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In recent years, research interest in plant water uptake strategies has significantly grown in many disciplines such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and the spatio-temporal dynamics significantly advanced from different disciplines across scales. Despite this progress, major limitations, i.e. to predict plant water uptake under drought or it´s impact at large-scales remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights in plant water uptake model structure.The main goal of this presentation is to highlight how the 4 dominant model approaches, e.g. Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabeling a holistic system understanding that e.g. embeds plant water uptake plasticity into a broader conceptual view of soil-plant feedbacks of water, nutrient and carbon cycling or reflects observed drought responses of plant-soil feedbacks and their dynamics under e.g. drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to e.g. different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.

Aims Soil priming affects soil N transformation and plant N availability, but few studies have investigated these interactions to date. Methods To address this, we reviewed the literature for studies quantifying soil priming, soil N transformation and plant N uptake. Results Gross N mineralization was strongly controlled by soil priming in studies with plants, while abiotic factors had a minor influence on gross N mineralization. In contrast, soil priming was negatively related to gross N mineralization and had a low explanatory power in incubation studies where substrates are added as surrogates for root exudates. These results indicate that plants support increased N mineralization and that this is not adequately reflected in incubation studies. Additionally, we observed a positive relationship between soil priming and the % of Norg-derived N uptake as well as total N uptake, which demonstrates that priming enhances the availability of N that was previously organically bound and that at least part of the N mineralized during priming was available for plant uptake. Conclusion Our results show that the effect of roots and rhizodeposition leads to a number of processes supporting N mineralization and availability through priming that are not well reflected in incubation studies. To fully capture the interactions between plant roots and their associated microbiota, we recommend focusing research on systems with plants. Additionally, the strong correlation between C and N transformation should be considered in biogeochemical modelling.

Upland rice is often cultivated on P poor soils with a high P fixing capacity or in areas where P fertilizers are unavailable. Therefore, rice genotypes with a high P acquisition efficiency (PAE) are needed. In order to identify mechanisms driving differences in rice genotypes’ PAE, root and rhizosphere traits such as the root surface area (RSA) or the P acquisition of contrasting soil P pools is of special interest.The objective of this study was to identify root and rhizosphere traits that can explain differences in PAE of two known (DJ123, Nerica4) and two recently developed promising (AB199, AB67) upland rice genotypes.A pot experiment with a high P fixing Andosol from Tsukuba, Japan, and four rice genotypes differing in PAE (high PAE: DJ123, AB199, AB67; low PAE: Nerica4) was performed. The genotypes were grown under low (3.5 mg P kg-1, LP) and high (70 mg P kg-1, HP) P conditions. For each pot, 1.5 MBq 33P labeled phosphoric acid was diluted in N:P:K (NH4NO3, NaH­2PO4, K2SO4) nutrient solution and homogenized into the soil using a handheld electric mixer. Plants were harvested 10 days after emerging (DAE) (estimated beginning of P uptake) and DAE 31. After each harvest, roots were scanned with an Epson flatbed scanner to measure the RSA. Shoots and roots were dried and their biomass as well as total P and 33P uptake was determined. A hedley fractionation (0.5 M NaHCO3, HCO3-P; 0.1 M NaOH, OH-P; 0.5 H2SO4, H2SO4-P) was conducted for bulk (BS) and rhizosphere soil (RS) to estimate 33P and P in the different soil P pools. Total P in soil was determined after pressure digestion using and ICP-OES.Plant shoot and root dry weight, P concentration in root and shoot, P uptake in root and shoot and RSA were higher in HP plants compared to LP plants. In the LP treatment, AB199 showed the highest shoot and root dry weight and 33P uptake 10 DAE, followed by DJ123, AB67 and Nerica4. Additionally, AB199 showed the highest P uptake per RSA, i.e. the highest PAE among all tested genotypes. This trend was confirmed for the second harvest, 31 DAE. However, differences in P uptake and PAE between DJ123 and AB199 were not significant. Nevertheless, these results indicate that the new variety AB199 might indeed by a promising upland variety for P limiting conditions. The hedley fractionation showed that the largest 33P fraction was the OH-P fraction (65-75%) followed by H2SO4-P (20-30%) and HCO3-P (5-7%). The H2SO4-P fraction was slightly lower in the rhizosphere soil in DJ123 and AB199, indicating that rhizosphere processes helped these genotypes to solubilize larger amounts of H2SO4-P, compared to the other two genotypes.We showed that the coupling of 33P labelling with measurements of soil P fractions and plant P uptake can help to identify parameters responsible for increased PAE of upland rice genotypes. This may assist in identifying valuable root and rhizosphere traits for breeding for enhanced PAE in upland rice.

