Dynamics of iron and aluminium in rice based ecosystem of detrital laterites of AEU 11and mitigation strategies

Abstract

A study entitled “Dynamics of iron and aluminium in rice based ecosystem of detrital laterites of AEU 11 and mitigation strategies” was carried out at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani during 2019-2023. The major objectives of the study were to assess Fe and Al dynamics in detrital laterites derived rice tracts of Northern Kerala (AEU 11), to suggest management strategies to mitigate acidity, and to maximize the yield of rice. The investigation was envisaged in six parts viz. survey and collection of geo referenced soil samples, characterization of geo referenced soil samples, generation of thematic maps using ARC GIS software, solution culture experiment, soil column studies for assessing the nutrient release pattern, pot culture experiment to screen the ameliorant for alleviating Fe & Al toxicity, field evaluation and validation. The salient research findings emanated are summarized below. In the first experiment, 10 geo referenced soil samples were collected from surface and subsurface depths of 0-15 cm (80 nos.) and 15-30cm (80 nos.) respectively from rice fields of AEU 11 (northern laterites). After surveying the study area, the soil samples were characterized for physical, chemical and biological parameters. Kalikkadavu series recorded the highest mean value for exchangeable acidity, potential acidity, pH dependent acidity, available Al content at 0-15cm and 15-30 cm. The highest available Fe content was observed in Kalikkadavu series at 0-15 cm and Naduvattom series at 15-30 cm. The order of contribution of Fe fractions to total Fe at 0-15 cm depth was observed as follows: Fe-re>Fe-or>Fe-ox>Fe-ex>Fe-c. At 15-30 cm, the order was as follows: Fe-re>Fe-ox>Fe-or >Fe-c>Fe-ex. Regarding Al fractions, the order of contribution of various Al fractions to total Al at both depths was observed as follows: Al-re> Al-ex>Al-c>Al-ox≥Al-or.The nutrient index of detrital laterite-derived rice soils of AEU 11 was observed to be medium to high for available N, available P, low to high for available K and medium to high for organic carbon content. Micronutrients like Fe, Mn, Zn and Cu were observed in the sufficiency range in all the soil series. From paired t-test analysis, significant variations of pHKCl, bulk density, organic carbon, labile organic carbon, carbon stock, available P, available Ca, available Fe, available Al, available B, pH-dependent acidity, and potential acidity were observed at two different depths of 0-15 cm and 15-30 cm. Principle component analysis (PCA) of surface soils showed that the first eight PCs eigenvalue ≥1 accounted for 73.914% of the total variance. In PC1, EC, organic carbon, labile carbon and available K had the highest factor loadings. Bulk density in PC2, mineralizeable carbon in PC3, exchangeable acidity in PC4, available Mn, Zn, and Al in PC5 available S in PC6, exchangeable H+ in PC7 and available Ca in PC8 reported highest factor loadings. PCA of soils at 15-30 cm showed that the first six PCs eigenvalue ≥1 accounted for 75.073 % of the total variance. In PC1, EC, organic carbon, carbon stock and available K had the highest factor loadings. Bulk density and mineralizeable carbon in PC2, water soluble carbon, exchangeable acidity and exchangeable H+ in PC3, exchangeable Al and exchangeable acidity in PC4, available P in PC5, and available Fe in PC5 exhibited highest factor loadings. The hierarchical cluster analysis of 80 locations was carried for two soil depths. At 0-15 cmand 15-30 cm, among the clusters, cluster II was the largest consisting of 57 soil samples including a majority of Naduvattom, Nanminda, Kalikkadavu, Kunnathura, Kunnamnagalam, and Payalam series. Analyzing the intra-cluster distance, it was found to be highest in Cluster V whereas, the inter-cluster distance was found to be highest between clusters III and IV. Interpretation of various statistical analysis revealed that Kalikkadavu series recorded the highest mean values for available Fe (979.61 ± 969.79 mg kg-1), Al (192.5 ± 65.25 mg kg-1), exchangeable acidity (1.66 ± 0.23cmol kg-1soil), potential acidity (45.8 ± 2.57 cmol kg-1) and pH dependent acidity. Hence this series was selected for conducting further experiments. A solution culture experiment was carried out using Hoagland nutrient solution to study the response of rice seedling variety Uma to 5 different levels of Fe and Al (control, 50 ppm, 450 ppm, 850 ppm, 1250 ppm).The relative decrease in root and root length with increase in Fe and Al concentration was observed. The control showed maximum root length of 8 cm and the minimum root length of 3 cm was observed in 1250 ppm Al. A maximum root length of 9.25 cm was observed in control and lowest value of 4.5 cm was observed in the treatment 1250 ppm Fe dose. Though the effect of Al doses on shoot length was non-significant, a decline in shoot growth was observed with increase in Al concentration. To conclude a significant reduction in root length and shoot length of rice seedlings was observed with increasing doses of Fe and Al. The representative Fe and Al rich soil samples from Kalikkadavu series were collected for setting up the soil column experiment. The treatments include KAU PoP, NPK as per PoP+ double dose of potassium (K), Phosphogypsm @ LR +NPK as per PoP, Phosphogypsm @ ½ LR + Lime @ ½ LR +NPK as per PoP, Calcium silicate as per LR+ NPK as per PoP, Rice husk ash + NPK as per PoP, Rice husk Biochar + NPK as per PoP, Vermicompost + NPK as per PoP and Absolute control. From the soil analysis, the lowest values for exchangeable acidity were noticed in rice husk ash (RHA) in the 21st week of sampling. The highest mean value for total Al was observed in control during the 1st, 7th, and 14th weeks of sampling while calcium silicate recorded the highest in the 21st weeks after sampling. Rice husk biochar (RHB) and RHA reduced the total Al content during the later stages of sampling. The lowest values for available Fe were noticed in RHA during the 1st and 7th week of sampling and RHB during the 14th and 21st weeks of sampling. A significant effect of RHB on lowering the available Al content was more pronounced during 1st, 14th, and 21st week while the lowest available Fe during the 7th week was exhibited by the treatment RHA. Regarding water analysis, the absolute control exhibited the highest value for exchangeable acidity at all the sampling periods while the lowest values were noticed by RHB treatment. A decrease in total Fe of water has been observed in all the treatments but the significant variation was observed only during 7th week. The significant effect of RHB in reducing total Al was noticed during 21stweek of sampling (0.267 mg kg-1).From the above results, a profound effect of rice husk ash, rice husk biochar, vermicompost and double dose of potassium on exchangeable acidity, Fe content and Al content was been noticed and thus these treatments were selected for conducting pot culture experiment. A pot culture experiment was conducted to evaluate the effectiveness of treatments to alleviate Fe and Al toxicity using rice as the test crop. From the experiment, the effect of RHB was found superior in reducing the Fe availability at growth stages. At active tillering stage, RHA recorded the lowest exchangeable acidity and at the harvesting stage, the effect of RHB was prominent. The highest dry matter yield of 126.83 g plant-1, grain yield of 48 g plant -1 and straw yield of 86.67 g plant-1, harvest index of 0.356 was noticed by RHB. From the interpretation of the results of pot culture experiment and by considering the potential ability of rice husk ash and rice husk biochar to reduce exchangeable acidity, available Fe and available Al content in soil under study, the aforesaid treatments were considered for field evaluation and validation. From the field experiment, the lowest value for available Fe was noticed in RHB (144.55 mg kg-1) at harvest stage. A highly significant and positive correlation was observed between available Fe and exchangeable Fe, oxide bound Fe, residual Fe and with total Fe content. With respect to available Al, lowest mean value was observed in RHB at panicle initiation and harvest stage. At harvest, the effect of RHA as on reducing exchangeable acidity was more pronounced. Available Al maintained highly significant and positive correlation with exchangeable Al, oxide bound Al, residual Al, and total Al content. The total Fe content of shoot, root and grain at active tillering and harvest were significantly low in plants treated with RHB. The total Al content of shoot at AT was significantly lower in plants treated with RHB while in roots; KAU PoP recorded the lowest value. At harvest, RHB outperformed by exhibiting the lowest value for Al content in shoot and root. In shoots, the highest Fe uptake was noticed by farmer’s practice. A significantly lower mean value was noticed by RHB. Regarding the Fe uptake by grain, RHB recorded the highest mean value. In shoot and root, a significantly lower Al uptake was noticed by farmer’s practice. The highest mean value for Al uptake in shoot and root was noticed in absolute control. Regarding plant observations, a significantly higher value of plant height, productive tillers was noticed by RHB at all stages. RHA recorded a significantly higher value for filled grains per panicle which was found statistically on par with RHB. A significantly higher value of grain yield, straw yield, and dry matter yield was observed in RHB which was found statistically on par with RHA. The significant mean value for Fe-deposited roots was found low in RHB. The highest lignin content, chlorophyll content, total sugars, and root spread were noticed in RHB. Also, RHB recorded a significant lower value for proline content. From the above results of the field experiment, the effect of application of rice husk biochar at 5 tha-1 on alleviating Fe and Al toxicity was more pronounced. Though the effect of RHB on soil nutrient status, crop nutrient content, nutrient uptake, plant growth characters and crop yield was found on par with the application of rice husk ash 2 tha-1, superior effect of rice husk biochar was more pronounced. From the above experiments, it can be concluded that the most common yield-reducing factors of rice production in northern laterites are high soil acidity preceded by Fe and Al toxicity. From the investigation, basal application of rice husk biochar at 5 tha-1 along with urea, rajphos, and muriate of potash can be recommended to alleviate Fe and Al toxicity, augment soil fertility, and enhance rice productivity in detrital-derived rice fields of northern Kerala