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

Spatial heterogeneity of nitrification contributes to tree–grass coexistence in West African savannas

Abstract In savannas, the coexistence between trees and grasses is determined by complex mechanisms based on water partitioning and disturbances. But little is known about the contribution of other resources, such as soil nitrogen (N). In West African savannas, nitrification inhibition by grasses and nitrification stimulation by trees create spatial heterogeneity in nitrification fluxes and N stocks. Savanna trees can also extend part of their roots in the surrounding open area to absorb N. To investigate the role of the spatial heterogeneity of nitrification in tree–grass coexistence, we used a two‐patch model that simulates N dynamics between an open patch (without trees) and a tree clump patch (trees with grasses under their canopy). The open patch was characterized by a low nitrification rate, while the tree clump patch was characterized by a high nitrification rate. Both patches were connected through horizontal fluxes due to soil horizontal exploration by tree roots. We tested coexistence for different spatial tree distributions, as they are known to strongly influence savanna dynamics. Our results show that the spatial heterogeneity of nitrification induces spatial partitioning between ammonium (NH 4 + ) and nitrate (NO 3 − ) promoting tree–grass coexistence. As nitrification inhibition by grasses leads to high NH 4 + availability in the open, the possibilities of coexistence are optimized when trees have different preferences in the open versus under their canopy. Thus, tree–grass coexistence is observed when grasses prefer NH 4 + , while trees prefer NH 4 + in the open and NO 3 − under their canopy. Contrary to random tree distribution, tree clumping enhances tree–grass coexistence. Intraspecific aggregation strengthens the effect of spatial heterogeneity, which decreases interspecific competition and favours tree–grass coexistence. On the contrary, increasing the surface explored by tree roots in the open tends to increase tree–grass competition. This enhances the competitive ability of trees for N acquisition and consequently favours tree invasion. Synthesis . This study shows that this new coexistence mechanism based on mineral N partitioning into NH 4 + and NO 3 − can be determinant in the functioning of West African humid savannas. This mechanism likely interacts with mechanisms based on disturbances, but such interactions should be studied using new models.Résumé Dans les savanes, la coexistence entre les herbes et les arbres est déterminée par des mécanismes complexes basés sur le partage de l'eau et les perturbations. Cependant, il y a très peu d'informations sur la contribution d'autres ressources telles que l'azote (N) du sol. Dans les savanes d'Afrique de l'Ouest, l'inhibition de la nitrification par les herbes et la stimulation par les arbres créent une hétérogénéité spatiale au niveau des flux de nitrification et des stocks de N. Les arbres de savane peuvent également étendre leurs racines dans la zone ouverte environnante pour récupérer l'azote. Pour étudier le rôle de l'hétérogénéité spatiale de la nitrification sur la coexistence herbe‐arbre, nous avons utilisé un modèle à deux patches qui simule les dynamiques d'azote entre le patch ouvert (sans arbres) et le patch bosquet (des arbres avec des herbes sous leur canopée). Le patch ouvert est caractérisé par un faible taux de nitrification alors que le patch bosquet est caractérisé par un fort taux de nitrification. Les deux patchs sont connectés par des flux horizontaux causés par l'exploration horizontale du sol par les racines d'arbres. Nous avons évalué la coexistence pour différentes distributions spatiales d'arbres, car elles sont connues pour influencer les dynamiques des savanes. Nos résultats montrent que l'hétérogénéité spatiale de la nitrification conduit à un partage spatial entre l'ammonium (NH 4 + ) et le nitrate (NO 3 − ) favorisant la coexistence herbe‐arbre. Étant donné que l'inhibition de la nitrification par les herbes augmente la disponibilité de NH 4 + dans la zone ouverte, les possibilités de coexistence sont optimisées quand les arbres ont des préférences différentes dans la zone ouverte versus sous leur canopée. La coexistence est donc observée quand les herbes préfèrent l'NH 4 + , pendant que les arbres préfèrent l'NH 4 + dans la zone ouverte et le NO 3 − sous leur canopée. Contrairement à la distribution aléatoire des arbres, la distribution agrégée des arbres facilite la coexistence herbe‐arbre. L'agrégation intraspécifique renforce l'effet de l'hétérogénéité spatiale, qui diminue la compétition interspécifique et favorise la coexistence herbe‐arbre. Cependant, augmenter la surface explorée par les racines d'arbres dans la zone ouverte a tendance à augmenter la compétition herbe‐arbre. Cela améliore la capacité des arbres à acquérir l'N et par conséquent favorise l'invasion des arbres. Synthèse . Cette étude montre que ce nouveau mécanisme de coexistence basé sur le partage de l'N en NH 4 + et NO 3 − peut être déterminant dans le fonctionnement des savanes humides d'Afrique de l'Ouest. Ce mécanisme interagit probablement avec des mécanismes basés sur les perturbations, mais ces interactions devraient être étudiées grâce à de nouveaux modèles

