This study was designed to examine the fixation pattern and kinetics of zinc
(Zn) in chelated (ethylenediaminetetraacetic acid, EDTA) and non-chelated
mixed micronutrient systems of semi-arid alkaline soils from the Southern High
Plains, USA. Soils were characterized for a suite of chemical and physical
properties and data obtained from extraction experiments fitted to various
kinetic models. About 30 % more plant-available Zn was fixed in the
non-chelated system within the first 14 days with only about 18 %
difference observed between the two systems by day 90, suggesting that the
effectiveness of the chelated compounds tended to decrease over time. The
strengths of the relationships of change in available Zn with respect to
other micronutrients (copper, iron, and manganese) were higher and more
significant in the non-chelated system (average
The soil, a subject of interdisciplinary study (Brevik et al., 2015), has
numerous ecological functions, among which is the storage and cycling of
plant-needed nutrients (Smith et al., 2015). Micronutrient fixation, a
process that leads to the reduction of plant-available portion of
micronutrients, through the interactions with other soil constituents, limits
crop productivity in most parts of the world (WHO, 2000). Plant-available
portion of micronutrient is generally controlled by a number of factors such
as soil organic matter (OM), clay, calcium carbonate (CaCO
Given the ease of fixation in soil, micronutrients such as Zn are often recommended to be applied in the forms of organic or synthetic chelates to enhance their availability to plants. The chemistry and effectiveness of chelated micronutrient compounds have also been previously documented under certain soil conditions and types (Sekhon, 2003; Luo et al., 2005; Chiu et al., 2005; Lucena et al., 2008). However, due to the heterogeneous nature of soils, it could be misleading to apply findings from one soil type and region to another, thus the need for soil- and site-specific studies.
As with other arid to semi-arid regions of the world, the Southern High Plains (SHP) of the USA is currently facing numerous environmental challenges such as drought, soil salinization, and wind erosion that limit agricultural productivity (Mehta et al., 2000; Stout, 2001; Allen et al., 2005). This region is typified by semi-arid climatic conditions with characteristically alkaline soil types. Thus, the characteristically high pH soils and climatic conditions of this region favor micronutrient deficiency. Recent field observations have also confirmed cases of limited crop productivity resulting from low level of micronutrients, particularly Zn in some important agricultural soils in Texas High Plains. In spite of the agronomic significance of the soils of this region, there is still little information on the chemistry of micronutrients in the semi-arid alkaline soils of this region (Udeigwe et al., 2016). Examining the fixation kinetics of micronutrients in these soils is vital for understanding micronutrient dynamics for further development of nutrient management tools for long-term agricultural and environmental sustainability. Kinetic parameters obtained from such efforts can be used for comparisons among micronutrients as well as among soil types. A more systematic approach to study micronutrients in soil systems will encompass examining their chemistry in a mixed system of a number of other micronutrients. Simple relationships developed from such examinations could be used for future predictive purposes.
A major limitation to previous studies (Manouchehri et al., 2006; Reyhanitabar and Gilkes, 2010; Abbas and Salem, 2011) on micronutrient fixation was that the experimental conditions (e.g. sample size, reaction times, experimental duration) limit the application of findings to field settings. An extensive literature search on these semi-arid soils indicates that limited resources are available to address the following: (i) how much of applied plant-available Zn will be present at a specific time, (ii) the reaction rates and mechanism of Zn fixation, (iii) how these compare to those of other micronutrients, and (iv) how they vary among chelated and non-chelated micronutrient compounds. The objective of this study was to examine the fixation pattern and kinetics of plant-available Zn in chelated (ethylenediaminetetraacetic acid, EDTA) and non-chelated mixed systems of selected agriculturally important soils of the SHP, USA. The experimental conditions of this study will facilitate the easier application of findings to field settings. Findings are intended to enhance the understanding of Zn chemistry and management in the semi-arid regions. This study is a part of a broad project on elucidating micronutrient fate in semi-arid alkaline soils. The first part of the project, which was focused on Cu, has been published (Udeigwe, et al., 2016) and will be referenced appropriately in this study.
