Zouhaier Noumi

Decoupling of soil nutrient cycles as a function of aridity in global drylands

The biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability. The increase in aridity predicted for the twenty-first century in many drylands worldwide may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.

Aridity Modulates N Availability in Arid and Semiarid Mediterranean Grasslands

While much is known about the factors that control each component of the terrestrial nitrogen (N) cycle, it is less clear how these factors affect total N availability, the sum of organic and inorganic forms potentially available to microorganisms and plants. This is particularly true for N-poor ecosystems such as drylands, which are highly sensitive to climate change and desertification processes that can lead to the loss of soil nutrients such as N. We evaluated how different climatic, abiotic, plant and nutrient related factors correlate with N availability in semiarid Stipa tenacissima grasslands along a broad aridity gradient from Spain to Tunisia. Aridity had the strongest relationship with N availability, suggesting the importance of abiotic controls on the N cycle in drylands. Aridity appeared to modulate the effects of pH, plant cover and organic C (OC) on N availability. Our results suggest that N transformation rates, which are largely driven by variations in soil moisture, are not the direct drivers of N availability in the studied grasslands. Rather, the strong relationship between aridity and N availability could be driven by indirect effects that operate over long time scales (decades to millennia), including both biotic (e.g. plant cover) and abiotic (e.g. soil OC and pH). If these factors are in fact more important than short-term effects of precipitation on N transformation rates, then we might expect to observe a lagged decrease in N availability in response to increasing aridity. Nevertheless, our results suggest that the increase in aridity predicted with ongoing climate change will reduce N availability in the Mediterranean basin, impacting plant nutrient uptake and net primary production in semiarid grasslands throughout this region.

The relative contribution of short-term versus long-term effects in shrub-understory species interactions under arid conditions

Abstract Plant–plant interactions (competition and facilitation) in terrestrial ecosystems include: (1) short-term effects, primarily quantified with experimental removals; and (2) long-term effects, mostly due to soil weathering processes, primarily quantified with observational methods. It has been argued that these effects are likely to vary in contrasting directions with increasing drought stress in arid systems. However, few studies have used appropriate methodology to assess both types of effects and their variation across nurse species and environmental conditions, in particular in arid systems. This knowledge is crucial for predicting variation in the mediating role of facilitation with climate change and assessing the importance of nurse effects in ecological restoration. In the arid climate of central-south Tunisia, understory species’ biomass, abundance and composition and soil parameters were compared in shrub-control, shrub-removed and open areas for three shrub species and in two habitats with contrasting soil moisture conditions. Long-term effects were dominant, positive for all three shrub species and associated with increasing nutrient content in shrub patches. Short-term effects, mainly related to water consumption, were weaker, mostly negative and dependent on shrub species. Additionally, long-term effects were less positive and short-term effects more negative in the dry habitat than in the wet habitat. Our study provides evidence of the primary influence of positive (facilitative) long-term effects in this arid system. However, the net effects of shrubs could be less beneficial for other species with increasing aridity under climate change, due to both a decrease in positive long-term effects and an increase in negative short-term effects.

Dynamics of Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan in arid zones of Tunisia

Abstract Acacia tortilis subsp.raddiana is one of the most uncommon species and is still present in North African regions. Indeed, the region of Bled Talah, in the south of Tunisia, constitutes today an ecosystem where the species continues to grow on a large surface of 16,488 ha. There is little information on the long-term dynamics of Acacia trees. The overall objectives of this study are to know the total individuals of Acacia trees which help to assess the density, to study the structure of Acacia population, to estimate the age of Acacia population and to determine the effects of A. tortilis canopy on soil properties. In this context, all individuals of A. tortilis covering 5,114 ha in the National Park Bov Hedma were systematically counted. Many dendrometric parameters were measured. In our study, age determination of A. tortilis is based on the counting of rings. Our results show that the population of A. tortilis subsp. raddiana in the region of Bou Hedma is characterised by an irregular structure, involved in a regressive dynamic. They show also that A. tortilis trees improve soil water availability and the nutrient status of soil.

Seed germination characteristics of Periploca angustifolia Labill. and Rhus tripartita (Ucria) Grande: effects of temperature, salinity and water stress.

Abstract Seed germination of two woody species (Periploca angustifolia and Rhus tripartita), indigenous plants from arid and semi—arid regions of Tunisia, were assessed under controlled conditions by determining the optimum temperature of germination and simulating drought stress using PEG 6000 and salt stress using different concentrations of NaCl. The germination response of the seeds in darkness were determined over a wide range of temperatures (10 °C to 30 °C), salinity (0 mM to 400 mM) and water stress (0 MPa to—3 MPa). Our results suggest that germination percentage is influenced by salt and osmotic stress at 25 °C and 15 °C respectively for P. angustifolia and R. tripartita which represent their optimum temperature of germination. However, studied species responded differently to stress induced by NaCl and PEG 6000. P. angustifolia manifested better adaptation to drought conditions compared to R. tripartita. These findings form the basis for future trials involving the use of indigenous shrubs in the restoration of rangelands.

Effects of exotic and endogenous shrubs on understory vegetation and soil nutrients in the south of Tunisia

This study was conducted in southern Tunisia in the growing seasons of 2013 and 2014, and aimed to compare the effects of exotic and endogenous shrub species (Haloxylon persicum and Retama raetam, respectively) on understory vegetation and soil nutrients. For each shrub species, the canopied sub-habitat (under the shrub crown) and un-canopied sub-habitat (in open grassland area) were distinguished. The concentrations of soil nutrients (organic matter, total nitrogen and extractable phosphorus) were found to be significantly higher (P<0.05) under R. raetam canopy than under H. persicum canopy and in open area. The result also showed that the presence of shrubs improved all the values of understory vegetation parameters (floristic composition, density, total plant cover and dry matter) and all these values were significantly higher under endogenous species canopy than under exotic species canopy. These results highlighted the positive effect of endogenous shrubs on understory vegetation and soil nutrients compared to the exotic ones in the Saharan areas of Tunisia.