The causes of the selection of biological nitrification inhibition (BNI) in relation to ecosystem functioning and a research agenda to explore them

International audienceBiological nitrification inhibition (BNI) has already led to several studies mainly focused on underlying molecular mechanisms and applications to agriculture. We argue that it is also important to study BNI more systematically from the ecological and evolutionary points of view to understand its implications for plants and soil nitrifiers as well as its consequences for ecosystems. Therefore, we propose here a dedicated research agenda identifying the most critical research questions: (1) How is BNI distributed across plant phylogeny and why has it been selected? (2) What are the costs-to-benefits balance of producing BNI compounds and the relative impacts on BNI evolution? (3) Can we understand the evolutionary pressures leading to BNI and identify the environmental conditions favorable to BNI plants? (4) How has BNI coevolved with plant preference for ammonium vs. nitrate? (5) Diverse BNI compounds and various inhibition mechanisms have been described, but implications of this diversity are not understood. Does it allow inhibition of various groups of nitrifiers? (6) Does this diversity of BNI compounds increase the efficiency, spatial extension, and duration of BNI effect? (7) What are the impacts of BNI compounds on other soil functions? (8) Can field experiments, coupled to scanning of the diversity of BNI capabilities within plant communities, evaluate whether BNI influences plant-plant competition and plant coexistence? (9) Can field quantification of various nitrogen (N) fluxes assess whether BNI lead to more efficient N cycling with lower losses and hence increased primary production? (10) Can the impact of BNI on N budgets and climate (through its impact on N2O emissions and its indirect impact on carbon budget) be evaluated at the regional scale? We discuss why implementing this research program is crucial both for the sake of knowledge and to develop applications of BNI for agriculture

Demography of the dominant perennial grass species of a humid African savanna

International audiencePerennial grasses are the main source of fuel during fires in savannas. The demography of these grasses likely varies between species although they have the same general architecture and coexist in savannas. However, very few studies compare their demography. Similarly, their demography likely varies between years because of the variability in weather condition and fire intensity. We described and compared the demography and life-cycle of the four dominant perennial grass species (Andropogon canaliculatus, Andropogon schirensis, Hyparrhenia diplandra and Loudetia simplex) of the Lamto savanna (Ivory Coast) and assessed the influence of their demographic features (stasis, fecundity, growth, fragmentation and retrogression) on this demography. Grass species were monitored over three consecutive years (2015-2016, 2016-2017 and 2017-2018) on three 5 m × 10 m plots. We used a size-classified matrix model with 5 circumference classes (3-10 cm, 10-20 cm, 20-35 cm, 35-50 cm and >50 cm). Results showed differences in the age-based parameters, asymptotic growth rates (λs) and elasticities of the λs of the grass species. The population of A. canaliculatus, A. schirensis and H. diplandra were slowly declining while L. simplex was significantly declining. There were noticeable year-to-year variations in the demography of these four species. The most important demographic parameter influencing λs was the stasis in all species, while retrogression and fragmentation contributed to a relative homogeneity of ages between size-classes 2 to 5. This study provides new insights about the demography of Guinean savanna grasses that could be used to describe the mechanisms of their coexistence, and inform fire management policies