Integrating plant control of nutrient cycling within root foraging strategies 

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The below-ground zone of inuence

The zone of inuence (ZOI) has been dened as the area over which a plant alters its environment, either above- or below-ground (Casper et al., 2003). The dierence with the rhizosphere is that here other mechanisms than the direct eect of living roots are involved. At the scale of the belowground ZOI, plant inuence on soil functioning emerges from the combination of the dierent eects of individual roots and their respective rhizospheres and the development of the root system.
A rst approximation of the dimension of the belowground ZOI is given by measuring the distribution of root biomass at a given time. This can be done either directly by excavating whole roots systems (e.g. (Guevara et al., 2009)) or by sampling the soil with a regularly distributed pattern (Lata et al., 2000), or indirectly by using tracers (Casper et al., 2003; Hartle et al., 2006). Maximum lateral spread, and rooting depth give an idea of the whole volume explored by plants (Schenk and Jackson, 2002; Casper et al., 2003).
In a given volume of soil, the surface of interaction between plant and soil is a function of root density (mass of roots per mass or volume of soil) but is more accurately described by root length density (cumulated length of roots per mass or volume of soil). Roots can vary in their specic root length (SLR root length developed per unit of root biomass). At a ner scale the exchange surface of roots can be enhanced by structures such as root hairs (Gregory, 2006). Cluster roots that are especially found in the Proteaceae family, is an extreme case of such surface development (Lambers et al., 2006). Root architecture the topological organisation of root length within the soil volume gives additional information to the understanding of plant-soil interaction in the belowground ZOI (Hodge et al., 2009; Pagès, 2011). In particular, it determines potential overlapping between the rhizospheres of dierent portions of roots (Pagès, 2011).
Root demography the dynamic of growth and senescence of roots within the belowground zone of inuence also has to be considered. The belowground ZOI is a volume in which a plant can easily grow new roots or where its dead roots can be found. The temporal dynamics of the root system can be evaluated in situ by the use of rhizotrons (Gregory, 2006). The lifespan of roots is highly variable, some roots remaining through the whole plant life and other structures such as cluster roots being short-lived (Eissenstat and Yanai, 1997). Considering root mortality, a growing root can benet from the inuence of a dead root within the soil explored (see subsection 1.2.1). The organisation of soil aggregates favours the growth of a young root in the reliquary rhizosphere of a dead root.

The above-ground zone of inuence

The aboveground ZOI is a priori simpler than the belowground ZOI. Aboveground, plants aect the most supercial layers of soil, through the deposition of aerial litter, the interception of litter, dust or rainwater and a micro-climatic eect. The aboveground ZOI is generally smaller than the belowground ZOI (Casper et al., 2003). Aboveground architecture is much more constrained physically than belowground architecture: contrary to branches, roots can grow at several decameters from plant stem Mordelet et al. (1996); Schenk and Jackson (2002). The aboveground zone of inuence should be relatively stable with time, apart from phenological changes largely determined by seasonality (summer vs. winter or wet vs. dry season) or perturbation such as re or herbivory (Abbadie et al., 2006).

 Extended above-ground zone of inuence

One can also consider an extended denition of the aboveground ZOI that includes plant interaction with herbivores. If a plant favours the presence of a herbivore, e.g. through palatability or shading, it may indirectly favour a positive feedback on soil functioning over an area larger than that directly inuenced by its canopy, but under which its roots can grow. Grazing lawns are an example where plants, in interaction with herbivores, create a zone functionally distinct from surrounding tall grass areas (McNaughton, 1984).

Linking plant control of nutrient cycling to root foraging strategies

From the previous section, soil exploration can be characterized by the size and shape of the belowground ZOI, while soil occupation is the outcome of the dynamics of the rhizosphere within the ZOI and the interaction between the above- and belowground ZOIs. Here I will show how soil exploration and soil occupation can be articulated within root foraging strategies. In a rst subsection, I will list plant traits that may aect the intensity of soil occupation and exploration and are likely to be selected within a nutrient acquisition strategy. Second, I will consider the multiple eect of roots on soil and their implications in the relationships between root density and nutrient cycling within the soil. Last, I will consider the integration of plant-soil interactions at the scale of the below-ground ZOI.

