl, basal respiration and fluorescein diacetate activities were significantly higher under perennial grasses (A. gayanus and A. ascinodis) than annual (A. pseudapricus and L. togoensis) and bare soil. Nitrate levels were higher under A. gayanus that had the lowest root biomass. Total carbon, total nitrogen, microbial biomass and acid phosphatase were higher under the perennial grass A. ascinodis. pHwater was lower under A. pseudapricus. Finally, ammonium and β-glucosidase activities weren’t significantly different between species. Overall, these results appear complex certainly due to factors of variability that remain to be identified. Nevertheless, they support the general hypothesis that perennial grasses have a greater influence on soil than annuals. Soil biological parameters (Basal respiration, microbial biomass, Beta-glucosidase, fluorescein-diacetate and acid phosphorus) exhibited strong relationship with soil pH, total C and N

Contrasting impacts of grass species on nitrogen cycling in a grazed Sudanian savanna

International audienceWe investigated the impact of perennial and annuals grass species on nitrogen cycling in a Sudanian savanna of Burkina Faso. We also analysed how the local context in terms of grazing and soil properties modifies these impacts. We selected four plots differing both by the intensity of grazing by cattle and soil depth, and used soil and grass biomass 15N as integrative indicators of N cycle. If perennials are able to foster a more efficient nitrogen cycling there should be lower 15N abundances in their biomass and soil. If soil depth and cattle pressure significantly modify nitrogen fluxes, soil depth and cattle pressure should influence 15N signatures. Our results suggest that perennial grasses are more conservative for nitrogen (inhibition of nitrification, less leaching via a perennial root system, slower cycling). The increase in leaf δ15N with N concentration is steeper in Loudetia togoensis than in the three other grasses. No significant difference was found between the 15N signatures of the four plots. Our results on 15N signatures and the fact that perennial grasses are much more abundant in the plots that are less grazed and have deeper soils, confirm that the switch from perennial to annual grasses is linked to a degradation in soil fertility and pasture quality. This suggests that 15N signatures can be used as indicators of fertility