Soil samples of interest were collected from three different crop production sites in western Texas. These soils were identified using the Web Soil Survey of the Natural Resources Conservation Services (NRCS). Surface (0–15 cm) and subsurface (15–30 cm) soil samples were collected from three important soil series in the SHP. The soil series include the Amarillo (A), Pullman (P), and Mansker (M), and their descriptions are presented in Table 1. The selected depths are the typical ones examined in most soil fertility and nutrient management studies (Havlin et al., 2013). Soil samples were randomly collected from approximately 12–15 spots at each representative site, using a digging spade marked at 0–15 and 15–30 cm depths. Samples from the same depth at each site were combined to get a composite sample of about 10 kg. Samplings were sometimes restricted to a defined area of approximately 5–7 ha to avoid crossing into a different soil series.
Soil classification and identification of the studied semi-arid alkaline soils of the Southern High Plains, USA.
Udeigwe et al. (2016)
From each original (untreated) soil sample, a subsample of approximately
2 kg was ground, passed through a 2 mm sieve, and stored in plastic bags at
a room temperature of approximately 23
Soil sample from each depth was thoroughly mixed and a representative portion
taken to fill a 4 L plastic pot. Each pot was planted with sorghum
(
Preparation of the DTPA extractant and the extraction procedure used followed
the method described by Lindsay and Norvell (1978). The DTPA extraction
technique is the most commonly and broadly used approach for estimating
plant-available micronutrient cations such as Fe, Mn, Cu, and Zn (Liang and
Karamanos, 1993). In brief, 20 mL of DTPA extracting solution was added to
10 g of air-dried soil sample in a 50 mL plastic tube. All tubes were
shaken on a reciprocal shaker for 2 h at approximately 25
Statistical analyses were performed using the Statistical Analysis Software
(SAS 9.4, SAS Institute, Cary, NC). Differences among means, where
applicable, were examined using PROC GLM and mean comparison conducted using
Fisher's least significance difference (LSD) at
Selected chemical (pH, EC, OM, and CaCO
Percent estimates of fixed plant-available Zn determined after the first 14 days (designated as short term), and 90 days (long term) are presented in Table 2. Individual soils and depths were examined; however, findings reveal no justifiable differences among the soils worth discussing. Thus, the findings are summarized as averages of all soils within a given depth and for both depths. The percent amount fixed was approximated using the differences between days 2 and 14, and days 2 and 90 for the 14 days and 90 days examinations. Comparison was made between the chelated and non-chelated micronutrient treatments. Average values from the three soil series examined revealed that within the non-chelated system, approximately 31.3 and 41.3 % of the added Zn was fixed in the 0–15 cm and 15–30 cm depths, respectively, after the first 14 days. When compared to the chelated system, these numbers were drastically reduced to 5.1 and 6.8 % in the 0–15 and 15–30 cm depths, respectively. After the first 14 days, the averages for both depths were 36.3 and 6.0 % for non-chelated and chelated systems, respectively. The findings clearly indicate that chelating with EDTA reduced the amount of plant-available Zn fixed by soil constituents (Lopez-Valdivia et al., 2002; Chiu et al., 2005; Alvarez and Gonzalez, 2006). This observed difference between the chelated and non-chelated partly supports the high fixation of Zn encountered in most alkaline soil, particularly when applied non-chelated and why Zn is often the most deficient micronutrient in most alkaline soils (Alloway, 2008).
Diethylenetriaminepentaacetic acid (DTPA)-extractable Zn
over the long term (90 days) in the non-chelated system fitted to
Diethylenetriaminepentaacetic acid (DTPA)-extractable Zn
over the long term (90 days) in the chelated system fitted to
Average % (with standard deviation) plant-available Zn fixed after 14 and 90 days in the non-chelated and chelated systems of the semi-arid alkaline soils of the Southern High Plains, USA.
Mean values within a column in a given Zn system with the same
lowercase letter and mean values within a column between the Zn systems with
the same uppercase letter are not statistically different (Fisher's LSD
The amount of Zn fixed by the end of the experimental period of 90 days (long-term fixation) was also examined. About 51.1 and 61.4 % of available Zn was fixed after 90 days in the non-chelated system within the 0–15 and 15–30 cm depths, respectively. These numbers when compared to the chelated system were 30.7 and 45.1 % for the 0–15 and 15–30 cm depths, respectively. Average fixation for both depths in these semi-arid soils after 90 days was 56.2 % for non-chelated system and 37.9 % for chelated system, a difference of approximately 18 % compared to the 30 % observed in the short-term fixation (14 days). The narrower differences in Zn fixation observed on the long term could be attributed to the half-life of the chelating agent, EDTA. A half-life of 39 to 59 days for EDTA in doses of 0.8 to 1.6 mmol in a heavy metal phytoextraction study was estimated by Meers et al. (2005), suggesting that the effectiveness of EDTA on micronutrient mobilization will decrease over time, causing more micronutrient to be fixed by other soil constituents. This can also be partly attributed to the dissolution of calcite in these alkaline soils, which has been shown to consume EDTA, thus reducing its effectiveness (Papassiopi et al., 1999). Although not significant, the slightly higher fixation of Zn observed in the subsurface soil could be partly attributed to higher clay content. Clay interaction with metal cations such as Zn, which could reduce the amount of plant-extractable Zn, has been widely documented (Sparks, 2003; Eze et al., 2010; Udeigwe et al., 2015).
