Biswapati Mandal

Beneficial effects of blue-green algae and Azolla, excluding supplying nitrogen, on wetland rice fields: a review

Abstract The role of blue-green algae (BGA) and Azolla in supplying N to rice fields is well documented. In addition, they also bring about, directly or indirectly, a number of changes in the physical, chemical and biological properties of the soil and soil-water interface in rice fields. For example, BGA liberate extracellular organic compounds and photosynthetic O2 during their growth, while Azolla prevent a rise in the pH, reduce water temperature, curb NH3 volatilisation and suppress weeds; and both of them contribute biomass. On decomposing, they influence the redox activity and result in the formation of different organic acids in soil. All such changes brought about by BGA and Azolla in soil may ultimately influence plant-available nutrients and also soil characteristics. An attempt has been made in this review to highlight these effects of BGA and Azolla in rice fields and discuss their possible implications relating to management and productivity of rice-field systems.

Assessing Soil Quality Under Long‐Term Rice‐Based Cropping System

Abstract Productivity of the rice‐based cropping system is low, and it continues to decline in India because of worsening soil‐related constraints. Keeping in view the importance of soil quality in rice‐based intensive cropping system, the present investigation was undertaken with the objective of identifying several biological, chemical, and physical indicators of soil quality using data collected from a long‐term experiment being conducted since 1972 on rice‐wheat‐jute cropping system in Indo Gangetic alluvial soils of India. The experiment was laid out in a randomized block design with five treatments, under long‐term fertilizer experiment [i.e., control (no fertilizer and manure); 100% of the recommended dose of nitrogen (100% N); 100% of the recommended dose of N and phosphorus (100% NP); 100% of the recommended dose of N, P, and potassium (100% NPK); and 100% of the recommended dose of N, P, K, and farm yard manure (100% NPK+FYM]. Soil samples were collected after the harvest of rice during the 2002 experiment and were analyzed for physical, chemical, and biological parameters. On the basis of the long‐term yield data, sustainable yield index was calculated. Multivariate statistical techniques were used to determine the smallest set of chemical, physical, and biological indicators that account for at least 95% of the variability in the total data set. The total soil N, available P, dehydrogenase activity, and mean weight diameter of the aggregates were the most important indicators in this case study. A multiple regression was run to evaluate the efficacy of minimum data set (MDS) taking sustainable yield index as goal (r2=0.69). Each MDS was transformed into score. The soil quality index (SQI) was calculated by using weighing factors derived from principal component analysis for each scored MDS variable. The highest SQI was found in 100% NPK+FYM treatment followed by 100% NPK, 100% NP, 100% N, and control treatment, respectively. To compare the soil aggradation or degradation, an undisturbed fallow soil was taken as reference, and it has been observed that 100% NPK+FYM and 100% NPK showed positive change in soil quality that is aggradation of soil quality, but the other three treatments, 100% N, 100% NP, and control, showed negative change of soil quality and indicates degradation of the system.

Soil organic carbon pools and productivity in relation to nutrient management in a 20-year-old rice–berseem agroecosystem

Labile fractions of soil organic C (SOC) can respond rapidly to changes in C supply and are considered to be important indicators of soil quality. An attempt is made in this paper to investigate into the dynamics of total organic C (Ctot), oxidisable organic C (Coc), very labile C (Cfrac 1), labile C (Cfrac 2), less labile C (Cfrac 3), non-labile C (Cfrac 4), microbial biomass C (Cmic), mineralizable C (Cmin) and particulate organic C (Cp) in relation to the system productivity of a 20-year-old rice (Oryza sativa L)–berseem (Trifolium alexandrium L) cropping system with different management strategies [no fertilization, only NPK and NPK + FYM (farmyard manure) applied in different seasons] in the hot humid, subtropics of India. Cultivation over the years caused a net decrease, while balanced fertilization with NPK maintained the SOC. About 62% of the C applied as FYM was stabilized into SOC. The passive pool (Cfrac 3 + Cfrac 4) constituted about 55% of the Ctot. A larger proportion (63%) of applied C was stabilized in the passive pool of SOC. Of the analysed pools, Cfrac 1, Cmic, Cp and Cmin were influenced most by the treatments imposed and explained higher per cent variability in the yield of the crops.

Assessment of potential hazards of fluoride contamination in drinking groundwater of an intensively cultivated district in West Bengal, India

We assessed the potential of fluoride (F) contamination in drinking groundwater of an intensively cultivated district in India as a function of its lithology and agricultural activities. Three hundred and eight groundwater samples were collected at different depths from various types of wells and analyzed for pH, EC, NO3–N load and F content. A typical litholog was constructed and database on fertilizer and pesticide uses were also recorded for the district. The water samples were almost neutral in reaction and non-saline in nature with low NO3–N content (0.02 to 4.56 μg mL−1). Fluoride content in water was also low (0.01 to 1.18 μg mL−1) with only 2.27% of them exceeding 1.0 μg mL−1 posing a potential threat of fluorosis. On average, its content varied little spatially and along depth of sampling aquifers because of homogeneity in lithology of the district. The F content in these samples showed a significant positive correlation (r=0.12, P= ≤0.05) with the amount of phosphatic fertilizer (single super phosphate) used for agriculture but no such relation either with the anthropogenic activities of pesticide use or NO3–N content, pH and EC values of the samples was found. The results suggest that the use of phosphatic fertilizer may have some role to play in F enrichment of groundwater.

