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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Soil Management Influences on Zinc Desorption for Rice and Maize Nutrition

Dep. of Agric. Chem. and Soil Sci., Bidhan Chandra Krishi Viswavidyalaya, Kalyani 741 235, West Bengal, India

biswa{at}klyuniv.ernet.in

	ABSTRACT

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In a rice-based cropping system soils are often subjected to different moisture regimes, which may influence desorption of adsorbed Zn and thus limit Zn availability to crops. Laboratory and greenhouse experiments were conducted to study the effect of moisture regimes with or without organic matter addition on changes in desorption of adsorbed Zn in soils and its utilization by rice and maize plants. Three different moisture regimes, flooded–dried, alternate wetting and drying and preflooding, with (50 g kg^-1) and without (0 g kg^-1) added organic matter were imposed in two Alfisols and two Inceptisols of West Bengal, India. Percent desorption of adsorbed Zn was significantly higher in Alfisols (64.5%) than in Inceptisols (45.5%). Desorption was also significantly higher under flooded–dried (61.4%), alternate wetting and drying (67.1%), and preflooding (47.3%) moisture treatments than in the control (43.4%). Organic matter application enhanced desorption under flooded–dried and alternate wetting and drying but decreased it under preflooding. The variation in Zn desorption among soils and moisture treatments is the result of changes in soil pH, Fe-oxides, bonding energy constants, and free energies for Zn adsorption. Greenhouse experiments showed that dry matter yield and uptake and utilization of Zn for maize were higher under flooded–dried. For rice, yield and Zn accumulation were higher under preflooding treatments compared to the control in which the soils were not subjected to these pre-plant moisture treatments. Soil-zinc data and plant response were in close agreement, except in Inceptisols for rice under preflooding with added organic matter treatment. Results indicated a more efficient use of Zn fertilizer where maize followed rice, and where rice was grown after preflooding the soils.

Abbreviations: CEC, cation-exchange capacity

	INTRODUCTION

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WIDESPREAD ZN DEFICIENCY for different crops has been reported from different parts of the world, including from India. To correct such deficiency, Zn is often applied to the soil as fertilizer. Its concentration in soil solution and its availability to crops is controlled by sorption–desorption reactions at the surfaces of soil colloidal materials (Swift and McLaren, 1991). Although desorption rather than adsorption likely controls the amount and rate of release of Zn into soil solution for plant uptake, only a few studies have examined the process in detail (Brummer et al., 1983; Dang et al., 1994). Desorption of Zn into soil solution is controlled by the energy with which it is adsorbed onto the soil colloidal surfaces. This in turn depends on the soil characteristics, particularly pH, cation-exchange capacity (CEC), the nature and content of the clay and different oxides of Fe, Al, and Mn, and CaCO₃ (Harter, 1991; Hazra and Mandal, 1996). Singh et al. (1997) reported that a multiple regression model that includes terms for CEC, pH, and crystalline Al-oxides explained 93% of the variability of added Zn desorption in some soils of New Zealand, but our understanding of the factors affecting the desorption of Zn from soils is far from complete.

Rice (Oryza sativa L.) is the major food grain crop in Southeast Asian countries, including India. Rice fields are often subjected to different moisture regimes: (i) continuously flooded, where the crop is grown under irrigation; (ii) intermittent flooding and drying, where the crop is grown under rainfed conditions; and (iii) preflooded before transplanting, when there is an early monsoon. These different soil moisture regimes bring about varying changes in soil physico-chemical and electro-chemical properties such as pH, Eh, electrical conductivity, CaCO₃ content, and amorphous and crystalline oxides of Fe and Mn, which are further accentuated by application of organic matter or manures, a common practice followed by rice farmers in this region.

