The challenge for agriculture will be to supply the food global demand via a sustainable way. The scope of expanding the agricultural lands to meet this increasing demand is limited in many parts of the world under current and predicted climatic regimes. Additionally, relaying on the high-input agricultural system could not be sustainable given the predicted limitation of water and nutrient resources for the near future. Given the limited water and nutrient resources, an important strategy to meet the increasing demand must focus on the selection of plant varieties with higher performance and higher water and nutrient use efficiency under drought and nutrient limited condition.(Shen et al. 2013). To do so, a proper mechanistic understanding of the transport of water and nutrient within soil-plant-atmosphere continuum under limited condition plays a major role in the success of suchprogram. Among different potential candidates affecting water and nutrient use efficiency in the plant, the rhizosphere, a tiny region at the immediate vicinity of roots in soilhas recently received large attentions as a new avenue to maximize efficient and sustainable use of limited water and soil nutrient resources(Gregory 2006; Dotaniya and Meena 2015). Also Zhang et al. (2010) revealed that rhizosphere management can help scholars in order to maximize efficiency of root and rhizosphere processes in nutrient acquisition.
The interface between the root surface and the soil, i.e., the rhizosphere, plays a critical role in water and nutrient acquisition by plant. This tiny region (thickness of a few millimeters) is engineered by both plant roots and microorganism and acts as the main terminal through which plants obtain water and nutrients(Hinsinger et al. 2009; Jones et al. 2009). Increasing evidences have been suggesting that plants actively modify their surrounding soil, the rhizosphere, to improve their access to water and nutrients(unpublished data of Zarebanadkouki et al. 2018).The most significant compounds in the rhizosphere is plant root exudates which play a key role in the interactions between soil microbes and plant roots (Naeem etal.2017).Although at first glance this may seem a waste of energy, it has been shown that the carbon exuded into the soil helps roots to take up nutrients (Hinsinger et al. 2009) and promote positive feedbacks between plants and microorganism(Hinsinger et al. 2009 ;Kuzyakov et al. 2000). These organic components create a distinct region around the roots having different biophysical and biochemical properties than the soil far from roots surface, the so called the bulk soil. The plant roots secreted compounds which serve significant roles as chemical attractants and repellents in the rhizosphere (Bais et al. 2001). Root exudates in the rhizosphere can regulate the soil microbial community in their vicinity, encourage beneficial symbioses, and change the chemical and physical properties of the soil (Nardi et al. 2000). Some scholars revealed that the root-soil contact and soil micro-aggregates can be increased and stabilized due to the presence of root secretions in the rhizosphere. Also, root exudates can protect the roots from desiccation, and able to select the adsorption and storage of ions (Rougier 1981; Bengough et al. 2006; Hawes et al. 2000).
Root exudates include i) low-molecular-weight compounds such as monosaccharides, amino acids and organic acids and ii) high-molecular weight compounds such as mucilage. A large number of studies have been focused on low molecular weight carbon components because they include majority of root exudates (estimated to be around 70%). These studies mainly focused on the biochemical properties of the rhizosphere. Briefly is has been shown that these low molecular weight components influence the microbial activities in the soil (Kuzyakov et al. 2000) and therefore increase nutrient availability in soil(Hinsinger et al. 2009). In contrast, less attention has been dedicated to the role of high molecular weight components, in particular mucilaginous fraction of the rhizodeposits referred as the mucilage, onplant performance. These is mainly due to the fact of rhizospheric studies were initiated and motivated by soil microbiologist and soil chemists and these components include less than 30% of root exudates.
1.2. Mucilage and its postulated functions in the soil
Mucilage is a gel-like substance consists of roots and microorganism exudations and the root borer cells. The mucilage constitutes a complex environment at the root-soil interface and suggested to play a crucial role in soil-plant water and nutrient relation and to have the potential to increase plant drought tolerance. An illustrative image of hydrated mucilage covering the crown roots of maize some hours after a rain event is shown in figure 1.
