Rhizobox Technology for Sustainable Agriculture–Acquired Implications

Evaluation on the effect of rhizobox technology in the rhizosphre soil of agricultural crops is necessary for sound and safe agriculture. This article confers the rhizobox technology-induced information about rhizosphere so far conducted since 1987s and on. Combined with its special features and construction technique, rhizobox-affected rhizosphere processes like pH changes, patterns of nutrient distribution, heavy metal movement and microbial activities are extensively analyzed. Finally, the established rhizobox model of 1987 encompassing its modified versions are detailed, followed by its potential application in rhizosphere studies of plant nutrition, soil chemistry, and soil biology for sustainable agriculture.

Introduction

Root affected soil is the rhizosphere [1]. After that huge amount of research work has been conducted globally to investigate the rhizosphere processes in plant soil system studies because of its numerous physical, chemical and biological importance on plant growth and development [2] encompassing the substantial reviews [3], [4]. However, only a few works have focused on the separation of rhizosphere and bulk soils [5]–[9]. So, in general, rhizosphere research lacks the demarcation of rhizosphere and bulk soils followed by the clear extent of the rhizosphere effect. However, for the last three decades, rhizosphere research technology established and developed by Youssef and Chino [10], [11], named the rhizobox tool, has made tremendous development in rhizosphere research. Because this rhizobox system allows one to sample rhizosphere and bulk soils located at prescribed distances with intense root mat formation to insight rhizosphere affected phenomena. In turn, this very specificity of the rhizobox system leads scientists to obtain quantitative data on nutrient distribution, heavy metal movement and microbial activities encompassing relevant pH changes in association with appreciable plant growth as insighting rhizosphere processes are difficult in general [12]. Thus, the rhizobox technology is a box type container used for sampling rhizosphere and bulk soils at defined distances [10], [11]. Consequently, this technology leads rhizosphere scientists to investigate the intensely developed root mat-affected rhizosphere phenomena like determination of pH changes, nutrient distribution and heavy metal movement, including microbial activity precisely [13]. Apart from this, rhizosphere implicates largely soil management, agro-climate and sustainable agriculture [3], [14] and soil fertility [15]. So, rhizosphere study-based findings revealed through rhizobox technology would serve as a practical guide to enlarge and broaden our ideas and thoughts to enhance the sustainable agriculture.

Uniqueness of the Rhizobox System [11]

Naturally question arises what is rhizobox? What features make it exceptional? It has several compartments which are made of a plastic frame having a nylon cloth stretched across it provided with radius pore (<25 πm), which allows water and nutrients to all the compartments arranged on the left and right-hand sides of the central compartment (CC) of the rhizobox. The remarkable feature of the rhizobox tool is that the plants are grown on the top of the CC and its nylon cloth fruitfully confines the root growth within itself and thus confirms the distinctly rhizosphere affected individual soil sampling at demarcated distances of the various compartments of the rhizobox. Consequently, rhizosphere-influenced phenomena like pH changes, nutrient distribution, heavy metal movement and microbial activities of the rhizosphere and bulk soils are ensured with their respective evidential range of rhizosphere effect.