After rice harvest, the soils contain relatively large amounts of the amorphous form of the oxides of Fe and Mn (Sah and Mikkelsen, 1986; Quang and Dufey, 1995), which have larger surface areas, and hence greater adsorptive capacity as compared with the crystalline forms. Hazra et al. (1987) observed that >84% of total Zn in soils occurred in relatively inactive, clay lattice-bound form, while a smaller fraction of the total occurred as water soluble plus exchangeable (1.1), organic complexed (1.6), amorphous sesquioxides–bound (11.1), and crystalline sesquioxides–bound (2.0) forms, respectively. Transformation of applied Zn to soils followed the relative order of preponderance of these different forms. Of these, water soluble, exchangeable, and organic complexed forms played the most important role in Zn nutrition of rice with little contribution from the amorphous sesquioxides–bound form (Mandal and Mandal, 1986). Rice is often followed by upland crops like maize (Zea mays L.) and wheat (Triticum aestivum L.). Since Zn deficiency is more widespread in rice, the element is often applied to soils for rice and the residual amount is used by subsequent upland crop. The adsorption of the added Zn, its subsequent desorption, and its subsequent availability to rice and the following maize or wheat crop are likely to be influenced by the changes in chemical and electro-chemical properties of the soils that result from different moisture regimes in the rice fields.

A laboratory study was undertaken to investigate the effects of flooded–dried, alternate wetting and drying, and preflooding moisture regime treatments with and without added organic matter on the desorption of adsorbed soil Zn in West Bengal, India. Greenhouse experiments under limited soil moisture treatments were also conducted with maize and rice to examine the relationship between the utilization of added Zn by these two crops and the results of desorption studies.

	Materials and methods

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Four surface (0–15 cm) soil samples, two Inceptisols [Hooghly (Typic haplaquepts) and Midnapore (Lithic ustochrepts)] and two Alfisols [Birbhum (Ultic paleustalfs) and Bankura (Rhodic paleustalfs)], from rice growing zones of West Bengal (India) were used. They were analyzed for their physico-chemical properties, which included pH, organic C, clay, and CaCO₃ (Table 1) following standard methods as described by Jackson (1973) and Black (1965). Amorphous and crystalline oxides were estimated by extraction with a solution of 0.2 M ammonium oxalate (pH 3.0) and 0.1 M ascorbic acid, respectively (Shuman, 1985). Free Fe-oxides of the soils were also estimated by citrate–bicarbonate–dithionite method (Mehra and Jackson, 1960). The reducible Mn-oxides were extracted by 0.1 M NH₂OH·HCl (pH 2.0) (Chao, 1972).

For the alternate wetting and drying treatment, the soils were subjected to eight 9- to 10-d cycles of wetting and drying. The time period for each cycle was kept short because of high prevailing atmospheric temperature (40–45°C) under open sun, which sufficiently dried the soil samples within these periods. Thereafter the soils were ground, sieved (80 mesh), and preserved in air-tight containers under refrigerated conditions for use in desorption studies.

For the preflooding treatment, however, 5-g portions of the soils that were untreated or treated with organic matter were placed in 50-mL (18 mm deep x 150 mm high) Corning glass tubes and incubated under flooded (5 ± 0.5 cm standing water) conditions at room temperature (30 ± 5°C) in the laboratory for 15 d; thereafter they were used for desorption studies. There was also a control series in which the soils were not subjected to any of the above mentioned moisture regime treatments. Deionized water was used throughout the experiment for maintaining the different moisture regimes. Soils were again analyzed for important physico-chemical properties after they were subjected to different treatments following the methods mentioned earlier.

Desorption Study
Portions of the 5.0-g soil samples from flooded–dried and alternate wetting and drying treatments were placed in polyethylene centrifuge tubes (100-mL capacity), equilibrated with 50 mL solution of 0.01 M KClO₄ containing three levels of Zn (0.0, 0.1, and 0.2 mg g^-1 soil) as ZnSO₄·7H₂O. Equilibration was achieved by vigorously shaking (150 oscillations per min) the contents for 1 h using a rotatory shaker at 5-h intervals for a period of 48 h in an incubator at 25°C. The contents were then centrifuged, and the Zn concentration in the supernatant solution was determined to calculate by difference the amount of Zn adsorbed by the soils. After decanting off the supernatant solution, the soils were washed thoroughly with alcohol and then extracted with DTPA extractant to determine the desorption of adsorbed Zn (Lindsay and Norvell, 1978). While calculating the amount of Zn adsorbed and the fraction of it desorbed by DTPA, the results of the blank (0 mg g^-1 Zn) treatment were always taken into consideration.

