Dielectric Spectroscopy and Application of Mixing Models Describing Dielectric Dispersion in Clay Minerals and Clayey Soils.

The number of sensors, ground-based and remote, exploiting the relationship between soil dielectric response and soil water content continues to grow. Empirical expressions for this relationship generally work well in coarse-textured soils but can break down for high-surface area and intricate materials such as clayey soils. Dielectric mixing models are helpful for exploring mechanisms and developing new understanding of the dielectric response in porous media that do not conform to a simple empirical approach, such as clayey soils. Here, we explore the dielectric response of clay minerals and clayey soils using the mixing model approach in the frequency domain. Our modeling focuses on the use of mixing models to explore geometrical effects. New spectroscopic data are presented for clay minerals (talc, kaolinite, illite and montmorillonite) and soils dominated by these clay minerals in the 1 MHz-6 GHz bandwidth. We also present a new typology for the way water is held in soils that we hope will act as a framework for furthering discussion on sensor design. We found that the frequency-domain response can be mostly accounted for by adjusting model structural parameters, which needs to be conducted to describe the Maxwell-Wagner (MW) relaxation effects. The work supports the importance of accounting for soil structural properties to understand and predict soil dielectric response and ultimately to find models that can describe the dielectric-water content relationship in fine-textured soils measured with sensors.

Effect of Organic Matter on Sorption of Zn on Soil: Elucidation by Wien Effect Measurements and EXAFS Spectroscopy.

Soil organic matter (SOM) is the major factor affecting sequestration of heavy metals in soil. The mean free binding energy and the mean free adsorption energy and speciation of Zn in soil, as affected by SOM, were determined by employing Wien effect measurements. The presence of SOM markedly decreased the Zn binding energy in soils in the following order: Top (5.86 kJ mol(-1)) < Bottom (8.66 kJ mol(-1)) < Top OM-free (9.44 kJ mol(-1)) ≈ Bottom OM-free (9.50 kJ mol(-1)). The SOM also significantly decreased the adsorption energy of Zn on black soil particles by reducing nonspecific adsorption of Zn on their surfaces. The speciation of Zn in soils was elucidated by extended X-ray absorption fine structure spectroscopy and microfocus X-ray fluorescence. The results obtained by linear combination fitting of EXAFS spectra revealed that the main forms of Zn in soil were outer-sphere Zn, Zn-illite, Zn-kaolinite, and HA-Zn. As the SOM content increased, the proportion of HA-Zn among the total immobilized Zn increased, and the proportion of nonspecific adsorbed Zn decreased. The present results implied that SOM is an important controlling factor for the environmental behavior of Zn in soils.

Dry Matter Accumulation in Maize in Response to Film Mulching and Plant Density in Northeast China.

Film mulching in combination with high plant density (PD) is a common agronomic technique in rainfed maize (Zea mays L.) production. However, the effects of combining colored plastic film mulching and PD on dry matter accumulation (DMA) dynamics and yield of spring maize have not been thoroughly elucidated to date. Thus, a 2-year field experiment was conducted with three mulching treatments (no mulching (M0), transparent plastic film mulching (M1), and black plastic film mulching (M2)) and five plant densities (60,000 (D1), 67,500 (D2), 75,000 (D3), 82,500 (D4), and 90,000 plants ha-1 (D5)). A logistic equation was used to simulate the DMA process of spring maize by taking the effective accumulated air temperature compensated by effective accumulated soil temperature as the independent variable. The results showed that compared with M0 treatment, the growth period of M1 and M2 treatments was preceded by 10 and 4 days in 2016, and 10 and 7 days in 2017, respectively. The corrected logistic equation performed well in the characterization of maize DMA process with its characteristic parameter (final DMA, a; maximum growth rate of DMA, GRmax; effective accumulated temperature under maximum growth rate of DMA, xinf; effective accumulated temperature when maize stops growing, xmax; effective accumulated temperature when maize enters the fast-growing period, x1). Plastic film color mainly affected DMA by influencing xinf. PD mainly affected DMA by affecting GRmax and x1. During the first slow growing period, the DMA of M1 treatment was the largest among the three mulching treatments, however, during the fast growing period, the DMA of M2 treatment accelerated and exceeded that of M1 treatment, resulting in the largest final DMA(a) and yield. When the PD was increased from D1 to D4, the maximum growth rate (GRmax) continued to increase, and the effective accumulated temperature when maize enters the fast growing period (x1) continued to decrease, which substantially increased the final DMA(a) and yield. The application of M2D4 treatment can harmonize the relevant factors to improve the DMA and yield of spring maize in rainfed regions of Northeast China.

A new method to estimate adsorption energies between cations and soil particles via wien effect measurements in dilute suspensions and an approximate conductivity-activity analogy.

Measurement of the Wien effect (increased electrical conductivity at strong electrical fields) in soil suspensions is proposed as the basis of a new method to characterize energy relationships between cations and soil particles. The simplified theory behind the method, the working principle of the short high-voltage pulse apparatus, and the measuring procedure are outlined briefly. The new method was used to evaluate the adsorption energies of two monovalent (Na, K) and two divalent (Ca, Cd) cations on yellow-brown soil and black soil particles, assuming an analogy between the activity of the cations and their contribution to the electrical conductivity of the suspension. Both the mean free bonding energies, deltaGbo, and the adsorption energies, deltaGad, of the cations for these two soils increased in the order: Na < K < Ca < Cd. Under the conditions of our experiments, estimated deltaGbo ranged from 4.7 to 6.4 kJ mol(-1) and from 7.0 to 8.2 kJ mol(-1) for mono- and divalent cations, respectively. The bonding energies obtained with the new method were similar to those determined previously by ionic activity measurement. The determined mean adsorption energies of cations desorbed at a field strength of 100 kV cm(-1), for example, ranged from 0.7 to 1.2 kJ mol(-1) and from 1.9 to 2.3 kJ mol(-1) for mono- and divalent cations, respectively.