Changes in the concentration of plant-available Zn over the experimental
period of 90 days were compared to those of other micronutrients in both the
chelated and non-chelated systems (Table 3). The relationships were examined
among individual soil and depth; however, there was no remarkable differences
among soils worth discussing. Thus, the findings are summarized as averages
for the soils at each depth and for both depths combined within each Zn
system (chelated and non-chelated). The strengths of the relationships and
gradients of change between Zn and each of the other micronutrient elements
(Cu, Fe, and Mn) were examined using regression analyses. Within each depth,
the amount of available Zn positively changes with each of the other
micronutrients, although to a varying degree within the chelated and
non-chelated micronutrient systems. Overall, the strengths of the
relationships were higher in the non-chelated systems (average
Regression equation and coefficient of determination (
Fixation kinetics of chelated and non-chelated Zn in these soils
were further examined by fitting the data obtained from kinetic experiments
to various kinetic models. A number of kinetic models (Table 4) were
examined, based on the experimental conditions of this study and evidence
gathered from previous related studies (Dang et al., 1994; Reyhanitabar and
Gilkes, 2010). The criteria used for evaluating best fit among the models
were coefficient of determination (
Kinetic models used in the examination of chelated and non-chelated Zn fixation in the studied semi-arid alkaline soils of the Southern High Plains, USA.
Coefficient of determination (
SE denotes standard error.
The data from Zn kinetic experiments were fitted to the zero-order, first-order,
second-order, and power-function models and findings summarized in Table 5. In all
the models,
The fixation kinetics of non-chelated Zn following the power-function model over the zero-, first-, and second-order models, are an indication of a more complex reaction type. Inferences drawn from the short- and long-term experiments substantiate the need to apply Zn micronutrient to these semi-arid soils in the chelated form as significantly less chelated Zn was fixed particularly within the first 14 days. Findings also highlighted the importance of timing in Zn micronutrient management in these soils even when chelated micronutrient compounds are used. The simple linear relationships of change in plant-available Zn relative to other micronutrients (Cu, Mn, and Fe) could be used as predictive tools. The kinetic parameters obtained from the kinetic experiments could be used for approximating how much of added Zn micronutrient will be available at a given time, particularly for the non-chelated Zn material since its fixation was reasonably described by the power-function model.
Findings from this study provide a basis for developing applications for comparing fixation pattern of Zn to those of other micronutrients in a given soil and also among soils. Of interest, the applications developed from this study provide a basis for a more mechanistic approach to evaluating and comparing the fixation patterns and effectiveness of different micronutrient compounds in any given soil system. A database of the reaction rate constants derived for different Zn micronutrient compounds could be used as a tool for making a more informed decision on Zn management on these semi-arid soils, an application that can be extended to soils of other regions.
The application of kinetic models to Zn fixation could be used to further the understanding of its chemistry and behavior in the soils of the semi-arid to arid climates. The reduction of plant-available Zn more closely followed the power-function models over the zero-, first-, and second-order models in these soils, suggesting a more complex reaction type. Timing is an important practice in Zn management for these semi-arid soils, even when chelated compounds are used. Evidence gathered from this study suggests that kinetic model application to Zn fixation provides a sound basis for evaluating Zn dynamics among soil types and for comparing different Zn micronutrient compounds. The experimental setup and conditions of this study will facilitate the easier application of findings to field settings. This study provides useful background information that will enable future studies on the examination of the reaction mechanisms involved in zinc fixation in chelated and non-chelated systems of these semi-arid soils.
The authors thank the College of Agricultural Sciences and Natural Resources, Texas Tech University, for assisting with the research enhancement funds that partly supported this study. Edited by: A. Cerdà Reviewed by: two anonymous referees