Most often, yield variability can be associated with differences in topography, soil properties and other environmental factors across agricultural landscapes. Ensuring high yield levels while simultaneously minimizing the risk of N-losses through inadequate use of fertilizers is especially difficult due to the high spatial variability of N transformations originating from those heterogeneities. Thus, understanding of N transformation in heterogeneous agricultural landscapes is key to an efficient and sustainable crop management.To assess the impact of soil heterogeneity on N transformation processes, a novel field design was established in Tempelberg, North-East Germany. The experimental area was categorized in high yield and low yield potential zones based on historic yield and soil textural maps with field sizes of half a hectare. To display the small-scale soil heterogeneity within the patches, measurements were done along transects of gradients of yield potential.We hypothesized that low yield soils with sandy texture, low soil water holding capacity (WHC) and locations at lower elevations within the field are associated with low N2O emissions and high N-leaching. In contrast, we expected high yield potential soils located at higher altitudes with a loamy texture to be characterized by high WHC, high N2O emissions and low N-leaching. Additionally, we postulated that edge effects across the transects may play a role due to patch design. We present results of the monthly N2O emission measurements done in fields cultivated with rape seed (Brassica napus L.), sunflower (Helianthus annuus L.) and maize (Zea mays L.), measured with NFT-NSS closed chambers, over a period of 6 months. 15N balances were calculated in the same fields tilled with sunflower (Helianthus annuus L.) and maize (Zea mays L.), by 15N tracer application and evaluation at three time points over growing season. Combination of N transformation processes and gaseous N fluxes in addition with WHC and topography allows for the identification of factors controlling soil N transformation and N availability in agricultural landscapes with high spatial variability of soil properties.Soil dependent N2O measurements were observed across each transect. Elevation, texture as well as soil water content (SWC) showed a clear influence on N2O emissions. High emissions were measured in plots characterized by a loamy texture, high SWC and locations at higher elevations. In addition, lower emissions were measured at the edge point of the given transect, which could be described as an edge effect.Evaluations of 15N tracer application results showed significant higher 15N leaching in low yield soils, which tend to have a higher sand content. High yield soils showed lower N-leaching. A strong dependence on soil texture and SWC was visible in the field cultivated with sunflower (Helianthus annuus L.): plots with higher sand content and lower SWC located at the center of the transect showed a higher N-leaching. This finding is in agreement with measured N2O emissions, which were noticeably lower in these areas.In conclusion, soil heterogeneity in agricultural fields originating from differences in soil texture, SWC and topography show a clear impact on N transformations and N emissions.