In the preflooding treatment, at the end of the 15-d period of incubation, the soil along with the supernatant water was quantitatively transferred from the incubation tubes to polyethylene centrifuge tubes using 10 mL of deionized water. Zinc was added as before through KClO₄ solution. The volume of KClO₄ solution and Zn concentration in the soil were, however, adjusted so that the level of Zn application (0.0, 0.1, and 0.2 mg Zn g^-1 soil) was maintained accordingly. The adsorption and subsequent desorption of adsorbed Zn in the soils was studied following the procedure described above. Each soil Zn concentration–treatment combination was replicated three times. While presenting the data for desorption of adsorbed Zn by DTPA for Alfisols and Inceptisols, the results for all other treatments in terms of moisture regime, organic matter, and level of Zn application were pooled together.

Greenhouse Experiment
With Maize
This experiment was conducted with an Alfisol (Bankura) and an Inceptisol (Midnapore). Four-kilogram portions of the soils, treated and thoroughly mixed with either 0 or 50 g kg^-1 organic matter as cellulose, were placed in a total of 36 6-L earthen pots fitted with stopcocks at the bottom and lined with polyethylene sheeting. They were kept flooded for 90 d, after which the standing water (5 ± 0.5 cm) was drained off by opening the stopcock. Thereafter the soils were allowed to dry under open sun for 30 d. The soils were then loosened and five seeds of maize (cv. Vikram) were sown in each pot. The pots were thinned to two seedlings on the tenth day and they were allowed to grow for 45 d. The flooded (90 d) followed by drying (30 d) soil moisture regime was imposed before sowing maize in order to simulate the natural field condition under which the crop is grown after flooded rice. There was a control series where the soils were not subjected to the above moisture treatment.

With Rice
This experiment was conducted with all the four soils used for the laboratory experiment. Pots were prepared as described before and the soils were kept under water for 15 d. Thereafter two heels comprised of six rice seedlings (cv. IET 5656) were transplanted in each pot. Thus there were eighteen seedlings (3 pots x 3 seedlings x 2 heels) per treatment. There was also a control series in which the rice seedlings were transplanted immediately after flooding the soil, a practice followed by farmers when the monsoon rains or availability of irrigation water is delayed. The plants in both the cases were allowed to grow for 45 d after transplanting.

Basal doses of N (100 kg ha^-1), P (60 kg ha^-1), K (60 kg ha^-1), and Zn (0 and 20 kg ha^-1 in the form of ZnSO₄·7H₂O) was applied both to maize and rice. Each treatment was replicated three times. Deionized water was used throughout the experiments for irrigating and flooding the soils (the depth of standing water being maintained constantly at a height of 5.0 ± 0.5 cm for rice). After their specified period of growth (45 d), the plants (maize or rice) were removed with roots intact by a water jet, washed sequentially by tap water, HCl (0.01 M), and then double-distilled water. Shoots and roots were separated, their dry matter weights recorded, and samples were subsequently digested in triacid mixture (HNO₃:H₂SO₄:HClO₄, 9:1:3) for analysis of Zn in the tissues (Jackson, 1973). Zinc and Fe in soil extracts and plant digests were analyzed using an atomic absorption spectrophotometer (Pye Unicam SP 9 800, Cambridge, UK). Statistical analyses of the data were done following completely randomized design, whereby the treatments were factorial in nature. Analysis of variance was performed by dividing the treatments into two groups, with and without organic matter. Such division of treatments into two groups caused a partial confounding of effects for organic matter and yielded only an overall effect of organic matter but not its interaction effects with other treatments used.