Mucilage absorbs large volumes of water and nutrients(Kroener et al. 2014;McCully and Boyer 1997;Marschner 2013;Lynch 2007). Exudates like mucilage can absorb nutrient either directly via mobilization of ions(Marschner 2013;Lynch 2007) or indirectly by feeding microbially mediated biogeochemical cycling of nutrients)Philippot et al. 2013).Several hypothetical functions have been attributed to mucilage such as being the sources of energy and nutrients for soil microorganisms, reducing frictional resistance to root penetration (Iijima et al. 2003; Iijima et al. 2004), facilitating root water uptake under drying soil condition by avoiding big drop in soil water potential and soil hydraulic conductivity (Carminati et al. 2017; Benard et al. 2018), preventing the root desiccation in drying soil condition by keeping the soil wetter(Ahmed et al. 2014)facilitating water uptake and maintain the transpiration and the photosynthesis in dry soils by acting as a hydraulic bridge between roots and the soil(Benard et al. 2018; Carminati et al. 2017), reducing the risk of gap formation between root and soil under drying condition by enhancing rhizosheath formation (Watt et al. 1994; North and Nobel 1997).It is revealed that mucilage can develop the symbiotic relationship with the soil-dwelling fungi(Moody et al. 1988). This relationship can help plants to increase the water and nutrient uptake from the soil and the plant cannot get enough nutrients and water without this relationship(Gianinazzi-Pearson et al. 1996).
So far most of the studies focusing on the rhizosphere and mucilage mainly focused on the role of roots exudates in improving the availability of water and nutrients in soil. In contrast, the transport (the flux) of water and nutrients from soil to the roots has received less attentions. This is mainly due to technical difficulties of measuring fluxes of water and nutrients across the root-soil interface. Here, we propose a research package to study the effect of root exudates and in particular mucilage on transport of nutrient within roots and soil under drying soil condition. The hypothesis is that mucilage plays a big role in transport of nutrient across the root soil-interface. Keeping the soil wetter and acting as a hydraulic bridge between soil particles and roots, mucilage facilitates nutrient uptake as soil dries. Transport of nutrient in soil and then into the roots is many occurring by diffusion and mass flow. These two processes are strongly affected by soil water content and both decreases nonlinearly as soil dries limiting nutrient uptake by plants.
Carminati et al. (2017) conceptualized that the combination of increased viscosity and decreased surface tension is the key to explain the hydraulically-related function of root mucilage. The concept was inspired by the observation of liquid phase configuration within soil pores (Fig. 2). Light transmission microscopy of mucilage within the pores showed that mucilage forms a filament-like structure within the soil pores as it dries. Water, with its low viscosity and high surface tension, could not from this filament-like structure within the pores. As soil dries the liquid bridge between soil particles shrinks and its surface to volume ratio increases. In this condition surface tension becomes the dominant forces stretching further the liquid bridge and eventually it breaks up to reduce the air-water interface. In a viscous liquid, such as mucilage and EPS, viscosity opposes to the stretching of the bridge and, eventually, it prevents its break-up. The Ohnesorge number explains the persistence of liquid filaments observed during soil drying.
Oh=?/???r
where ? is viscosity, ? is density, ? is surface tension and r is a characteristic length corresponding to the radius of the filament.Castrejon-Pita et al. )2012) showed when a critical viscosity is reached, its force dominates over surface tension and the break-up of liquid bridges is prevented. Carminati et al. )2017) estimated the Oh number to be 15 for the mucilage of plants which suggests the dominant role of viscosity over surface tension and inertia on mucilage thinning. Benard et al. )2018) showed that for liquid with dominant viscosity (water-mucilage mixture), liquid phase stays connected as soil dries. At the low mucilage content, a 1D isolated filament will be formed between the particles. At the higher mucilage content, the 1D elongated filaments grow into larger hallow cylindrical-shaped structure between two particles and with further increase of mucilage content, the hollow cylinders merge into 2D interconnected lamellae that span throughout the pore space. In conclusion, the authors speculated that the presence of mucilage alters the configuration of liquid phase within the soil pores increasing soil water holding capacity and soil hydraulic conductivity of the soil as it dries. The significant of these effects is strongly affected by the concentration of mucilage and the size and geometry of the soil particles (assumed that particle size and pore size are correlated).