Soil C priming is a short-term change in the turnover of soil organic matter caused by the addition of easily available organic C to the soil. The increase in SOM decomposition during priming is likely to affect not only C- but also gross N mineralization from SOM because large amounts of soil N is stored in SOM that is decomposed during priming. In order to assess whether soil C priming results in an increase in gross and mineralization and finally in enhanced plant N availability and uptake, we searched the literature for studies relating soil C priming to soil N cycling. In order to assess the effect of soil priming on soil N cycling we included studies quantifying soil C priming (PE) and gross N mineralization (GNM) in plant systems and in incubation setups. Secondly, we searched for studies measuring GNM in dependence to addition of C to the system. The third data set comprised studies, quantifying PE and the % of soil N derived N uptake as well as total N uptake of plants. Finally we included studies that quantified soil priming and enzyme activities in the respective soil samples. In order to be able to compare PE to GNM, % of soil N derived N uptake and soil enzyme activities respectively, we calculated the excess of GNM, % of soil N derived N uptake and soil enzyme activities by subtracting the parameter values of the control from the treatment values. We found a significant positive relation between soil C priming and GNM for studies with plants (R2=0.21) indicating that soil priming caused by root exudation increased soil N mineralization. In agreement with this, activities of enzymes related to the N cycle were positively related to priming (p=0.09), though, due to the small number of studies, the enzyme results must be interpreted with caution. In contrast to plant studies, the relation between soil C priming and GNM was significantly negative for incubation studies (R2=0.06). These contrasting results for plant and incubation studies indicate that incubation studies might not adequately reflect processes occurring in the rhizosphere. It is possible that plants attract particularly N mineralizing microbes for example by exudation of signaling compounds, a process that would not be reflected in incubation studies. We also found a significant positive relation between soil C priming and the % of soil N derived N uptake by plants (R2=0.56) and total plant N uptake (R²=0.21) indicating that at least part of the N mineralized during priming was available to, and taken up by the plants. In conclusion, the results of our meta-analysis indicate that rhizosphere C priming positively feeds back to plant N nutrition by causing increased N mineralization in the rhizosphere that facilitates plant N uptake.

Soil drying negatively impacts several rhizosphere processes, but plant roots are capable of alleviating changes in rhizosphere water content by releasing mucilage. We propose that enhanced water retention in the rhizosphere due to mucilage and microbial extracellular polysaccharides allows for fast diffusion of enzymes and substrates, and thus high enzyme activity in the vicinity of roots. To assess the effect of diffusion on enzyme activity, the relation between phosphatase activity and volumetric soil water content (VWC) was quantified in sterile soil. Then, barley plants were grown in rhizoboxes and subjected to a drying cycle, while VWC and phosphatase activity were monitored by neutron radiography and soil zymography. The relation between phosphatase activity and VWC was well described by a diffusion model (R2 = 0.64), demonstrating the importance of diffusion for enzyme activity. This finding was confirmed in the experiment with plants where phosphatase activity strongly decreased upon soil drying. Enzyme activity decreased less in the rhizosphere than in the bulk soil: the ratio between phosphatase activity in the rhizosphere and bulk soil was 10 when the soil was close to saturation and 63 when the soil contained 5% water. The relationship between phosphatase activity and local soil WC were well fitted by the diffusion model (rhizosphere: R2 = 0.54, bulk: R2 = 0.63), emphasizing the effect of diffusion on soil enzyme activity. Our results indicate that soil VWC has a significant effect on soil enzyme activity by increasing diffusion of enzymes and substrate. The higher retention of water in the rhizosphere maintains high enzyme activity around roots in drying soils, which might be beneficial for plant nutrient acquisition.

Root exudation is an important carbon (C) and energy source for soil microorganisms but quantifying its spatial distribution is challenging. We tested whether 14C imaging (analogue of previous autoradiography) can be used to quantitatively estimate the spatial distribution of root exudates in the rhizosphere. First, the attenuation coefficient of 14C β− rays in soil and in water was measured and expected gradients of 14C in the rhizosphere were modelled. Secondly, barley plants were pulse labelled in 14CO2 atmosphere and the origin (roots or root exudation) and locations of 14C signal in soil were detected with imaging. The attenuation coefficient of 14C was 148 cm−1 for soil and 67 cm−1 for water, corresponding to a maximum distance that 14C β− rays pass through a dry soil of 0.37 mm. Based on the measured coefficients we calculated the effect of exudation intensity, root radius and root position on the imaged 14C signal. The distribution of the imaged signal was strongly affected by: a) 14C activity in the root, b) root radius, c) distance from the root surface to the imaging screen, d) amount of root exudates in the soil, and e) presence of an air gap (or a region with high porosity) between the soil and the imaging screen. Neglecting the effects of these factors (a-e) may cause biases in the estimation of root exudates using 14C imaging of the rhizosphere. The 14C imaging approach should therefore be accompanied by accurate measurement of these factors and calculation of the β− ray transmission through the soil.

