L. N. Mandal

ZINC FRACTIONS IN SOILS IN RELATION TO ZINC NUTRITION OF LOWLAND RICE

Soil zinc fractionation studies revealed that more than 90% of the total zinc occurred in the relatively inactive clay lattice bound form and that a small fraction, viz., 0.26, 0.74, 1.58 and 0.71% of the total occurred as water-soluble and exchangeable, organic complexed, amorphous sesquioxide, and crystalline sesquioxidebound forms. The applied zinc was transformed to the latter four forms following their relative order of preponderance in native soils and constituted on an average 3.68, 12.17, 19.85, and 5.33%, respectively, of the applied amount. Waterlogging caused an increase in the organic complexed and amorphous sesquioxide-bound forms of native soil zinc with a concomitant decrease in the content of the other forms, suggesting the occurrence of a dynamic equilibrium of these forms in soil. Simple correlation and multiple regression analyses showed that the zinc fractionation data obtained from the soils after incubation were more relevant than the same obtained from the cropped soil in predicting zinc concentration of lowland rice plants. The path coefficient analyses revealed that organic complexed zinc played the most important role in zinc nutrition of lowland rice and lent support to the validity of the zinc fractionation scheme suggested by Murthy (1982) and modified by us in this regard.

Distribution of zinc fractions and their transformation in submerged rice soils

Distribution of different forms of Zn in 16 acid alluvial rice growing soils of West Bengal (India) and their transformation on submergence were studied. The results showed that more than 84% of total Zn occurred in the relatively inactive clay lattice-bound form while a smaller fractionviz. 1.1, 1.6, 11.1 and 2.0 per cent of the total occurred as water-soluble plus exchangeable, organic complexed, amorphous sesquioxide-bound and crystalline sesquioxide bound forms, respectively. All these four Zn forms showed significant negative correlations with soil pH (r=−0.48**, −0.39*, −0.61** and −0.67**, respectively), while the latter two Zn forms showed significant positive correlations with Fe2O3 (0.68** and 0.88***) and Al2O3 (0.89*** and 0.75***) content of the soils. The different Zn forms were found to have positive and significant correlations amongst each other, suggesting the existence of a dynamic equilibrium of these forms in soil.Submergence caused an increase in the amorphous sesquioxide-bound form of Zn and a decrease in each of the other three forms. The magnitude of such decreases in water-soluble plus exchangeable and crystalline sesquioxide-bound forms was found to be correlated negatively with initial pH values of the soils and positively with the increase in the amorphous sesquioxide-bound form, indicating their adsorption on the surface of the freshly formed hydrated oxides of Fe, which view was supported by the existence of significant positive correlation between the increase in the amorphous sesquioxide-bound form of Zn and that in AlCl3-extractable iron. The existence of a positive correlation between the decrease in crystalline sesquioxide-bound Zn and that in Fe2O3 content in soil suggested that on waterlogging the soil Zn occluded in the cry talline sesquioxide was released as a result of reduction of Fe2O3.

DECREASE IN AVAILABILITY OF ZINC AND COPPER IN ACIDIC TO NEAR NEUTRAL SOILS ON SUBMERGENCE1

We investigated in the laboratory the changes in the diethylenetriaminepentaa-cetic acid (DTPA)-extractable Zn and Cu in 26 acidic to near neutral alluvial soils on submergence and observed that there was always a decrease in their contents. The percentage of decrease was significantly correlated with initial pH, organic C, clay, cation exchange capacity (CEC), and initial DTPA-extractable contents. The pH seemed to play the most important role in influencing the magnitude of decrease. Multiple regression equations showed that 40.06 and 39.44% variability in the magnitude of decrease of Zn and Cu, respectively, could be accounted for by some of the initial soil properties. On submergence, there was an increase in pH, CO32-, S2-, and DTPA-extractable Fe and Mn. The percentage of decrease in DTPA-extractable Zn and Cu was positively correlated with the magnitude of increase in pH (not with Cu), CO32-, S2-, and Fe and was negatively correlated with Mn. Of the variability in the percentage of decrease in Zn and Cu, 57.30 and 59.30% could be accounted for by the change in these parameters, thus lending support to the hypothesis that the decrease in availability of Zn and Cu in acidic to near neutral soils on submergence is due to their precipitation as hydroxide, carbonate, sulfide, and iron compounds. The results of path coefficient analysis revealed that increase in pH directly contributed 36.96% of the percentage of decrease in DTPA-extractable Zn in soil on submergence. The total contribution of pH, comprising both direct and indirect effects through CO32-, S2-, and Fe, was 56.40%, suggesting that mechanisms other than those mentioned are operative to cause decrease in Zn availability in soils on submergence. Further research is necessary for their elucidation.