Fig.2: a) Mucilage in soils visualized using a light transmission microscope. Mucilage was extracted from chia seeds, mixed with soil particles of varying diameter, stained with ink and let dry for ca. 48 hours. Thin filaments (indicated by blue arrows) connecting sand particles at 0.036% mucilage concentration (gram of dry mucilage per gram of dry soil). b) Hollow cylinder between glass beads mixed with chia mucilage. c) Conceptual model of mucilage drying between soil particles. At low mucilage content, liquid bridges between soil particles merge into 1D thin filaments which do not break up because of the high extensional viscosity of the mucilage polymers. At high mucilage content, the filaments merge into quasi 2D structures, such as hollow cylinders and 2D lamellae that during drying, create an additional matrix that holds water and that maintains the continuity of the liquid phase across the rhizosphere(fromBenard et al. 2018).
1.2. Preliminary works and conceptual frameworks
This proposal benefit from ongoing researches of researchers at the department of Soil Physics at the University of Bayreuth. In this group, recently a series of complementary techniques has been used to understand the mechanistic impact of mucilage on hydraulic properties of soils. The findings provided bunch of experimental evidences supporting the hypothesis that plant roots engineer hydraulic properties of their surrounding soil (by release of mucilage) to maintain their access to soil resources at varying soil conditions. The main findings are summarized below:
1) Mucilage maintains film flow of water under drying condition and limits vapor diffusion
An evaporation experiment was used to monitor drying rate (evaporative flux) of soils treated with different concentration of mucilage. HyProp was used to perform this experiment enabling to measure water flux and soil water potential at varying depth during an evaporation process. To identify the effect of mucilage on hydraulic properties of soil, the data of HyProp were used to numerically simulate water flow during evaporation by solving water flow equation (Richards equation). Then the soil retention curve and the soil hydraulic conductivity curve were inversely parameterized to best reproduce the measured average soil water contents and the matric water potential at varying depths. For the case of soil retention curve, mucilage increased water holding capacity of the soil at any given water potentials compared to the control soil (Fig. 3). For the case hydraulic conductivity, mucilage reduced hydraulic conductivity of near saturated soil, but it avoids big drop in hydraulic conductivity of soil as it dries resulting in a higher soil hydraulic conductivity drying condition. These effects differed among different soil types and different concentration of mucilage. Only if enough mucilage was added there was an effect on soil retention curve and hydraulic conductivity curve. Such a critical concentration was interpreted as the concentration when mucilage started to form a new matrix inside the porous medium. Interestingly, the critical concentration was higher in coarse textured soils, suggesting that in soils with big pores more mucilage is needed to increase the water retention (Kroener et al. 2018).
Fig. 3: a) Water retention and b) hydraulic conductivity curve of a silty soil without (blue) and amended with 2.5 mg g -1 mass of dry mucilage per mass of soil of mucilage (red; extracted from chia seeds); solid lines indicate the mean of three replicates and grey areas indicate the 95% confidence interval of three replicates.
2) Key function of mucilage is altering the spatial configuration of liquid phase within soil pores
The effect of mucilage on soil hydraulic properties could be explained by its impact on the spatial configuration of liquid phase within the soil pores during a drying cycle which is influenced by intrinsic properties of mucilage such as network formation, high viscosity and low surface tension (Benard et al. 2018). Figures 4and 5 show the conceptual effect of mucilage on hydraulic conductivity of the soil. Mucilage increases viscosity of the liquid phase in the soil and therefore reduces the near saturated hydraulic conductivity of the soil. As soil dries and its concentration increases and depending on its concentration in the liquid phase it may form 1D filaments structure, 2D filaments structure and interconnected surfaces within the soil pores. Such structure maintains the continuity of liquid phase as soil dries avoiding big drop in hydraulic conductivity of soil. In contrast to maintaining the connectivity of liquid phase, such a structure could reduce the vapor diffusion within the soil. Liquid configuration was visualized using an X-ray microtomography with spatial resolution of 0.3µm in a soil (glass bids) treated with different concentrations of mucilage (Fig. 6). The results showed that in drying soil mucilage forms interconnected surfaces within soil pores.
Fig. 6: top) X-ray tomography of dry maize root mucilage structures within glass beads; bottom) 3D segmentation of dry mucilage structures (red) after reconstruction of CTs. These results showed that mucilage formed interconnected surfaces of approximately 1 µm thickness within the pore space of glass beads (blue; 0.1 – 0.2 mm in diameter).