Construction

Depicted pictogram in Fig. 1 is the structure of the rhizobox. Basically, it consists of several compartments, which are articulated plastic frames of nylon cloth stretched across. Aided with dichloromethane, the plastic frames are firmed on one side, having the dimensions of 200 mm length, 200 mm width and 5 mm thickness. Compartments arranged on the left and right-hand sides of the central compartment (CC) are sealed with dichloromethane. Devised compartments on either side of the CC are named 0–5 mm, 5–10 mm, and 10–15 mm and a large compartment attached to each end of the large compartment designated as greater than 15 mm compartment contains soil that may never be affected by root activity called bulk soil. Subsequently, all the compartments of the rhizobox are packed with test soil. For instance, the large compartment frame is filled with air dry sieved and treated soil by laying horizontally on a flat table, followed by filling the 10–15 mm compartment by overlaying on the large compartment. Similarly, in the same way, other new compartments/remaining compartments are overlaid on the old ones one by one as the formers are filled with treated soil (Fig. 1a). However, later rhizobox, as pictorially shown in Fig. 1b, was modified for suitable and concrete studies. So, the altered rhizobox is composed of several compartments made of a plastic frame with a nylon cloth (200 mm/200 mm/1 mm) having the same radius pore. Here the CC consists of a plastic frame (200 mm/200 mm/2 mm) having the same features. Compartments arranged on the left side of the CC are designated as L₁, L₂, L₃, L₄, L₅, and L₅₀, and those on the right-hand sides of the CC are designated as R₁, R₂, R₃, R₄, R_5, indicating a distance of 1 mm, 2 mm, 3 mm, 4 mm, 5 mm from the left and right-hand sides of the CC, respectively. Large bulk soil compartments made of a plastic frame (200 mm/200 mm/50 mm) attached to each end of the rhizobox are designated as L₅₀ and R_50, indicating a distance of 50 mm from the CC to either side of the rhizobox. Thus, this rhizobox technology reveals the definite range of rhizosphere (0–50 mm) affected phenomena like pH changes, nutrient distribution, and heavy metal movement, including microbial activity demarcating the rhizosphere and bulk soils (dispensed/situated at specific distances).

Fig. 1. Design of rhizobox: (a) schematic diagram of a rhizobox from [10] where CC means central compartment (zero compartment), (b) schematic diagram of a rhizobox from [16] and [17] where Z is CC.

Rhizobox Induced Rhizosphere Processes

Plant Growth

Plants grow remarkably (Fig. 2) at defined growth period being confined at the CC [10], as rhizosphere influences plant culture and growth to a great extent [18]. Reports on the beneficial effects of rhizosphere on plant growth and development are evident [19] as saliently rhizosphere exerts a tremendous role on plant growth [20]. Better plant culture and growth is the salient feature of induced rhizosphere effect [3], [14], [21] although it varies largely depending on the crop grown.

Fig. 2. Increase in plant (wheat) height in centimeter per week after plantation in the rhizobox. Plants were grown in the central compartment (0). Values are means of height of four plants of both boxes [18].

Root Growth and Traits

With reference to an intense root mat formation (Fig. 3), there are well recognized findings [3], [14], [21], [22]. These authors also suggested that the profound root development in rhizobox technology is the typical feature. Additionally, convincing evidence on remarkable root development across rhizosphere has been well documented [23]. So, one of the interesting aspects in rhizosphere studies is the dense root mat affected rhizosphere phenomena [10], [18].

Fig. 3. Roots developed, after two months of transplanting in the C.C (0 mm) of oxamide and ammonium sulphate treatment [22].

pH Changes

Rhizosphere induces changes on the various rhizosphere processes because of differential rhizosphere conditions and rhizobox exerts an important role on pH change of both rhizosphere and bulk soil (Fig. 4) where rhizosphere effect remains extended up to a distance of 5 mm [10]–[22]. In experiment with barley and wheat grown on rhizobox tool, Youssef and Chino [11] and Miah et al. [18] found similar pattern of rhizosphere pH change in association with same range of rhizosphere effect. However, pH changes in the rhizosphere largely depend on kind of treatment imposed, type of soil used, and crop variety tried.

Fig. 4. Wheat rhizosphere effect on pH change as measured after 15 days of plantation in the central compartment (CC) designed as zero compartments. L and R compartments were arranged on the left- and right-hand sides of the central compartment. The numbers indicate the distance from the root e.g., L₂.

Nutrient Status

Changes in pH across the rhizosphere (Figs. 5 and 6) are crucial for nutrient acquisition and enhance nutrient movement and distribution [3]. As for relationship between rhizosphere pH change and mobility of nutrients, there are evidential reports [4], [21]. In this regard, rhizobox technology induced data on pH change and nutrient dynamics revealed that such phenomena may affect on nutrient behavior being dispensed up to distances of 15 mm and 5 mm, respectively [11], [18]. Thus, pH change in the rhizosphere is important not only because of its diverse effect on plant nutrition in general but also because of its unique role on nutrient dynamics in particular.