Effect of phosphorus application on transformation of zinc fraction in soil and on the zinc nutrition of lowland rice

We have studied in the laboratory the effect of different levels of P application on the transformation on native as well as of applied zinc in a rice-growing soil under two moisture regimes viz., flooded and nonflooded. Application of P caused a decrease in the water soluble plus exchangeable and organic complexed with a concomitant increase in the amorphous and crystalline sesquioxide bound forms of native soil zinc. Application of P also caused a decrease in the transformation of applied Zn into the water soluble plus exchangeable and organically complexed and an increase in the amorphous and crystalline sesquioxide bound forms of zinc. The above effects of P were more pronounced in soil under flooded than under nonflooded moisture regimes. The water soluble plus exchangeable and the organically complexed forms of Zn are considered to play an important role in Zn nutrition of lowland rice, while the role of the amorphous and crystalline sesquioxide bound forms are less important in this regard.The results of greenhouse experiments showed that P application caused a progressive decrease in the Zn concentration in shoot and root. This was attributed at least partly to the decrease in the water soluble plus exchangeable and organically complexed forms of Zn and an increase in the amorphous and crystalline sesquioxide bound forms in soil due to P application.

EFFECT OF PREFLOODING ON TRANSFORMATION OF APPLIED ZINC AND ITS UPTAKE BY RICE IN LATERITIC SOILS

Lateritic soils usually contain high amounts of oxides of iron, which are known to have strong zinc-binding capacity. When such soils are used for growing flooded rice and zinc is used as fertilizer, much of it is likely to be fixed by oxides of iron if it is applied immediately after flooding the soil. But if such application of zinc is made after keeping these soils preflooded for a few days, its fixation may be lowered due to the changes occuring in the oxides of iron during the preflooding period. Keeping the above in view we investigated the effect of different periods (0 and 15 d) of preflooding on the transformation of applied zinc in four lateritic ricegrowing soils into its different forms, namely (i) water soluble plus exchangeable (WSEX), (ii) organically complexed (OC), (iii) manganese oxide bound (MnOX), (iv) amorphous iron oxide bound (AMOX), and (v) crystalline oxide bound (CRYOX). Results showed that the transformation of applied zinc into WSEX, OC and AMOX forms was more, and that transformation into MnOX and CRYOX forms was less, when zinc was applied after keeping the soils preflooded for 15 days as compared to such transformation when zinc was applied immediately after flooding (0 d preflooding). This was found to be related to the changes in the forms of oxides of iron and manganese during the period of preflooding. Results of a greenhouse experiment showed that uptake of applied zinc by rice was more when it was applied after 15 d than after 0 d preflooding. Results suggest that use efficiency of applied zinc for rice in lateritic soils can be improved if its application is delayed by 15 d after flooding the soils.

Effect of growth and subsequent decomposition of blue-green algae on the transformation of iron and manganese in submerged soils

N2-fixing blue-green algae (Cyanobacteria), besides enriching soils with N and organic carbon, may modify a number of chemical and electro-chemical properties of the soils resulting in a change in availability of some micronutrient elements. Keeping this in view, an experiment was conducted to study the effects of growth and subsequent decomposition of blue-green algae on changes in the different forms of Fe and Mn in four soils under submerged condition. A mixed algal culture containing Anabaena, Nostoc, Cylindrospermum, and Tolypothrix was used as inoculum. It was allowed to grow for 2 months, after which the soils were sequentially extracted with (i) M NH4OAc (pH 7.0), (ii) M K4P2O7, (iii) 0.1 M NH2OH.HCl (pH 2.0), (iv) 0.2 M (NH4)2C2O4 (pH 3.0) and (v) 0.1 M ascorbic acid to obtain water-soluble plus exchangeable, organically bound, easily reducible, amorphous oxides-and crystalline oxides-bound forms of Fe and Mn, respectively, both during the growth as well as the subsequent in-situ decomposition of the algal biomass in soils. Iron and Mn in the extracts were estimated by atomic absorption spectrophotometry.The results showed that growth of blue-green algae in submerged rice soils caused a decrease in the NH4OAc-extractable forms of Fe and Mn with concomitant increases in all the other four determined forms of the elements. Such decreases and/or increases in different forms of Fe and Mn in soils were explained as being due to release of O2, addition of organic matter and liberation of extracellular organic compounds by the blue-green algae during their growth. The decomposition of algal biomass resulted in an increase in the NH4OAc-, K4P2O7- and (NH4)2C2O4-extractable forms of Fe and Mn with a simultaneous decrease in the NH2OH · HCl- and ascorbic acid-extractable forms. Development of strong reducing conditions and formation of organic acids with chelating properties were suggested as being the cause of the above changes. The implication of these changes in the forms of Fe and Mn for the Fe and Mn nutrition of rice plants were discussed.