In a parallel experiment, configuration of liquid phase during a drying cycle was visualized using a light transmission microscopy technique. It was found that mucilage keeps the connectivity of soil pores under drying condition (Fig. 7). Here, thin section of soil was mixed by mucilage extracted from chia seeds and let dry under room condition while the liquid phase was monitored under the microscope. As soil dries, the matric potential dropped down to a point that water could not be held in the pore by capillary pressure anymore but thanks to the presence of mucilage the continuity of water film was maintained.
3) Mucilage maintains diffusive transport of solutes within a drying soil
Given that mucilage maintains the connectivity of liquid phase during soil drying (as it was proven by x-ray tomography in figure 6, it should maintain diffusive transport of nutrients as soil dries. This hypothesis is tested in a simple experiment as following: a phosphor imaging technique was used to visualize redistribution of 137Cs radionuclide in soils treated with different mucilage contents (Fig. 8). A container with size of 5×2×1 cm was partitioned into two parts and the first parts was filled with a control soil that was initially rewetted by equilibrating it with different water potentials. The other half was equilibrated with 137CS solution of 0.1M at the same water potential as the first half. Spatial redistribution of 137Cs throughout the sample was visualized and then quantified using a phosphor imaging technique. Then the spatial concentration of 137Cs was numerically simulated by solving a diffusion equation and adjusting the diffusion coefficients to best reproduce the measurements. The results showed that presence of mucilage prevent big drop in diffusion coefficients of soil at drying condition suggesting that mucilage may favor nutrient uptake (Fig. 9).
Fig. 8: Phosphor imaging of 137Cs diffuion in a sandy soil treated with and without mucilage extracted from Chia seeds. Containers of size 5×2×1 cm were filled with soil treated with and without mucilage. The soil used to fill the first 2.5 cm of containers was wetted with 137Cs solution and the other 2.5 cm with water. Diffusion was monitored at different water contents (?) and different times (t).
Fig. 9: Diffusion coefficient of 137Cs as function of mucilage cotent (zero: control, 2.5 mg g-1 ) and soil water contents. Mucilage prevent big drop in diffusion coefficient as soil dries.
In a simple scenario the data of diffusion coefficients were used to simulate the transport of nutrient into a single root. The soil domain was conceptualized as following figure:
Assuming a steady state flow and neglecting the effect of mucilage on adsorption/desorption of nutrients following equation was solved to describe the concentration of nutrient at the root surface during a soil drying cycle.
Where cw is the concentration of nutrients in the liquid phase (mg cm3), ? is volumetric soil water content cm3 cm-3, t is time, r is radial distance from root surface cm, D is the diffusion coefficient cm s-2, jr is the radial flux of water in the soil cm s-1. Here, hydraulic properties of soil and the rhizopshere were parameterized according to Kroener et al. (2016). Concentration of nutriients at the root surface was evalutated under follwing different condtion: 1) with presence and without presence of the rhizopshere, 2) high and low root water uptake rate, 3) and high and low concentration of nutriient in the soil solution (to mimic macro and micro elements).
The results showed that for nutrient with high concentration in the soil solution, mucilage delays the buildup of solutes at the root surface, reducing the risk of salinity stress (Fig. 10). For the nutrient with low concentration in the soil solution, mucilage reduces nutrient depletion at the root surface by keeping the rhizosphere wet and hydraulically connected as soil dries.
Fig. 10: Simulated cocnentration of nutriient at the root surface during a drying soil cycle based on different scenarios 1) with presence and without presence of the rhizopshere, 2) high and low root water uptake rate. Left figure shows the case of nutirient with high concentration in the soil solution. Right: shows the case of nutirient with low concentration in the soil solution.
Objectives
The general objective of this project is to understand the how mucilage affects the transport of nutrients under drying soil condition and whether it has potentials to improve nutrient acquisition from soil under drying soil condition.