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.

Zinc adsorption in soils as influenced by different soil management practices

Zinc adsorption was studied in four soils (two Alfisols and two Inceptisols) under three different moisture regimens, viz., flooded-drying (FD), alternate wetting and drying (AWD), and preflooding (PF), each with and without added organic matter (0.50 and 0%). Results showed that the FD and AWD moisture treatments caused a marked decrease, whereas the PF treatment caused a marked increase, in the magnitude of Zn adsorption by the soils compared with controls (no moisture and no organic matter treatment). When organic matter was combined with all three moisture regime treatments, however, there was a significant increase in Zn adsorption compared with the no-organic matter control. Other adsorption-related parameters of the soils, such as adsorption maxima, bonding energy constant, free energy change for Zn, and Zn-hydroxide potential, affirmed the pattern of changes in Zn adsorption following such treatments. All such changes have been explained on the basis of the changes in soil physico-chemical properties, viz., pH, CaCO 3 equivalent, and Fe oxides as a result of treatments. Amorphous Fe oxides and soil pH were, however, found to play a major role in this regard. Results indicated the need for split application of Zn under AWD and FD regimes, but the reverse is true under PF treatments.

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.

Nitrate and fluoride contamination in groundwater of an intensively managed agroecosystem: A functional relationship

A study was conducted to assess the potential of nitrate-nitrogen (NO(3)-N) and fluoride (F) contamination in drinking groundwater as a function of lithology, soil characteristics and agricultural activities in an intensively cultivated district in India. Two hundred and fifty two groundwater samples were collected at different depths from various types of wells and analyzed for pH, electrical conductivity (EC), NO(3)-N load and F content. Database on lithology, soil properties, predominant cropping systems, fertilizer and pesticide uses were also recorded for the district. The NO(3)-N load in groundwater samples were low ranging from 0.12 to 6.58 microg mL(-1) with only 8.7% of them contained greater than 3.0 microg mL(-1) well below the 10 microg mL(-1), the threshold limit fixed by WHO for drinking purpose. Samples from the habitational areas showed higher NO3-N content over the agricultural fields. The content decreased with increasing depth of wells (r=-0.25, P< or =0.01) and increased with increasing rate of nitrogenous fertilizer application (r=0.90, P< or =0.01) and was higher in areas where shallow- rather than deep-rooted crops (r=-0.28, P< or =0.01, with average root depth) are grown. The NO3-N load also decreased with increasing bulk density (r=-0.73, P< or =0.01) and clay content (r=-0.51, P< or =0.01) but increased with increasing hydraulic conductivity (r=0.68, P< or =0.01), organic C (r=0.78, P< or =0.01) and potential plant available N (r=0.82, P< or =0.01) of soils. Fluoride content in groundwater was also low (0.02 to 1.15 microg mL(-1)) with only 4.0% of them exceeding 1.0 microg mL(-1) posing a potential threat of fluorosis. On average, its content varied little spatially and along depth of sampling aquifers indicating little occurrence of F containing rocks/minerals in the geology of the district. The content showed a significant positive correlation (r=0.234, P=< or =0.01) with the amount of phosphatic fertilizer (single super phosphate) used for agriculture. Results thus indicated that the groundwater of the study area is presently safe for drinking purpose but some anthropogenic activities associated with intensive cultivation had a positive influence on its loading with NO(3)-N and F.

Transformation of zinc in soils under submerged condition and its relation with zinc nutrition of rice

Laboratory and greenhouse experiments were conducted with two soilsviz., laterite and alluvial to study the transformation of applied Zn in soil fractions under submerged condition in the presence and absence of added organic matter and its relationship with Zn nutrition of rice plants.The results showed that application of organic matter caused a decrease in the concentration of Zn in shoot and root of rice plants and helped in translocating the element from root to shoot. The per cent utilization of applied Zn by plants was also found to increase by the application of organic matter. The transformation of applied Zn in different fractions in soils showed that a major portion (53.6–72.6%) of it found its way to mineral fractions leaving only 1.0–3.3, 6.6–18.9, 11.0–21.6 and 2.3–8.8% of the applied amounts in water soluble plus exchangeable, organic complexed, amorphous sesquioxides and crystalline sesquioxides bound fractions respectively. Application of organic matter favoured such transformation of applied Zn into these fractions except the mineral and crystalline sesquioxides bound ones.Simple correlation and multiple regression analyses between applied Zn in different soil fractions and fertilizer Zn content in plants showed that organic matter application increased the predictability of fertilizer Zn content in plants which has been attributed to the higher per cent recovery of applied Zn in plant available fractions in soils in presence of added organic matter.

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.