Aims Soil organic carbon (C) efflux is tightly linked to the rhizosphere, where soil microorganisms rapidly decompose organic compounds released from roots. Recently, imaging approaches have greatly improved our understanding of small-scale C-turnover heterogeneity and promoted the term ‘rhizosphere hotspots’ for highly active areas. However, despite often assumed, the effect of these hotspots on total soil C balances is still unknown. We aim to bridge this gap by correlating rhizosphere imaging data to soil respiration on individual plant scale. Methods We grew 17 maize (Zea mays L.) plants in rhizoboxes filled with sandy arable soil. After four weeks, the plants were labelled with 14CO2 and root exudation was visualized and quantified by 14C-imaging one day after labeling. The evolved CO2 was trapped in NaOH and 14CO2 as well as total CO2 was quantified before and after labelling. Enzyme activity (β-glucosidase) was quantified by soil zymography. Results Bulk soil β-glucosidase activitiy negatively correlated to total CO2 efflux, and was the most important predictor (R2 = 0.55). Total and rhizosphere specific 14C-activity were solely correlated to 14CO2 efflux (r = 0.51, r = 0.58). A combination of bulk soil β-glucosidase activity, rhizosphere-14C activity and root biomass, explained about 75% of variance in CO2 efflux. Conclusions This indicates that root exudation and enzyme-activity hotspots are suitable predictors for total soil respiration, particularly when combined with root biomass to account for three-dimensional variation, and that hotspots on the rhizosphere scale are directly linked to larger scale C balances.

International audience; Soil respiration is tightly linked to rhizosphere processes, which are characterized by high spatial variability. We tested whether planar optodes can be applied to capture this spatial variability of CO2 in the rhizosphere. Maize (Zea mays) was grown in rhizoboxes and CO2 concentration around roots was measured at three volumetric soil water contents (VWC) using planar optodes. Gradients of CO2 were clearly visible around root tips but less pronounced around mature root parts probably due to high root respiration and microbial activity around tips. Saturating the soil from 21% VWC increased the measured CO2 concentration from 0.23 to 1.47 μmol CO2 L−1. Statistical comparisons between VWC levels indicated that small changes in soil WC might strongly affect CO2 measurements. However, provided that the soil moisture is kept constant, optode measurements of CO2 can be used in moist soil samples to quantify relative differences in CO2 concentration between treatments or soil regions

Soil erosion has for a long time been considered as a process causing soil organic matter (SOM) loss, however, recent studies pointed out that erosion may increase soil carbon sequestration because only 10-30% of eroded topsoil material is transported into water bodies while the remaining 70-90% are transported in depositional settings. Soil erosion leads to variation in topsoil thickness and soil characteristics and leads to two different main types of erosion states develop along hillslope: the eroding and the depositional landform position. Disruption of aggregates and the transport of soil during erosion, likely leads to SOM loss in the eroding slope. In contrast, after deposition, the eroded material can be protected if it is incorporated into soil aggregates or sorbed to mineral surfaces, leading to an increase in SOM in the depositional landform position.So far, there has been no study evaluating literature results on the effect of erosion on carbon and nutrient distribution in soils. We therefore reviewed the literature for the influence of erosion on carbon/nutrient contents and stocks in erosion affected landscapes. While 32 studies reported results on the enrichment of eroding sediments in carbon (C), nitrogen (N) and phosphorus (P), 39 studies reported results on carbon/nutrient contents and stocks in erosion affected landscapes.The average C enrichment ratio (sediment C/soil C) was 1.56 while N enrichment ratio was 1.54 and P-enrichment ratio was 1.77. This indicates that the fine soil fractions, that carbon and nutrients are mostly associated to, were preferentially moved during soil erosion. High element contents in the original soils, resulted in relatively low enrichment ratios which may allow the conclusion that in low C- and nutrient soils, a relatively high portion of the elements are stored in the fine soil fraction. C and N enrichment ratios showed a significant positive relation (R2=0.61), pointing to the strong ecological link of both elements.Carbon and nutrient contents were comparable for all landscape positons (upslope, backslope, footslope, depositional). This indicates that carbon and nutrients, lost during an erosion event, are replenished relatively fast in the eroded slopes. In contrast, erosion induced C, N and P stocks increased from the upper towards the depositional soil site, resulting in a 1.6, a 1.4 and 2.2 time increase in C, N and P stocks for the depositional site, compared to the upslope position.In conclusion, this meta-analysis indicates that carbon and nutrients are preferentially moved during soil erosion which might lead to loss in soil fertility and crop productivity after erosion events. However, similar C and nutrient contents along hillslopes indicate that elements are replenished relatively fast in eroded soils after the occurrence of an erosion event. Increased soil stocks toward the depositional site can therefore be explained by increased soil depths in lower hillslope positions. Changes in soil depth, rather than changes in C and nutrient contents are therefore more likely to explain soil fertility losses in eroding slopes compared to depositional sites.