The main hypothesis is that plant roots engineer their surrounding soils to optimize their access to water and nutrient resources particularly under drying soil conditions. These engineering areimplemented by exuding gel-like substances called the mucilage into the soil that increasescation exchange capacity of the soil, reduces the surface tension of soil solution, increases the viscosity of the soil solution and bridges soil pores by forming network-like surface within pores(Carminati et al. 2017). These modifications will have great impacts on the availibility of nutriients (adsorption and desorption) and the transport of nutriients (diffusive and convective transport), across the root soil-interface.
3. Work program incl. proposed research methods
3.1. Materials: Plants, Soil, Mucilage, and nutrient solution
Due to tiny scale of rhizosphere and the difficulty of sampling soil and nutrients in such a small region of soil, we will use a modelled rhizospheric soil to test our hypothesizes and concepts. At the end the consequent effects of rhizosphere on transport and uptake of nutrients by plants will be tested by growing plant mutants with high and low root exudation rates. Mutants are similar in all the features except one feature (amount of extracted mucilage). The modelled rhizospheric soil will be used to experimentally test and prove different aspects of our concepts. The modelled rhizospheric soil will be prepared in two different ways:
1) As a modelled rhizospheric soil we will use mucilage extracted from Chia seeds (Salvia hispanica L.) and mix it with a loamy soil collected from an agricultural field near university of Bayreuth(Ahmed et al. 2014). Although the chemical composition of mucilage extracted from chai seeds could be different than the one of exuded by roots but there are some important similarities between them that are the core of our concepts. Both mucilageincreases the viscosity and liquid phase, decreases the surface tension of liquid phase and forms a network like structure between the porous media (Fig 11).
Fig. 11: Chia seeds a) dry Chia, b) soaked Chia
2) As a more realistic rhizospheric soil, we also used rhizosheath, the adhering soil particles on the surface of roots (thanks to the root exudates and root hairs). We will grow maize mutants with high and low exudation rates in rhizoboxes of 40×40×2cm. The rhizoboxes will be designed in way that can be open from one side for rhizosheath collection at the end of growth period (Fig. 12). The rhizoboxes will be filed with a loamy soil. The plants will be grown for a period of 8 weeks and during this time soil will be kept at two different water contents. Some plants will be kept at an optimum soil water content (vol. 25 to 30%) some at water stressed condition (8 to 12%).
Here the idea is to test if plants grown under water stress condition will establish a more efficient rhizosphere to facilitate the transport of water and nutrient from soil to the roots. When plants are 8 weeks old we let plants to dry soil to a water content of 5% and then we opened the containers and collect the roots and adhering soil. The collected roots and rhizosheath will be gently shaken with a constant mechanical force to and then the remaining soil will be removed by applying a stronger force and finally by brushing it off from the roots.
Fig. 12. A schematic diagram of the rhizo-box.
3.2. Working packages
Work package 1: To monitor spatial distribution of liquid phase during a soil drying cycle in the selected rhizospheric soil
Objective: The core of our hypothesis is the effect of mucilage in altering configuration of liquid phase between soil pores as soil dries. The presence of mucilage maintains the connectivity of liquid phase as soil dries which in turnmaintain and facilitate the diffusive transport of nutrient under during condition.
Materials and Methods: As the first step we will study the spatial configuration of liquid phase under different soil drying conditions among three different selected rhizospheric soils and the control soil. To do so, we will use a similar experimental setup as the ones presented in figures 6 and 7. The modelled rhizospheric soil will be packed in cylindrical containers of radiusof 0.5 cm and length of 2 cm. The soil will be saturated with water and low to loss water from top by evaporation. Once soil water content reaches a given water content the top of the container will be covered a thin plastic film to minimize the evaporation rate and wait till water content reaches an equilibriumthroughout the soil. Then a micro X-ray tomography will be used to monitor spatial distribution of liquid phase between the soil. Then the captured images with an expected resolution of 0.3µm will be reconstructed to visualize the soil porosity and the spatial configuration of liquid phase. In parallel experiment
Work package2: To study the effect of mucilage on storage and exchange capacity of nutrients in soil
Objective: The objective of this study is to understand the impact of mucilage on keeping and leaching of nutrients in a loamy soil.