	Results

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Effect of Treatments on Zinc Desorption in Soils
There was a significantly higher percentage desorption of adsorbed Zn by DTPA in Alfisols (64.5%) than in Inceptisols (45.5%), irrespective of the treatments imposed (Table 2) . Desorption was much higher in soils subjected to flooded–dried without organic matter treatment (61.4%) than in the control (43.4%). Flooded–dried treatment caused a further increase in percentage desorption when combined with added organic matter (66.2%), the magnitude being more in Alfisols (78.5%) than in Inceptisols (53.8%).

Treatment Effects on Crop Response to Added Zinc
There was a greater positive response of dry matter yield for maize to added Zn in the soils subjected to flooded–dried (12.0%) as compared to the control (6.6%) (Table 3) . Combined application of organic matter and the flooded–dried treatment further increased (16.3%) plant growth. Zinc uptake by maize also recorded a higher increase in soils subjected to the flooded–dried than other treatments. This was further enhanced by its combination with organic matter. Percentage utilization of added Zn by maize (Table 4) was also higher under this treatment (3.1%) as compared to the control (2.5%). Organic matter application further enhanced the utilization (3.5%).

	Discussion

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The higher degree of desorption of adsorbed Zn by DTPA in the Alfisols vs. that in the Inceptisols is associated with lower values of bonding energy constants and free energy changes for Zn adsorption in the Alfisols (0.014 L mg^-1 and -24.1 kJ) than in the Inceptisols (0.566 L mg^-1 and -33.0 kJ) (Mandal and Hazra, 1997) (Table 7) . The Alfisol group of soils also contains kaolinite as the dominant clay, which has lower fixing capacity in contrast to the other group. That group contains predominantly smectite (Ghosh and Kapoor, 1982), which has a greater capacity for fixing or entrapment of added Zn (Shuman, 1980; Ma and Uren, 1998) in DTPA non-extractable forms. Moreover, the lower pHs of Alfisols vs. Inceptisols also favored greater desorption in the Alfisols (Anand et al., 1995).

Application of organic matter with the flooded–dried treatment resulted in a slight increase in the amounts of amorphous Fe-oxides in the soils compared to treatments of no organic matter or to the control (Table 7). In spite of this, organic matter caused an increase in the desorption of the adsorbed Zn by DTPA. The organic matter added upon decomposition during the 90-d period of flooding might have produced organic ligands (Stevenson, 1967), which on binding onto the amorphous Fe-oxides and other soil constituents may act as a bridge between Zn and the adsorbing planes. Such adsorption may involve a low energy of retention and thus favors desorption on extraction with DTPA. Decreases in bonding energy constants and free energy changes for Zn under the flooded–dried with organic matter treatment supports this contention. While studying metal extraction from aquatic sediments after drying, Malo (1977) reported that coating of Fe-oxide surfaces by the well humified organic matter (which is likely to be formed when the soils are kept under flooded condition for a long period) decreased their adsorptivity for metals. Metals bound under such conditions might be desorbed readily.

Similarly, the recorded increase in desorption under the alternate wetting and drying treatment, both with and without added organic matter, may also be explained by the observed decreases in bonding energy constant and free energy change for Zn adsorption under this treatment (Table 7). Ma and Uren (1997a,b), however, observed in calcareous soils that drying and rewetting at elevated temperatures (22 vs. 40°C) caused transformation of added Zn from available to less available forms. They suggested that drying and rewetting probably enhanced the dehydration of retained Zn and the diffusion of Zn cations into micropores in soils, which leads to decreased availability.

There was an increase in the desorption of adsorbed Zn under the preflooding treatment, despite increases in the bonding energy constants, free energy changes, and content of amorphous oxides and pH values of the soils. Organic matter application combined with the preflooding treatment, however, caused a significant decrease in the desorption, accompanied by increases in bonding energy constants, free energy changes, and content of amorphous oxides in the soils. These observations are in contrast with those obtained in the flooded–dried treatment. Behavior of Zn in flooded soils differs significantly from non-flooded soils (Mikkelsen and Kuo, 1976; Dutta et al., 1989), because of drastic changes in physical, chemical, and electro-chemical properties of soils under flooding (Ponnamperuma, 1972). These changes might have facilitated desorption of adsorbed Zn under the preflooding treatment. This agrees with the observations of Mandal et al. (1992), who reported increased recovery of added Zn in available forms and its subsequent uptake by rice plants under the preflooding treatment. Application of organic matter along with this treatment caused a flush of activity of microorganisms in soils, resulting in an immobilization of a part of the adsorbed Zn in the soils. This process might cause decreased desorption by DTPA solution. Recently, Mandal et al. (1997) observed an initial significant decrease in the recovery of added Zn and Cu in soils by DTPA when Sesbania rostrata Bremek. & Oberm. and Azolla microphylla Kaulf. were incorporated under preflooding treatment as compared to preflooding alone or to no preflooded controls.

