Activity Enhancement of MgCl2-supported Ziegler-Natta Catalysts by Lewis-acid Pre-treatment for Ethylene Polymerization

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Modeling the biomass allocation of tree resprout in a fire-prone savanna

International audienceYoung savanna trees can quickly grow back from belowground storage structures after topkill. This capacity is atolerance trait that confers persistence at the plant individual level, enabling them to survive diverse disturbanceregimes.We simulated the growth of a single resprouting stem (without and with fire) of a deciduous tree species thatallocates its photo-assimilates during the vegetation season to reserves, belowground and aboveground parts(leaves and stem). As savannas grow under highly seasonal climates, the model considers that trees are leaflessduring the dry season and following growth is only possible thanks to reserves. Stem architecture constrains theleaf biomass to be proportional to stem length rather than biomass. We compared the success of differentallocation strategies, with and without fire and according to the seasonality. To do so, the height of theresprouting stem after 50 yrs and the time to reach 2 m were modeled for three species of a humid savanna.The viable and faster growth strategies are those for which allocation to belowground parts is <40%. There isvery little sensitivity to allocation to reserves since successful growth is observed for allocation to reserves between 0.5% and 85% of photo-assimilates. In the literature and in our results, there is little impact of fire on thestem height or the time needed to escape the fire trap. Our model suggests that (1) allocation to leaves isdeterminant as leaves are the primary source of assimilates that can then be turned into fire-resistant structures(reserves and roots) and (2) fire only weakly slows down the plant growth compared to dry seaso

Modelling facilitation or competition within a root system: importance of the overlap of root depletion and accumulation zones

International audienceAims: The concept of intra-plant, inter-root competition considers the overlap of nutrient depletion zones around roots, but neglects the spatial pattern of root exudates that can increase nutrient availability. We tested the hypothesis that interactions between nutrient accumulation zones due to exudation by different roots can lead to intra-plant inter-root facilitation.Methods: We used the PARIS model (Raynaud et al. 2008) to simulate phosphorus uptake by a population of roots that are able to increase phosphorus availability by exuding citrate. We carried out several simulations with the same parameters but with increasing root density in order to study out if changes in root densities would alter nutrient uptake per unit root.Results: Emerging relationships between root uptake efficiency and root length density indicated cases of inter-root competition or facilitation. The sizes of the accumulation and depletion zones were calculated to explain these results. Our simulations showed a continuum between cases of inter-root competition and facilitation. Facilitation occurred at low exudation rates, when phosphorus supply was not saturated within the phosphorus depletion zone surrounding roots. Low exudation systems led to a lower phosphorus uptake per unit root length, but minimized phosphorus losses in the process.Conclusions: Based on our model, we derived conditions that allowed predicting whether competition, facilitation or no interaction, is the dominant interaction between roots within a root system, based on the different distances to which an isolated root alters P concentration and supply

Biodiversité et fonctionnement écologique des sols

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Biodiversité et fonctionnement écologique des sols

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Spatial heterogeneity in nitrification and soil exploration by trees favour source–sink dynamics in a humid savanna: A modelling approach

International audienceSavannas are structured ecosystems characterized by a grass layer interspersed with trees. Trees strongly modify their local environment and favour nutrient accumulation under their canopies. Tree roots can also forage horizontally far beyond the canopy projection to increase nutrient uptake. In the Lamto savanna (Cote d'Ivoire), grasses are able to inhibit nitrification while trees stimulate it.Here, we used a two-patch model simulating nitrogen (N) dynamics in a humid savanna between an open patch (without tree) associated with a low nitrification rate and a patch of tree clump associated with a high nitrification rate. The model also includes horizontal N fluxes between these two patches corresponding to horizontal soil exploration by tree roots. We analysed the impact of spatial heterogeneity in nitrification and soil horizontal exploration on N budget and plant biomass.Despite high N losses under trees due to nitrification stimulation by trees, our results show that the ability of trees to explore horizontally the open allows them to uptake more nutrients in total. This leads to an asymmetric N flux from the open to tree clumps, which contributes to nutrient enrichment under tree clumps and thereby to tree growth.Although trees have the ability to horizontally explore the soil to accumulate nutrients under their canopy, increasing the surface occupied by tree clumps increases N losses per hectare of savanna due to the increased nitrification under trees and the subsequent increase in NO3- leaching.While perennial savanna grasses show a restricted horizontal soil exploration to control nutrient availability, our results predict that the extension of tree roots outside their canopy increases their nutrient acquisition in the Lamto savanna. This study is the first one emphasizing the influence of horizontal exploration of trees and tree cover on savanna N budget and functioning. Overall, the proportion of tree cover and horizontal soil exploration are important factors to consider in savannas characterized by spatial heterogeneity in N cycling created by trees and grasses. These factors appear critical to the functioning of West African humid savannas and should be investigated in other savanna types.A free Plain Language Summary can be found within the Supporting Information of this article

