Small-scale heterogeneity in carbon dioxide, nitrous oxide and methane production from aggregates of a cultivated sandy-loam soil

Spatial variability in carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) emissions from soil is related to the distribution of microsites where these gases are produced. Porous soil aggregates may possess aerobic and anaerobic microsites, depending on the water content of pores. The purpose of this study was to determine how production of CO₂, N₂O and CH₄ was affected by aggregate size and soil water content. An air-dry sandy loam soil was sieved to generate three aggregate fractions (<0.25 mm, 0.25–2 mm and 2–6 mm) and bulk soil (<2 mm). Aggregate fractions and bulk soil were moistened (60% water-filled pore space, WFPS) and pre-incubated to restore microbial activity, then gradually dried or moistened to 20%, 40%, 60% or 80% WFPS and incubated at 25 °C for 48 h. Soil respiration peaked at 40% WFPS, presumably because this was the optimum level for heterotrophic microorganisms, and at 80% WFPS, which corresponded to the peak N₂O production. More CO₂ was produced by microaggregates (<0.25 mm) than macroaggregate (>0.25 mm) fractions. Incubation of aggregate fractions and soil at 80% WFPS with acetylene (10 Pa and 10 kPa) and without acetylene showed that denitrification was responsible for 95% of N₂O production from microaggregates, while nitrification accounted for 97–99% of the N₂O produced by macroaggregates and bulk soil. This suggests that oxygen (O₂) diffusion into and around microaggregates was constrained, whereas macroaggregates remained aerobic at 80% WFPS. Methane consumption and production were measured in aggregates, reaching 1.1–6.4 ng CH₄–C kg⁻¹ soil h⁻¹ as aggregate fractions and soil became wetter. For the sandy-loam soil studied, we conclude that nitrification in aerobic microsites contributed importantly to total N₂O production, even when the soil water content permitted denitrification and CH₄ production in anaerobic microsites. The relevance of these findings to microbial processes controlling N₂O production at the field scale remains to be confirmed.     

Dry matter partitions and specific leaf weight of soybean change with tree competition in an intercropping system

In 2004 and 2005, the yield, leaf area, dry weight and dry weight partitions of soybeans were determined at the Agroforestry Research Site (ARS) (est. 1987, Ontario, Canada). Soybean was intercropped with poplar (Populus deltoides x nigra DN-177 L., 556 m³crown tree⁻¹), silver maple (Acer saccharinum L., 308 m³), black walnut (Juglans nigra L., 148 m³) and pecan (Carya illinoensis Wangenh., 114 m³), or grown alone (monoculture). Yield of soybean was not different in either year between the monoculture and the black walnut or pecan intercrops. In the poplar and silver maple treatments, yield was 66 and 85% (2004 and 2005) lower than in the monoculture. Despite the fact that different tree species were used, there was a significant negative linear regression between yield and tree crown volume (R ² = 0.76, P = 0.0049 and R ² = 0.93, P < 0.0001 in 2004 and 2005, respectively). With increasing tree crown volume, soybean tended to partition more dry matter to the photosynthetic and reproductive parts and less to structural tissue and petiole. This demonstrates the phenotypic flexibility of the crop component in agroforestry systems. Contrary to theoretical predictions, soybean leaves were thicker as shade increased (increase by 6.2 × 10⁻⁴–1.2 × 10⁻³ mg cm⁻², per unit of crown volume), pointing to competitive interactions specific to tree-based intercrops.     

Strategie de lutte chimique contre la morelle jaune (Solanum elaeagnifolium Cav.) au Maroc*

La lutte chimique contre Solanum elaeagnifolium Cav. a fait l’objet de nombreux travaux de recherche, et ce grâce aux nombreuses expérimentations menées sur un grand nombre d’herbicides (doses, stades d’application, conditions climatiques, biotopes traités, etc.). Les résultats montrent que les herbicides appliqués en cours de végétation n’aboutissent générallement pas à un bon contrôle de l’adventice. En effet, les doses d’herbicides sélectifs de ces cultures sont peu efficaces contre S. elaeagnifolium. Le glyphosate, le sulfosate et l’aminotriazole – herbicides systémiques – se sont montrés très performants et sont préconisés principalement pour le traitement non sélectif des vergers d’oliviers, d’agrumes et d’arbres fruitiers ainsi que des parcelles en postrécolte des cultures (céréales, betterave à sucre, maraîchage). Ils nécessitent des conditions d’application particulières et sont conseillés surtout en cas de fortes infestations. Les phytohormones, l’imazapyr et le bromacile sont utilisés pour le désherbage des bordures de routes publiques boisées et/ou non boisées et éventuellement les parcelles non exploitées. Le bromacile peut être utilisé dans les vergers d’agrumes agés de plus de 4 ans, mais uniquement sur des zones fortement infestées compte tenu de son coût élevé. Par le biais du transfert de technologie chez les agriculteurs, ces derniers ont été convaincus de la possibilité de contrôler cette adventice par le glyphosate et d’autres herbicides lorsqu’un certain nombre de conditions sont respectées. En se basant sur ces résultats, des opérations de lutte chimique ont été menées à grande échelle par les organismes de développement.     