The greenhouse experiments were conducted primarily to examine whether the laboratory results of Zn desorption have any relevance to the Zn nutrition of crops. A greater response of dry matter yield of maize to applied Zn and its increased uptake by the plants under the flooded–dried treatment compared to the control (Table 3) showed that these were related to the higher desorption of adsorbed Zn under this treatment (Table 2). This was again reflected in a higher percentage utilization of added Zn by maize under this treatment (Table 4). Similarly, a greater dry matter yield of rice and a higher Zn uptake by the plants because of addition of Zn under the preflooding treatment (Table 5) is related to the higher desorption of adsorbed Zn in the soils by DTPA under this treatment (Table 2). The discrepancies in results between Inceptisols and Alfisols under preflooding with organic matter treatment with respect to Zn uptake and its percent utilization by rice can be explained as follows: Preflooding, with added organic matter, caused an initial decrease in desorption of adsorbed Zn due to microbial immobilization. As incubation progressed and decomposition of the added organic matter proceeded to completion (what happened in the greenhouse during growing of rice), soluble organic acids were produced. These acids can mobilize soil-bound Zn and also restrict the fixation of soluble Zn by soil components by chelating the element. Besides, mineralization of the immobilized Zn also took place at a later period (Mandal et al., 1997). All these might have contributed to increase bioavailability of the added Zn to rice plants in Inceptisols. This was also expected in Alfisols, but, in contrast, there was a decrease in uptake as well as in percentage utilization of added Zn caused by the organic matter addition in this group of soils. Zinc absorption by rice plants depends not only on the concentration of Zn but also on other factors, particularly the concentration of Fe²⁺ and Mn²⁺ in the soil solution. It is known that high concentrations of Fe²⁺ and Mn²⁺ in soil solution have an antagonistic effect on Zn absorption. Giordano et al. (1974) have shown the order of interference on Zn absorption and translocation to be as follows:

Depression of Zn absorption

Depression of Zn translocation

The Alfisols contained a significantly higher amount of Fe-oxides than the Inceptisols (32.2 vs. 21.1 g kg^-1, citrate–bicarbonate–dithionite extractable). Under the intense reducing conditions of preflooding with added organic matter treatment, these oxides underwent reduction and might have produced a very high concentration of Fe²⁺ in soil solution. Such high concentration of Fe²⁺ might have interfered with Zn absorption by rice plants and ultimately caused reduced Zn uptake and utilization. The results of desorption of adsorbed Zn by DTPA are, in general, reflected in the utilization of the added Zn by both maize and rice, except for rice in the preflooding combined with added organic matter treatment in Inceptisols. Results also indicated a better use of Zn fertilizers when maize is grown following rice, and when rice is grown after keeping the soils preflooded for a few weeks before transplanting.Black Evans White Ensminger Clark 1965; Hazra Mandal 1995; Willet 1991

	ACKNOWLEDGMENTS

 
The authors are thankful to Dr. Asim Chakraborty, Central Research Institute for Jute and Allied Fibres, Indian Council of Agricultural Research, Barrackpore, West Bengal, for his kind help in statistical analysis of experimental data.

Received for publication August 16, 1998.

	REFERENCES

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This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Mandal, B. Articles by Mandal, L.N. Search for Related Content PubMed Articles by Mandal, B. Articles by Mandal, L.N. Agricola Articles by Mandal, B. Articles by Mandal, L.N.