Materials and Methods:Three different modelled rhizospheric soil; prepared from mucilage extracted from chai seeds, collected as rhizosheath of maize mutant with low and high exudation rate will be used during this working package. As the first step the cation exchange capacity of these three rhizospheric soils will be determined according to the method of pH 7 buffered ammonium acetate. To do so, 2g air-dried soil will be placed into 50 ml centrifuge tubes and 20 mL of the 1 M NH4OAc will be added into tube. Then, we will shake it and allow to stand 16 hours. Finally. the samples will be centrifuged and Na, Ca, Mg, and K will be measured by ICP-OES (Reeuwijk, 2002).
In a parallel experiment the adsorption capacity of the soil in adsorbing K, Ca, P, Zn, Mn will be investigated. To do so, a service of batch experiment as following will be used: 5 g of each rhizospheric soil will be placed in a centrifugeflask and equilibrated with 50 ml of background solutions containing K, Ca, P, Zn and Mn(individual element) of o.1M. The flask will be shaken in a controlled temperature for a period of 24 h. The samples will be flittered via a filter paper and then centrifuged to gain a clear solution. Finally, the concentration of each nutrients will be analyzed using inductively coupled plasama with optical emission spectroscopy (ICP- OES). Also, the amount of nutrient adsorption in soil will be determined from the reduction of nutrient concentration in solution.The relation between concentration of each elements in the liquid phase (initial concentration of the solution) and the solid phase will be used to parameterize the kinetic of adsorption in each rhizospheric soil. In a parallel experiment the desorption isotherms will be studied. Desorption isotherms show how available are the elements stored in the solid phase to the liquid phase. The fresh background solutions (without nutrients) will be added to therhizospheric soil samples (5 g) and shaken for 24 h. Finally, the solution will be centrifuged and filtered to measure nutrients in the beckground solution by ICP method (Hararah et al., 2012). The nutrients adsorbed and desorbed at equilibrium qe (mg/g) will be measured via the following equation (Tan et al. 2009):
q_e=((C_0-C_e )*V)/W
The initial and equilibrium concentration of each nutrient are shown by Ce and C0(mg/L), respectively. Also, volume of the solution and the weight of the adsorbent (soil) are indicated via L and W (g).
A complimentary experiment as the ones described above will be carried out following the protocol proposed by Colombani et al. (2015): A polyethylene (PE) columns with an internal diameter of 1 cm and length of 5 cm equipped with a nylon membrane with mesh size of 0.5µm will be filled with each rhizospheric soil at a given bulk density to a height of 4 cm. The membrane limits the transport of particles out of the container while it allows the drain of water out of the container (Fig. 13).Then, soil will be infiltrated with nutrient solution (known volume and known concentrations) using a peristaltic pomp at a rate of 1 cm3 min-1. At different time intervals of 2 min the infiltrated water out of the containers will be collected and the volume and the concentration nutrientswill be determined by ICP method.
In a similar experiment the availability of stored nutrients in the solid phase to the plants will be investigated. Here each rhizospheric soil will be pre-equilibrated will the solution of 0.1M of each nutrient and then an infiltration experiment with deionized water will be carried out and the concentration of each nutrient will be determined at different time intervals of 2 min at the collected solution at the bottom of the containers. Here, depending on the results of experiment if nutrients are not leached out of the soil deionized water, we may infiltrate the pre-equilibrated soils with some weak acids.
Fig. 13. A schematic design of leaching experiment
Work package 3: To study the impact of mucilage on diffusive transport of nutrient in drying soil
Objective: The objective of this study is to determine the impact root exudations and particular the mucilage on the diffusive transport of nutrients under drying soil conditions.