Etat actuel de l’infestation par la morelle jaune (Solanum elaeagnifolium Cav.) au Maroc*

Signalée pour la première fois au Maroc en 1949, la morelle jaune (Solanum elaeagnifolium Cav.) est devenue à partir des années 80 une adventice préoccupante à l’échelle nationale. Dans un premier temps, l’infestation est restée limitée à la région du Tadla. Mais vingt ans ont suffi à la morelle jaune pour sortir de son berceau et se propager vers des régions éloignées. Des études et des enquêtes effectuées au Maroc ont permis de souligner la grande diversité et l’efficacité de facteurs de dissémination tels que: le fumier organique, les ovins, l’eau d’irrigation, les machines agricoles, les plants en motte et le vent. Toutefois, les trois premiers facteurs restent les plus importants. Suivant les conditions locales, certains de ces facteurs de dissémination deviennent prépondérants et peuvent être simultanés ou successifs.     

Extracts from Marine Macroalgae and Opuntia ficus-indica Cladodes Enhance Halotolerance and Enzymatic Potential of Diazotrophic Rhizobacteria and Their Impact on Wheat Germination Under Salt Stress

Soil salinity, which affects more than 6% of the earth’s land surface and more than 20% of its irrigated areas, is a major threat to agriculture. Diazotrophic bacteria are among the functional groups of soil microbiota that are threatened by this abiotic stress, as their activity is mostly inhibited by salt stress. Seventy bacterial strains with distinct characteristics were isolated from soils by using N-free Jensen’s selective medium. Based on their ability to produce metabolites of agricultural interest, four strains were selected and identified as Flavobacterium johnsoniae, Pseudomonas putida, Achromobacter xylosoxidans, and Azotobacter chroococcum. The selected strains were grown at different NaCl concentrations (0–600 mmol L~(-1) in N-free broth and 0–2 000 mmol L~(-1) in Luria-Bertani medium) in the presence and absence of glycine betaine (GB), aqueous and hydro-alcoholic extracts from marine macroalgae, Ulva lactuca and Enteromorpha intestinalis, and Opuntia ficus-indica cladodes. The selected bacterial strains, GB, and the aforementioned extracts were tested for their ability to promote the germination of wheat (Triticum durum) seeds at 0–300 mmol L~(-1) NaCl. Compared with the results obtained with the synthetic osmoprotectant GB, the extracts from O. ficus-indica, U. lactuca, and E. intestinalis significantly promoted bacterial growth and seed germination under salt stress.     

Impacts of small hill dams on agricultural development of hilly land in the Jendouba region of northwestern Tunisia

We examine the shifting farming system around the small hill dams in northwestern Tunisia. A socio-economic questionnaire was given to a sample of farmers using water from eight representative small hill dams. Cluster analysis was used to establish farmer groups before and after creation of the reservoirs. Before the creation of hill dams, farmers were grouped into two clusters. Afterwards, three new farming systems emerged. The main differences between these groups are farm area, cropping system, irrigation practice, livestock type and number, and income. Following construction of the small hill dams farmers increased the extent of fruit tree cultivation, introduced irrigation practices, and augmented livestock herd size and production. Farmer income increased with the availability of water in the hill dam reservoirs.     

Revue des moyens de lutte contre la morelle jaune (Solanum elaeagnifolium Cav.) au Maroc

Cette revue présente les méthodes de lutte contre la morelle jaune explorées au Maroc. Concernant la lutte culturale, l'emploi d'une culture étouffante comme la luzerne ( Medicago sativa L.) en début de rotation réduit sensiblement l'infestation de cette adventice grâce aux fréquentes coupes, à la grande compétition et à l'effet allélopathique qu'exerce cette culture sur la morelle jaune. En lutte mécanique, trois méthodes ont démontré leur efficacité : le labour profond à la charrue à trois disques, le sweep (outil de 2,5 m de largeur composé de trois lames en V) et la sape (outil manuel qui coupe la plante au niveau du sol). Le labour profond (> 30 cm) et le sweep sont préconisés en début d'été et en post-récolte des cultures. L'intervention à la sape, utilisée 2 à 3 fois par an au stade initiation florale dans différents biotopes, permet l'affaiblissement progressif de l'adventice à moyen terme. La luzerne, le labour profond et le sweep sont conseillés pour les infestations moyennes àélevées. En outre, les interventions mécaniques combinées aux herbicides (le glyphosate, le sulfosate, l'aminotriazole et les phytohormones) donnent souvent de bonnes efficacités. La combinaison de ces méthodes est vivement souhaitable pour les grandes superficies où l'infestation est moyenne à forte.     

Transgressive segregation for resistance to Pyrenophora teres in barley

Populations of F₁, F₂, F₁× parent 1 (BC1), F₁× parent 2 (BC2), and F₃ resulting from two crosses between four susceptible varieties of barley used in national and international breeding programmes were tested at the seedling stage for their resistance to an isolate of Pyrenophora teres which is virulent to the parental varieties. Infection type, average lesion size, and number of lesions per unit leaf area were used to assess disease reaction. In the two crosses, F₂ and especially F₃ generations (produced by self pollination of selected resistant F₂ plants) showed more resistance expressed by infection type and average lesion size than either parent. The frequency distributions of F₂ and F₃ generations derived from these crosses were continuous and showed transgressive segregation for resistance. The results indicated that the four cultivars used in this study possessed additive genes for resistance to P. teres. Thus useful resistance could be obtained following crossing of susceptible cultivars and selection in succeeding generations.