Plant roots nurture a wide variety of microbes via exudation of metabolites, shaping the rhizosphere's microbial community. Despite the importance of plant specialized metabolites in the assemblage and function of microbial communities in the rhizosphere, little is known of how far the effects of these metabolites extend through the soil. We employed a fluid model to simulate the spatiotemporal distribution of daidzein, an isoflavone secreted from soybean roots, and validated using soybeans grown in a rhizobox. We then analysed how daidzein affects bacterial communities using soils artificially treated with daidzein. Simulation of daidzein distribution showed that it was only present within a few millimetres of root surfaces. After 14 days in a rhizobox, daidzein was only present within 2 mm of root surfaces. Soils with different concentrations of daidzein showed different community composition, with reduced α‐diversity in daidzein‐treated soils. Bacterial communities of daidzein‐treated soils were closer to those of the soybean rhizosphere than those of bulk soils. This study highlighted the limited distribution of daidzein within a few millimetres of root surfaces and demonstrated a novel role of daidzein in assembling bacterial communities in the rhizosphere by acting as more of a repellant than an attractant.
The soil‐gas diffusion is a primary driver of transport, reactions, emissions, and uptake of vadose zone gases, including oxygen, greenhouse gases, fumigants, and spilled volatile organics. The soil‐gas diffusion coefficient, Dp, depends not only on soil moisture content, texture, and compaction but also on the local‐scale variability of these. Different predictive models have been developed to estimate Dp in intact and repacked soil, but clear guidelines for model choice at a given soil state are lacking. In this study, the water‐induced linear reduction (WLR) model for repacked soil is made adaptive for different soil structure conditions (repacked, intact) by introducing a media complexity factor (Cm) in the dry media term of the model. With Cm = 1, the new structure‐dependent WLR (SWLR) model accurately predicted soil‐gas diffusivity (Dp/Do, where Do is the gas diffusion coefficient in free air) in repacked soils containing between 0 and 54% clay. With Cm = 2.1, the SWLR model on average gave excellent predictions for 290 intact soils, performing well across soil depths, textures, and compactions (dry bulk densities). The SWLR model generally outperformed similar, simple Dp/Do models also depending solely on total and air‐filled porosity. With Cm = 3, the SWLR performed well as a lower‐limit Dp/Do model, which is useful in terms of predicting critical air‐filled porosity for adequate soil aeration. Because the SWLR model distinguishes between and well represents both repacked and intact soil conditions, this model is recommended for use in simulations of gas diffusion and fate in the soil vadose zone, for example, as a key element in developing more accurate climate change models.
Isoflavones play important roles in rhizosphere plant-microbe interactions. Daidzein and genistein secreted by soybean roots induce the symbiotic interaction with rhizobia and may modulate rhizosphere interactions with microbes. Yet despite their important roles, little is known about the biosynthesis, secretion and fate of isoflavones in field-grown soybeans. Here, we analyzed isoflavone contents and the expression of isoflavone biosynthesis genes in field-grown soybeans. In roots, isoflavone contents and composition did not change with crop growth, but the expression of UGT4, an isoflavone-specific 7-O-glucosyltransferase, and of ICHG (isoflavone conjugates hydrolyzing beta-glucosidase) was decreased during the reproductive stages. Isoflavone contents were higher in rhizosphere soil than in bulk soil during both vegetative and reproductive stages, and were comparable in the rhizosphere soil between these two stages. We analyzed the degradation dynamics of daidzein and its glucosides to develop a model for predicting rhizosphere isoflavone contents from the amount of isoflavones secreted in hydroponic culture. Conjugates of daidzein were degraded much faster than daidzein, with degradation rate constants of 8.51 d-1 for malonyldaidzin and 11.6 d-1 for daidzin, vs. 9.15 × 10-2 d-1 for daidzein. The model suggested that secretion of isoflavones into the rhizosphere is higher during vegetative stages than during reproductive stages in field-grown soybean.