Fig. 5. Distribution of: (a) Ca, (b) Mg and (c) K across the rhizosphere of wheat.

Fig. 6. Distribution of NH₄ (above) and NO₃ (below) across the rhizosphere of wheat.

Heavy Metal Distribution

Rhizosphere soil properties are different from those of bulk soil and thus are of vital necessity for heavy metal movement and dynamics [16]. Simultaneously, demonstrations of heavy metal solubility across the rhizosphere are also available [24], having no defined extent of rhizosphere effect [3]. However, in the plant rhizosphere, there are differential patterns of heavy metal movement and distribution with respect to the definite range of rhizosphere [17]. Namely, the rhizosphere effect varies from 4 to 7 mm distance depending on the treatment and crop variety [17], [25], and rhizobox technology is the most effective tool for determining the extent of rhizosphere effect with clear demarcation between rhizosphere and bulk soil being situated at defined distances for heavy metal movement and availability (Figs. 7a–7c).

Fig. 7. Relation between the relative percentage of soluble: a) Fe, b) Cu, and c) Mn and the distance from plant roots in clay loam soil from [26].

Microbial Activities

As for microbial functions namely β-glucosaminidase (Fig. 8), generalized agreement is that the rhizosphere and bulk soil properties are remarkably different [15] owing to dispensed up to few millimeter [3]. Because microbial numbers particularly bacteria and fungi [25] significantly influence the microbial activities across the rhizosphere [27], as major contribution for sustained soil health [28] resulting from their abundance and huge biological activities [3]. In aid with the rhizobox system, thus considerable progress on quantitative microbial activities for rhizosphere and bulk soil samples at prescribed distances have been well documented [10] and its modified version [27].

Fig. 8. Distribution of β-glucosaminidase across the rhizosphere of wheat. Values are means of compartments of both sides.

Rhizobox and Sustainable Agriculture

To a great extent, the agricultural sustainability depends on the rhizosphere induced [14] nutrient use efficiency of the plants as revealed with rhizobox technology [3]. However, till update plant growth as well as yield greatly depend on high input chemical fertilizers and rhizosphere because of its physical, chemical and biological role on enhancement of nutrient uptake efficiency of applied chemical fertilizers [15], [18] mainly because of rhizobox affected maximization of the applied nutrient use efficiency. This suggests that a rhizobox based rhizosphere research with emphasis on the minimization of chemical nutrient input is essential to retain sustainable agriculture. Additionally, for efficient nutrient uptake, rhizobox plays a unique role through the maintenance of intricate relationship among soil, root, rhizosphere and plant [29]. However, derivation of the efficient nutrient use by the plants largely depends on the plant types grown and fertilizer applied [17], [18]. Since soils differ in their characteristics according to their types, so it is very unlikely that only one plant species will grow well in this rhizobox system but for sustainable agricultural studies preferential candidates are gramineae members [14], [18].

Conclusions

Rhizobox technology, because of its uniqueness, enables one to quantify the nutrient dynamics, heavy metal distribution and biological activities across plant rhizosphere accompanied by related pH distribution and a defined range of rhizosphere influence. Naturally, by applying rhizobox technology, one could foresee acquiring quantitative data on the aforementioned and similar rhizosphere processes. Simultaneously, an understanding of rhizosphere biology and chemistry would also be enhanced through rhizobox technology-induced information on plant-soil system studies. The very feature that warrants its applicability for sustainable agriculture through rhizosphere research is the defined extent of rhizosphere influence on demarcated rhizosphere and bulk soils associated with the rhizosphere processes named above. Relevantly, this rhizobox technology is an effective and efficient tool for researchers who are interested and engaged in plant nutrition, soil chemistry and soil biology studies intended to sustain sound and safe agriculture. Mainly because of its unique role in the determination of nutrient uptake efficiency coupled with the minimization of chemical inputs, beneficial heavy metal movement and availability, significant microbial activities, along with preferential pH ranges required. Ultimately, these implications serve as the keys for soil health in general and for plant growth and development in particular, which are foundations for global agro-ecological sustainability.