Effect of inoculation ofAzospirillum lipoferum on nitrogen fixation in rhizosphere soil, their association with root, yield and nitrogen uptake by mustard (Brassica juncea)

SummaryA microplot field experiment was conducted in the presence or absence of P and N application to evaluate the influence of the seed inoculation of mustard (cv. Baruna T59) withAzospirillum lipoferum on N2-fixation in rhizosphere, association of the bacteria with the roots and grain yield and N uptake. Inoculation significantly increased the N content in rhizosphere soil particularly at early stage (40 days) of plant growth, which was accompanied by the increased association of the bacteria (A. lipoferum) in rhizosphere soil, root surface washing and surface-sterilized macerated root. A significant increase in grain yield and N uptake was also observed due to inoculation. Application of P particularly at the 20 kg. ha−1 level further enhanced the beneficial effect ofAzospirillum lipoferum inoculation, while N addition markedly reduced such an effect.

Transformation of zinc fractions in rice soils

We investigated in the laboratory the changes in the different forms of zinc in two soils (alluvial and laterite) under two different moisture regimes, viz., continuously submerged and alternately submerged, mith two levels of organic matter (starch application) (0 and 0.5% of soil). The soils were sequentially extracted at different periods after incubation with different extractants: (1) 1.0 M NH4OAc (pH 7.0), to extract water-soluble plus exchangeable Zn; (2) 0.05 M Cu(OAc)2, to extract organic complexed Zn; (3) 0.2 M acidified ammonium oxalate (pH 3.0), to extract amorphous sesquioxide-bound Zn; and (4) CBD solution, to extract crystalline sesquioxide-bound zinc. Zinc in the extract was estimated with the help of atomic absorptions spectrophotometry. The results showed that the NH4OAc-and the CBD-extractable zinc concentration in soils decreased below the initial contents; the magnitude of decrease was always higher under the continuously than under the alternately submerged moisture regime in both soils. Application of starch caused a further decrease in the concentrations of the two zinc fractions, particularly during the initial 15 d of incubation. The Cu(OAc)2-extractable fraction of zinc recorded a decrease during the initial period of incubation, the magnitude of decrease being higher under the continuously than under the alternately submerged moisture regime. Starch application further enhanced the decrease. The effect of continuous submergence and starch application, however, showed a reverse trend during the later period of incubation. The acidified ammonium-oxalate-extractable zinc fraction recorded a progressive increase with the period of incubation and attained a peak value after 15 d of incubation and thereafter remained almost unchanged. Continuous submergence and starch application, in general, favored the increase in Zn content. The decrease in the NH4OAc- and Cu(OAc)2-extractable zinc fractions has been attributed to their adsorption on the surface of amorphous sesquioxide in soil, and that in the CBD-extractable form has been sesquioxide unde the reduced condition developed in soil followed by subsequent adsorption on the amorphous sesquioxide which has a high scavenging power for zinc. The results further showed a net release from or transformation to some more resistant fractions of zinc as influenced by different soil environmental conditions. Such changes in different zinc fractions in soil have important implications for zinc nutrition of wetland rice.

Interaction of phosphorus and molybdenum in relation to uptake and utilization of molybdenum, phosphorus, zinc, copper and manganese by rice

SummaryThe effect of phosphorus and molybdenum alone and combined, on the uptake and utilization of Mo, Mn, Zn, Cu and P by rice (Var. IR-579) was studied in the greenhouse at varying levels of Mo (0, 2.5 and 5.0 ppm) and (0,100 and 200 ppm). Application of P increased the dry matter yield of shoot and root. Combined application of P and Mo increased the dry matter yield of shoot. Application of Mo increased the concentration of Mo and P in shoot. Applied P caused an increase in the concentration of Mo, Zn and P in shoot. Combined application of P and Mo resulted in an increase in concentration and uptake of Mo in shoot.

Effect of growth and subsequent decomposition of cyanobacteria on the transformation of phosphorus in submerged soils

The effect of growth and subsequent decomposition of cyanobacteria (inoculated and indigenous) on changes in P fractions was studied in four soils under submerged condition.The growth of cyanobacteria in soils caused an increase in organic P with concomitant decreases in Olsen-P, Al-P, Fe-P, and Ca-P, but little change in reductant-soluble Fe-P and occluded Al-P. Such changes have been attributed to the solubilization of different inorganic P fractions and subsequent assimilation of the released P by cyanobacteria. The decomposition of cyanobacterial biomass in soils caused an increase in Olsen-P with a simultaneous decrease in other P fractions, except the Ca-bound P. Development of intense reducing condition and formation of organic acids with chelating properties have been suggested as the cause of the above changes. Implications of such changes in P fractions due to the growth of cyanobacteria, and of the decomposition of the cyanobacterial biomass for the P nutrition of rice plants are discussed.