Root traits involved in root foraging strategies

A rst, relatively obvious trait that aects both soil exploration and occupation is the allocation of carbon, nutrients and energy to roots, from which depends the overall root system size (biomass) and activity. A given quantity of root biomass can be spread over a wide range of horizontal or vertical distance (Jackson et al., 1996; Hartle et al., 2006; Schenk and Jackson, 2002; Casper et al., 2003). Root lateral spread and maximum rooting depth give the ultimate border of the soil explored by plants (Schenk and Jackson, 2002; Casper et al., 2003). I focused in my work on the horizontal distribution of roots. The relative size of the below- and above-ground zones of inuence are also parameter that plant can adjust depending on the context. (Casper et al., 2003) showed that plants growing in arid soils tend to have a larger belowground ZOI compared to aboveground. The dierence of size of the ZOI is also a parameter involved in the creation of islands of fertility accumulation of carbon and nutrients below the plant canopy (Scholes and Archer, 1997).
The distribution of roots within the explored volume of soil is often heterogeneous. Part of this heterogeneity comes from an architectural development constraints that causes roots to concentrate near the plant stem (Casper et al., 2003). However, plants are also able to locally adjust root density and activities to the heterogeneity of nutrients (Hodge, 2004) and the presence of competitors (Gersani et al., 2001). Plant morphological and physiological plasticity potentially aect the degree of soil occupation. Some soil activities can be directly correlated to root density (e.g. (Lata et al., 2000)) and the geometry of the rhizosphere depends directly on the rates of uptake or exudation (see subsection 1.3.1).

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Table of contents :