Soil carbon (C) redistribution within cultivated landscapes is strongly controlled by soil erosion and sedimentation and it is widely appreciated that C is preferentially transported during erosion. In contrast, it remains elusive whether erosion induced transport of C and N is coupled although changes in the balance between C and N strongly affect soil C and N dynamics. We therefore reviewed the literature on carbon and nitrogen redistribution by erosion. Twenty-nine studies reported results on C and N enrichment of freshly eroded sediments after erosion events. Thirty-nine studies reported results on C and N contents and stocks along eroded slopes. Eroded sediments were enriched in C and N by 51.3% and 50.6% indicating that both elements are stored in soil fractions that are preferentially eroded. Slope gradient and soil texture strongly affected C and N enrichment. Decreasing C and N enrichment in fine-textured soils was counterbalanced by increasing erosion rates in these soils. This suggests similar SOM losses independent of textural class. The C/N ratio increased by 9.9%, pointing to preferential movement of C-rich particulate organic matter (POM) compared to N-rich mineral associated organic matter (MAOM). Breakdown of aggregates by rainfall energy possibly released POM which is then preferentially eroded. Soil C and N contents and total stocks showed similar percentage increases from upslope to depositional sites, indicating that downslope C and N stocks were largely driven by enrichment, rather by than increases in depth or bulk density. Altogether, our findings confirm that quantification of soil loss alone is not sufficient to estimate erosion-induced changes in soil fertility, because soil organic matter and plant nutrients are selectively moved during erosion. This leads to a shift in C and N dynamics in different slope positions and thus to an increase in the spatial variability of the C and N along the slope.

Background and Aims Although it is commonly accepted that root exudation enhances plant-microbial interactions in the rhizosphere, experimental data on the spatial distribution of exudates are scarce. Our hypothesis was that root hairs exude organic substances to enlarge the rhizosphere farther from the root surface. Methods Barley (Hordeum vulgare 'Pallas' - wild type) and its root-hairless mutant (brb) were grown in rhizoboxes and labelled with 14CO2. A filter paper was placed on the soil surface to capture, image and quantify root exudates. Key Results Plants with root hairs allocated more carbon (C) to roots (wild type: 13 %; brb: 8 % of assimilated 14C) and to rhizosheaths (wild type: 1.2 %; brb: 0.2 %), while hairless plants allocated more C to shoots (wild type: 65 %; brb: 75 %). Root hairs increased the radial rhizosphere extension three-fold, from 0.5 to 1.5 mm. Total exudation on filter paper was three times greater for wild type plants compared to the hairless mutant. Conclusion Root hairs increase exudation and spatial rhizosphere extension, which probably enhance rhizosphere interactions and nutrient cycling in larger soil volumes. Root hairs may therefore be beneficial to plants under nutrient-limiting conditions. The greater C allocation below ground in the presence of root hairs may additionally foster C sequestration.

Rhizodeposition is an important energy source for soil microorganisms. It is therefore crucial to estimate the distribution of root derived carbon (C) in soil and how it changes with soil water content. We tested how drought affects exudate distribution in the rhizosphere by coupling 14CO2 labelling of plants and phosphor imaging to estimate C allocation in roots. Rhizosphere water content was visualized by neutron radiography. A numerical model was employed to predict the exudate release and its spatiotemporal distribution along and around growing roots. Dry and wet plants allocated similar amounts of 14C into roots but root elongation decreased by 48% in dry soil leading to reduced longitudinal rhizosphere extension. Rhizosphere water content was identical (31%) independent of drought, presumably because of the high water retention by mucilage. The model predicted that the increase in rhizosphere water content will enhance diffusion of exudates especially in dry soil and increase their microbial decomposition. Root growth and rhizosphere water content play an important role in C release by roots and in shaping the profiles of root exudates in the rhizosphere. The release of mucilage may be a plant strategy to maintain fast diffusion of exudates and high microbial activity even under water limitation.