Effects of Mineral Nitrogen Partitioning on Tree–Grass Coexistence in West African Savannas

International audienceCoexistence between trees and grasses in savannas is generally assumed to be due to a combination of partial niche separation for water acquisition and disturbances impacting the demography of trees and grasses. We propose a mechanism of coexistence solely based on the partitioning of the two dominant forms of mineral nitrogen (N), ammonium (NH4+) and nitrate (NO3−). We built a mean-field model taking into account the capacity of grasses and trees to alter nitrification fluxes as well as their relative preferences for NH4+ versus NO3−. Two models were studied and parameterized for the Lamto savanna (Côte d’Ivoire): In the first model, the nitrification only depends on the quantity of available NH4+, and in the second model the nitrification rate is also controlled by tree and grass biomass. Consistent with coexistence theories, our results show that taking these two forms of mineral N into account can allow coexistence when trees and grasses have contrasting preferences for NH4+ and NO3−. Moreover, coexistence is more likely to occur for intermediate nitrification rates. Assuming that grasses are able to inhibit nitrification while trees can stimulate it, as observed in the Lamto savanna, the most likely case of coexistence would be when grasses prefer NH4+ and trees NO3−. We propose that mineral N partitioning is a stabilizing coexistence mechanism that occurs in interaction with already described mechanisms based on disturbances by fire and herbivores. This mechanism is likely relevant in many N-limited African savannas with vegetation composition similar to the one at the Lamto site, but should be thoroughly tested through empirical studies and new models taking into account spatiotemporal heterogeneity in nitrification rates

Multifunctionality is affected by interactions between green roof plant species, substrate depth, and substrate type

Summary Green roofs provide ecosystem services through evapotranspiration and nutrient cycling that depend, among others, on plant species, substrate type, and substrate depth. However, no study has assessed thoroughly how interactions between these factors alter ecosystem functions and multifunctionality of green roofs. We simulated some green roof conditions in a pot experiment. We planted 20 plant species from 10 genera and five families (Asteraceae, Caryophyllaceae, Crassulaceae, Fabaceae, and Poaceae) on two substrate types (natural vs. artificial) and two substrate depths (10 cm vs. 30 cm). As indicators of major ecosystem functions, we measured aboveground and belowground biomasses, foliar nitrogen and carbon content, foliar transpiration, substrate water retention, and dissolved organic carbon and nitrates in leachates. Interactions between substrate type and depth strongly affected ecosystem functions. Biomass production was increased in the artificial substrate and deeper substrates, as was water retention in most cases. In contrast, dissolved organic carbon leaching was higher in the artificial substrates. Except for the Fabaceae species, nitrate leaching was reduced in deep, natural soils. The highest transpiration rates were associated with natural soils. All functions were modulated by plant families or species. Plant effects differed according to the observed function and the type and depth of the substrate. Fabaceae species grown on natural soils had the most noticeable patterns, allowing high biomass production and high water retention but also high nitrate leaching from deep pots. No single combination of factors enhanced simultaneously all studied ecosystem functions, highlighting that soil-plant interactions induce trade-offs between ecosystem functions. Substrate type and depth interactions are major drivers for green roof multifunctionality. K E Y W O R D S ecosystem services, evapotranspiration, nitrogen and carbon cycles, soil-plant interactions, trade-offs, urban ecology, water retention 2358 | DUSZA et Al

Explore less to control more: why and when should plants limit the horizontal exploration of soil by their roots?

International audienceIn ecosystems limited by soil nutrients, some plants show a restricted horizontaldistribution of their roots. We explored the hypothesis that this particular pattern is aforaging strategy emerging from trade-offs between soil exploration (that increases the poolof nutrients available for plants) and the local control of nutrient cycling within the soil thatwe call soil occupation. We developed two general analytical models of the cycling of alimiting nutrient in a plant population that is not limited by water. They allowed to explorehow plant productivity is affected when roots do not exploit the whole soil available and todetermine the conditions for which plant nutrient stock is maximized when plants limittheir exploration of soil. We predict that a restricted exploration strategy can be beneficialwhen (1) there is at least one trade-off between a nutrient cycling parameter and soilexploration, (2) nutrient availability in the unexplored soil is poor and (3) the area of soilexplored by plants is stable over time. The exploration limitation strategy results inspatially heterogeneous and nutrient-conservative ecosystems. Our results should applywell to perennial tussock grasses within tropical nutrient-limited ecosystems and raisesinteresting cues for the construction of more sustainable agro-ecosystems. Overall, ourstudy underlines the importance of considering the multiplicity of root-soil interactions andof their scales when considering root foraging strategies