[1] Describing and predicting gas and solute diffusivities and electrical and thermal conductivities under variably saturated fluid conditions are necessary for simulating gas, solute, and heat transport in soils. On the basis of comprehensive data for gas (D p ) and solute (D s ) diffusivities and electrical (EC) and thermal (TC) conductivities for differently textured and variably saturated soils, we investigated analogies and differences between the four parameters. At fluid (water or air) saturation, relative parameter values for D p , D s , and EC were all well described by an excluded-volume expansion of Archie's first law. The cementation exponent in Archie's first law was close to 1.5 for all parameters. At fluid-unsaturated conditions, relative values of D p , D s , and EC (normalized at fluid saturation) were well described by an excluded-volume expansion of Archie's second law. In the case of relative TC, the saturation exponent in Archie's second law was substituted by the inverse of it for the three other parameters since water bridge effects dramatically enhance the TC with increasing moisture contents in relatively dry porous media. If appropriate but different expressions for a percolation threshold in Archie's second law were applied for the four parameters, a saturation exponent value of around 2.0 generally gave accurate predictions of all four parameters for differently textured soils. Finally, the excludedvolume expansion of Archie's second law was modified to also represent porous media with bimodal pore size distribution and well-described data for D p and D s in aggregated soil.Citation: Hamamoto, S., P. Moldrup, K. Kawamoto, and T. Komatsu (2010), Excluded-volume expansion of Archie's law for gas and solute diffusivities and electrical and thermal conductivities in variably saturated porous media, Water Resour. Res., 46, W06514,
T , ,of gases in the soil vadose zone are governed by gas diff usion and advection. Th e soil gas diff usivity, D p /D 0 (where D p is the soil gas diff usion coeffi cient [m 2 s −1 ] and D 0 is the gas diff usion coeffi cient in free air [m 2 s −1 ]), is the governing transport parameter for gas diffusion under gas concentration gradients, while k a (μm 2 ) is the governing transport parameter for advective gas transport under soil air pressure gradients. Accurate prediction of the variation of D p /D 0 and k a with ε plays a crucial role in simulating transport of volatile organic chemicals in soil (Poulsen et al., , 1999 and in investigating the exchange and emission of greenhouse gases at the soil-atmosphere interface (Kruse et al., 1996;Ball et al., 1997).Th e gas transport parameters are strongly aff ected by soil physical properties such as particle size distribution and bulk density, as well as pore structure parameters including ε, total porosity, and the pore connectivity-tortuosity of the air-fi lled pore space. Osozawa (1998) measured D p /D 0 and k a for four diff erently textured soils and suggested that the diff erences in measured D p / D 0 were mainly controlled by the air-fi lled pore space and pore network tortuosity, while the diff erences in measured k a were also governed by soil pore size distributions and the continuity of large-pore networks. In agreement with this, McCarthy and Brown (1992) observed higher k a values for structured soils than for relatively structureless soils. Fujikawa and Miyazaki (2005) reported that soil compaction led to an increase in D p /D 0 at the same ε value, probably caused by the decrease in inactive air-fi lled pore space for gas diff usion and lower volumetric water content (i.e., lower water blockage eff ects) (Moldrup et al., 2000). Th e presence of soil water greatly aff ects gas transport parameters by water blockage of gaseous fl ow paths at bottlenecks between pores, in addition to the reduction of air-fi lled porosity. Supporting this hypothesis, lower D p /D 0 in wet media compared with dry media at the same ε has been observed due to the water-induced changes of the pore shape and confi guration and the tortuosity of air-fi lled pores (Papendick and Runkles, 1965). Moldrup et al. (2000) therefore suggested to separate the tortuosity-connectivity for dry porous media and the additional water blockage eff ects on D p /D 0 for wet media in order to take the increased tortuosity of soil air-fi lled pore space in wet soil into account in predictive D p /D 0 models. Although these studies all imply eff ects of particle size distribution and soil compaction on D p /D 0 and k a , further
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