The physical properties of the rhizosphere are strongly influenced by root-exuded mucilage, and there is increasing evidence that mucilage affects the wettability of soils on drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage toward the contact region between particles. At low mucilage contents (milligrams per gram of soil), mucilage deposits have the shape of thin filaments that are bypassed by infiltrating water. At higher contents, mucilage deposits occupy a large fraction of the pore space and make the rhizosphere hydrophobic. This hypothesis was confirmed by microscope images and contact angle measurements. We measured the initial contact angle of quartz sand (0.5–0.63- and 0.125–0.2-mm diameter), silt (36–63-μm diameter), and glass beads (0.1–0.2-mm diameter) mixed with varying amounts of chia ( L.) seed mucilage (dry content range 0.2–19 mg g) using the sessile drop method. We observed a threshold-like occurrence of water repellency. At low mucilage contents, the water drop infiltrated within 300 ms. Above a critical mucilage content, the soil particle–mucilage mixture turned water repellent. The critical mucilage content decreased with increasing soil particle size. Above this critical content, mucilage deposits have the shape of hollow cylinders that occupy a large fraction of the pore space. Below the critical mucilage content, mucilage deposits have the shape of thin filaments. This study shows how the microscopic heterogeneity of mucilage distribution impacts the macroscopic wettability of mucilage-embedded soil particles.

Temperature effects on enzyme kinetics and on the spatial distribution of microbial hotspots are important because they are crucial to soil organic matter decomposition. We used soil zymography (in situ method for the two dimensional quantification of enzyme activities) to study the spatial distributions of enzymes responsible for P (phosphatase), C (cellobiohydrolase) and N (leucine-aminopeptidase) cycles in the rhizosphere (living roots of maize) and root-detritusphere (7 and 14 days after cutting shoots). Soil zymography was coupled with enzyme kinetics to test temperature effects (10, 20, 30 and 40 °C) on the dynamics and localization of these three enzymes in the root-detritusphere. The percentage area of enzyme activity hotspots was 1.9–7.9 times larger and their extension was broader in the root-detritusphere compared to rhizosphere. From 10 to 30 °C, the hotspot areas enlarged by a factor of 2–24 and Vmax increased by 1.5–6.6 times; both, however, decreased at 40 °C. For the first time, we found a close positive correlation between Vmax and the areas of enzyme activity hotspots, indicating that maximum reaction rate is coupled with hotspot formation. The substrate turnover time at 30 °C were 1.7–6.7-fold faster than at 10 °C. The Km of cellobiohydrolase and phosphatase significantly increased at 30 and 40 °C, indicating low affinity between enzyme and substrate at warm temperatures. We conclude that soil warming (at least up to 30 °C) increases hotspot areas of enzyme activity and the maximum reaction rate (Vmax) in the root-detritusphere. This, in turn, leads to faster substrate exhaustion and shortens the duration of hot moments.

Soil macropores serve as preferential pathways for water and solute transport as well as for root growth. They are often coated with organic material and known as "hotspots" of nutrient and C turnover. Differences in the SOM composition between macropores and soil matrix as well as between macropore types (biopores, cracks, pinhole fillings) imply potential differences in the microbial community composition and enzymatic activities. The objective of this work was to detect and assess the spatial distribution of enzyme activities related to C turnover, xylanase (XYL) and phenol oxidase (POX), and the composition of microbial communities in structural components of Luvisol Bt-horizons, developed from loess and glacial till. We applied conventional enzyme assays and phospholipid fatty acids (PLFA) analysis to study materials separated from different types of macropores and soil components as well as bulk soil samples. The spatial distribution of enzyme activities on surfaces of large soil core slices (20 cm in diameter) was quantified by soil zymography. Higher XYL activities were detected in separated burrow wall materials, clay-organic coatings, and pinhole fillings from both sites, as compared to the respective soil matrix or bulk soil samples. The XYL activities correlated with bacteria-specific PLFAs. POX activities were solely found increased for earthworm burrow walls from the loess-derived Bt-horizon, but not for burrows from till samples. Zymograms revealed particularly increased XYL activities at rooted earthworm burrows, emphasising evidence for hotspots of enzyme activity and C turnover. The zymography of POX was hampered by methodological restrictions.