I. Introduction et synthèse bibliographique 
Introduction Générale
1. Integrating plant control of nutrient cycling within root foraging strategies 
1.1. Introduction
1.2. Plants ability to control nutrient cycling
1.2.1. Direct control of nutrient availability
1.2.2. Interaction with soil microbes
1.2.3. Interaction with large herbivores
1.3. The spatial and temporal scales of plant-soil interactions
1.3.1. The rhizosphere
1.3.2. The below-ground zone of inuence
1.3.3. The above-ground zone of inuence
1.3.4. Extended above-ground zone of inuence
1.4. Linking plant control of nutrient cycling to root foraging strategies
1.4.1. Root traits involved in root foraging strategies
1.4.2. Relationships between local root density and soil functioning
1.4.3. Root foraging strategies at the whole plant scale: hypothesis of a trade-o between soil exploration and occupation
1.5. Conclusions
II. Pourquoi et quand les plantes devraient-elles limiter l’exploration du sol par leurs racines ? 
Introduction de la partie
2. Why and when should plant limit the exploration of soil by their roots? 
2.1. Abstract
2.2. Introduction
2.3. Material and Methods
2.3.1. Models descriptions
2.3.1.1. Spatial organization of the plant-soil system
2.3.1.2. Compartments of the nutrient cycle
2.3.1.3. Nutrient uxes in the plant-soil system
2.3.1.4. Model without nutrient uxes between unoccupied and occupied soil
2.3.1.5. Model considering a spatial dynamic of the zones of inuence .
2.3.1.6. The consequences of space exploration on nutrient cycling parameters
2.3.2. Parametrisation
2.3.3. Partial recycling eciencies and system closure
2.4. Results
2.4.1. Equilibrium and stability conditions
2.4.2. Conditions for which a reduced explorations optimizes plant biomass
2.4.3. Consequences of reduced soil exploration on the plant-soil system functioning
2.4.4. Role of the spatial dynamics between occupied and unoccupied soil
2.5. Discussion
2.5.1. When is it benecial for plants to reduce soil exploration by roots?
2.5.2. Generality of model predictions
2.5.3. Potential applications
Perspectives
III. Patrons d’exploration racinaire, eet îlot de fertilité et cycle de l’azote chez trois espèces de Poacées pérennes de savane. 
Introduction de la partie
3. Root exploration pattern and nutrient cycling in the plants-soil system of three savanna grasses 
3.1. Abstract
3.2. Introduction
3.3. Material and Methods
3.3.1. Study site
3.3.2. Experimental design
3.3.3. Sampling procedure
3.3.3.1. Quadrat selection
3.3.3.2. Roots and soil sampling
3.3.4. Analyses performed
3.3.5. Statistics
3.4. Results
3.4.1. Aboveground biomass pattern
3.4.2. Belowground exploration pattern
3.4.3. Soil content in C, N and P
3.4.4. Nitrogen cycling (plant and soil N stock and 15N)
3.5. Discussion
3.5.1. Plant soil exploration strategies
3.5.2. Absence of island of fertility eect ?
3.5.3. Species eects on nitrogen cycling
3.5.4. Conclusion
3.6. Acknowledgement
Perspectives
IV. Modélisation de l’impact de la distribution racinaire sur le contrôle du recyclage des nutriments à l’échelle de la rhizosphère et de la zone d’inuence souterraine 
Introduction de la partie
4. Modelling the impact of root distribution on the control of nutrient availability at the rhizosphere scale 
4.1. Abstract
4.2. Introduction
4.3. Material & Methods
4.3.1. Model Description
4.3.2. Numerical analysis
4.3.3. Upscaling rhizospheres to the below-ground zone of inuence
4.4. Results
4.4.1. Root density eects on nutrient uptake at the centimetre scale
4.4.2. Rhizosphere sizes as predictors of root interactions
4.4.3. Root foraging at the scale of the below-ground zone of inuence
4.5. Discussion
4.5.1. Inter-root competition and facilitation
4.5.2. Inferring optimal root strategies
4.5.3. The exploration/occupation trade-o
4.5.4. Conclusion
4.6. Acknowledgement
Perspectives
5. Discussion Générale 
5.1. Stratégies d’exploration racinaire et cycles des nutriments
5.1.1. Compétition et facilitation racinaire
5.1.2. Intégration des interactions racinaires à l’échelle de la zone d’inuence racinaire
5.1.3. Confrontation aux plantes réelles
5.1.4. Le compromis exploration/occupation : un outil heuristique pertinent ? .
5.2. Généralisation : stratégies d’acquisition des ressources et rétroactions plante-sol .
5.2.1. Un autre mode d’acquisition des ressources : les associations mycorhiziennes
5.2.2. Spécicité des interactions entre les plantes et les microorganismes du sol
5.2.3. Les rétroactions plantes-sol à l’échelle de la communauté de plantes
5.2.4. Application aux agro-écosystèmes ?
5.3. Conclusion
Bibliography 
V. Annexes 
6. Appendix to Chapter 2 
6.1. Equations and stability conditions for model 2
6.1.1. Model description
6.1.2. Stability of the equilibrium
6.2. Trade-os equations and parameterization of the model
6.2.1. Trade-o equations
6.2.2. Parameterization
6.2.3. trade-o calibration
6.3. Detailed analysis of model 1 with a functional trade-o between exploration and uptake
6.3.1. Stability conditions
6.3.2. Calculation of optimal soil exploration xP
6.3.3. Variation of soil nutrient stocks D and N with soil exploration x
6.3.4. Variation of total nutrient stocks T with soil exploration x
6.4. Generalization of the results of model 1 for other trade-os
6.4.1. Functional trade-o between soil exploration and mineralization .
6.4.2. Functional trade-o between soil exploration and lixiviation
6.4.3. Coupled trade-os
7. Appendix to Chapter 3 
7.1. Root scan analysis
7.2. Patterns of root exploration
7.3. Soil content in C and nutrients
7.4. plant and soil C:N
7.5. N isotopic data
8. Appendix to chapter 4 
8.1. Relationships between root length density and uxes of phosphorus whithin the soil
8.2. Relationship between root length density nroot and rhizP =rhizS
8.3. Upscaling to the whole plant
Résumé 

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