Materials and Methods:In this working package a similar experimental setup as described in figure 8 and figure 15 will be used.The idea is to compare diffusion coefficient of different elements as function of concentrations of mucilage at varying soil water contents. To do so, aluminum containers of size 2 (width) ×5 (length)×1 (depth) cm will be packed with three rhizospheric soils and the base loamy soil (loamy soil used for growing plant and the collection of rhizosheath) as the control. The soil will be packed at a given bulk density. Each container will be partitioned into two parts and the first part will be filled with a given rhizospheric soil that is pre-equilibrated at a given water content using a nutrient solution containing a given nutrient at concentration of 0.1M. The other half parts will be filled with the same rhizospheric soil at is equilibrated at the same water content with deionized water. To study the diffusive transport of nutrients in soil we need to exclude any convective transport of nutrients. To do so we need to pre-equilibrate the rhizospheric soilsat the same water potential. In another words, to compare different treatments (rhizospheric soil and water content) we need to consider the soil water potentials rather than the soil water contents. Moradi et al. 2011 and Carminati et al. 2010 by monitoring soil water contents around the roots of transpiring plant grown in soil showed that rhizospheric soil is water than the bulk soil at any given water potential during a drying cycle. Therefore, the rhizospheric soil and control soil used here will be first saturated with water or a given nutrient solution. Then it will be placed in a porous plate apparatus which allows us to equilibrate soil to any given water potential. We will equilibrate soils at matric potentials of -60 (moisture condition of FC in Germany), -300, -1000, and -3000 cm for a period of 1 week.
Then the pre-equilibrated soils will be packed into the container. The containers will be covered by a thin layer of plastic film to reduce evaporation from the soils. Then the concertation of nutrients at different positions of soil at different time intervals of 5, 24, 48, 72, 96, 144 and 240 hourswill be determined by two following methods:
i)A phosphene imaging technique will be used tomonitor the spatial distribution of 137Cs, 32P and 40K in the soil.
ii) A soil sampling technique will be used to measure concentrations of nutrient that their radionuclide is not available such as K, Ca, Zn and Mn. Here soil will be sampled at position intervals of 0.5 cm and the concentration of nutrients in the liquid phase will be determined after extracting the soil solution using ICP method.
Fig. 15. A schematic diagram of thecontainer portioned in two parts
The measured profile of concertation at different potation of soil and different times will be inversely simulated by solution a diffusion equation and adjusting the diffusion coefficient. Then a simple scenario as described in figure 10 will be used to simulate concertation of different nutrients in the soil solution during a soil drying cycle as nutrients are taken up by a single root.
Work package 4: To study the potential impact of mucilage in facilitating the nutrient acquisition under drying soil condition.
Objective: The objective of this study is to investigate the impact of root exudations and particularly the mucilage on nutrient uptake under dry soil conditions. Here the concertation (and content) of different macro and micro elements in biomass and the rhizospheric soil of two maize mutants; one with less exudation rate and one with enhance exudation rate will be compared.
Here in this work package the predicted effect of mucilage on the concentration of macro and micro elements in the plant biomass and the rhizospheric soils obtained from a modeling exercise in working package of 2 will be tested by growing plant mutant with less and enhanced exudation rate. The hypothesis is that maize mutant with enhanced root exudation rate will have i) a higher concentration of nutrients in its biomass under different drying soil conditions ii) the risk of salinity stress by building up of a high concertation of macro elementswill be less in the rhizosheath of maize mutant with enhanced root exudation rate as soil dries iii) and the risk of nutrient deficiency will be less in maize mutant with enhanced root exudation rate as soil dries.
Materials and Methods: Maize mutants will be grown in rhizoboxes 40×40×2 cm which can be opened from one side at the end of experiment. The rhizoboxes will be filled with the same loamy soil as the one will be used in the previous experiments. Prior to the filling of the rhizoboxes, soil will be enriched by nutrients at two different levels; one will be optimum level of nutrient concentrations and deficiency level of the concentration which will be thirty percent of the optimum condition. The optimum level and stress level include the initial concertation of nutrients in the used loamy soil. The rhizoboxes will be filled with soil at a given bulk density and one pregerminated seed will be placed at a depth of 0.5 cm in the middle of each rhizoboxes. The plants will be irrigated every day from top for one week to establish the plants and then two irrigation treatment will be applied: for each maize mutant one group will be kept at the soil water content of 0.25-0.3 cm3 cm-3and the other group will be kept at a soil water content of 0.8 to 0.12 cm3 cm-3.
In brief 24 maize mutant with low exudation rate and 24 maize mutants with high exudation rate will be grown. Among each mutant12plants will be grown in a loamy soilwith optimum nutrient concentrationand 12 in soil under deficient nutrient level. Among each nutrient level 6 plant will be kept at optimal soil water content and 6 plant at soil water stressed condition. Plant will be grown for a period of 8 weeks rhizosheath and plant biomass will be collected and the concentration of nutrients in both will be determined.