Background Recent advances in imaging techniques now make it possible to visualize the biogeochemical and physical environment around the roots, the rhizosphere. Detailed images of pore space geometry and water content dynamics around roots have demonstrated the heterogeneity of the rhizosphere compared with the soil far from the roots. These findings have inspired new models of root water uptake which aim to describe such small-scale heterogeneity. However, the question remains of how far these image-based findings have really advanced our understanding of how roots extract water from soils. Scope The rhizosphere processes affecting root water uptake are reviewed. Special attention is dedicated to the role of mucilage exuded by roots. Mucilage increases the soil moisture at negative water potentials and it keeps the rhizosphere wet when plants take up water, possibly maintaining the hydraulic connection between roots and soil. However, mucilage becomes viscous and hydrophobic upon severe drying and it limits the water fluxes across the rhizosphere during the rewetting phase. The role of mucilage in maintaining the hydraulic contact between the root surface and the surrounding soil, thereby softening the drops in water potential around the roots in dry soils, remains to be demonstrated. Conclusion Despite detailed images of water content, water fluxes and soil structure in the rhizosphere, a general understanding of how the rhizosphere affects root water uptake is still lacking. The missing elements of the puzzle are the gradient in water potential around roots. Measurements of the xylem water potential at varying soil water potentials and transpiration rates supported by numerical models of root water uptake would allow the estimation of the water potential across the rhizosphere. Such measurements are crucial to comprehend how water enters the roots.

We tested whether root exudation confers high P acquisition efficiency (PAE) in rice by quantifying 13C allocation and rhizosphere phosphatase activity for three contrasting genotypes. Phosphatase activity per root surface was similar for all genotypes, indicating that it is not conferring PAE in rice. DJ123 allocated more 13C from shoots to roots compared to Nerica4 and Chomrong Dhan in accordance to increased root biomass of this genotype. The allocation of 13C from roots to the soil was similar between genotypes indicating no difference of root exudation. However, DJ123 showed lower PAE compared to the other two genotypes. We conclude that in the tested genotypes, root systems size rather than root exudation confers PAE.

Mucilage secreted by roots alters hydraulic properties of soil close to the roots. Although existing models are able to mimic the effect of mucilage on soil hydraulic properties for specific soils, it has not yet been explored how the effects of mucilage on macroscopic soil hydraulic properties depend on soil particle size. We propose a conceptual model of how mechanistic pore-scale interactions of mucilage, water, and soil depend on pore size and mucilage concentration and how these pore-scale characteristics result in changes of macroscopic soil hydraulic properties. Water retention and saturated hydraulic conductivity of soils with different ranges of particle sizes mixed with various mucilage concentrations were measured and used to validate the conceptual model. We found that (i) at low mucilage concentrations, the saturated conductivity of a coarse sand was a few orders of magnitude higher than that of a silt, (ii) at an intermediate concentration, the hydraulic conductivity of a fine sand was lower than of a coarse sand or a silt, and (iii) at a high concentration, all soils had a hydraulic conductivity of the same magnitude. At low matric potentials, mucilage increased the water content in all soilsin all soils. In coarser soils, higher mucilage concentrations were needed to induce an increase in water content of >0.05 g g at low matric potentials. This study shows how pore-scale interactions between mucilage, water, and soil particles affect bulk soil hydraulic properties in a way that depends on soil particle size. Including such effects in quantitative models of root water uptake remains challenging.

The rhizosphere is a hot-spot for biogeochemical cycles, including production of greenhouse gases, as microbial activity is stimulated by rhizodeposits released by roots and mycorrhizae. The biogeochemical cycle of nitrogen (N) in soil is complex, consisting of many simultaneously occurring processes. In situ studies investigating the effects of roots and mycorrhizae on gross N turnover rates are scarce. We conducted a 15N tracer study under field conditions in a spruce forest on organic soil, which was subjected to exclusion of roots and roots plus ectomycorrhizae (ECM) for 6 years by trenching. The forest soil had, over the 6-year period, an average emission of nitrous oxide (N2O) of 5.9 ± 2.1 kg N2O ha−1 year−1. Exclusion of roots + ECM nearly tripled N2O emissions over all years, whereas root exclusion stimulated N2O emission only in the latest years and to a smaller extent. Gross mineralization–ammonium (NH4 +) immobilization turnover was enhanced by the presence of roots, probably due to high inputs of labile carbon, stimulating microbial activity. We found contrasting effects of roots and ECM on N2O emission and mineralization, as the former was decreased but the latter was stimulated by roots and ECM. The N2O emission was positively related to the ratio of gross NH4 + oxidation (that is, autotrophic nitrification) to NH4 + immobilization. Ammonium oxidation was only stimulated by the presence of ECM, but not by the presence of roots. Overall, we conclude that plants and their mycorrhizal symbionts actively control soil N cycling, thereby also affecting N2O emissions from forest soils. Consequently, adapted forest management with permanent tree cover avoiding clearcutting could be a means to reduce N2O emissions and potential N leaching; despite higher mineralization in the presence of roots and ECM, N2O emissions are decreased as the relative importance of NH4 + oxidation is decreased, mainly due to a stimulated microbial NH4 + immobilization in the mycorrhizosphere.

Despite the importance of mucilage for soil–plant relations, little is known about the effect of soil drying on mucilage exudation. We introduce a method to collect mucilage from maize growing in wet and dry soils. Mucilage was collected from brace roots. The amount of mucilage exuded did not change with soil water content and transpiration rate. Mucilage exuded in dry soils had a higher degree of hydrophobicity, suggesting that the wetting properties of mucilage change in response to soil drying.

As plant roots take up water and the soil dries, water depletion is expected to occur in the rhizosphere. However, recent experiments showed that the rhizosphere was wetter than the bulk soil during root water uptake. We hypothesise that the increased water content in the rhizosphere was caused by mucilage exuded by roots. It is probably that the higher water content in the rhizosphere results in higher hydraulic conductivity of the root–soil interface. In this case, mucilage exudation would favour the uptake of water in dry soils. To test this hypothesis, we covered a suction cup, referred to as an artificial root, with mucilage. We placed it in soil with a water content of 0.03 cm3 cm–3, and used the root pressure probe technique to measure the hydraulic conductivity of the root–soil continuum. The results were compared with measurements with roots not covered with mucilage. The root pressure relaxation curves were fitted with a model of root water uptake including rhizosphere dynamics. The results demonstrated that when mucilage is added to the root surface, it keeps the soil near the roots wet and hydraulically well conductive, facilitating the water flow from dry soils towards the root surface. Mucilage exudation seems to be an optimal plant trait that favours the capture of water when water is scarce.

Plant roots exude approximately 10% of the carbon assimilated through photosynthesis into the soil, a process referred to as rhizodeposition. Here, we show that the mucilaginous fraction of the rhizodeposits, referred to as mucilage, plays a crucial role on soil–plant water relation and it has the potential to increase plant drought tolerance. Mucilage is a gel that can absorb large volumes of water, altering the physical properties of the rhizosphere and maintaining the rhizosphere wet and conductive when the soil dries. It is hypothesized that mucilage acts as a hydraulic bridge between roots and the soil, facilitating root water uptake and maintaining transpiration in dry soils. By employing a simplified model of root water uptake coupled with mucilage dynamics, we found that in a sandy soil the benefit of mucilage in maintaining root water uptake commenced to manifest when the soil matric potential dropped below approximately −0.8 MPa. This critical matric potential varied with transpiration rate, root length, and exudation rate. Below the critical potential, mucilage maintained photosynthesis and resulted in a net gain of carbon. In summary, rhizodeposition modifies the physical soil environment and has an impact on transpiration and photosynthesis. In other words: water for carbon, but also carbon for water.