Long-term application effects of chemical fertilizer and compost on soil carbon under intensive rice–rice cultivation

From a long-term fertilizer experiment on rice–rice cropping in Typic Endoaquept, established at the Central Rice Research Institute, Cuttack, India in 1969, effects of application of composted manure (5 Mg ha⁻¹ year⁻¹) and chemical fertilizers (N, NP, NK, and NPK twice in a year), in series without compost (C₀) or with compost (C₁) on changes in soil carbon and microbial pools were examined by comparing the soils archived in 1984 and those sampled in 2004. Mean concentrations of soil organic carbon (SOC) varied between 5.5 and 7.6 g kg⁻¹ in 1984, and 6.8 and 10.8 g kg⁻¹ in 2004, respectively. Temporal increases in the total amounts of carbon, which reflect the carbon sequestration potential of the soil followed the order: C₁ + NK > C₁ + NP = C₁ + NPK > C₁ + N = C₁-control > C₀ + NP = C₀ + NK > C₀ + NPK > C₀-control > C₀ + N. Fractions of H₂O–C and K₂SO₄–C were higher in 1984, especially in those soil treated without compost. A reverse trend was observed in case of KMnO₄–C and carbohydrate–C fractions. The continuous application of compost enhanced microbial biomass carbon as well as active microbial biomass carbon in 2004. Long-term application of chemical fertilizers in combination, rather than N alone, had beneficial effects on soil carbon and microbial pools. Compost application, even once a year, invariably led to higher increments in both soil carbon and microbial pools and the combinations of chemical fertilizers with compost generally showed comparable effects in the long-term.     

Placement effects on rice residue decomposition and nutrient dynamics on two soil types during wheat cropping in rice–wheat system in northwestern India

Crop residues are important sources of organic matter and plant nutrients and known to affect soil quality. Tillage affects residue decomposition and nutrient cycling processes. A 2 year field study was aimed to investigate the decomposition and nutrient dynamics from surface-placed and incorporated rice (Oryza sativa L.) residue on two soil types using the nylon mesh bag technique over wheat cropping cycle in rice–wheat system in northwestern India. A single-pool first-order exponential decay function showed R ² larger than 0.9. Over the wheat growing cycle about 20% of buried rice residue and 50% of surface-placed residue remained undecomposed. Nitrogen exponential decay models were significant at P < 0.01. At wheat boot stage (1,100 growing degree days, DGD) 12 kg N ha⁻¹ (27% of the initial N) was released from the buried residue losing 60–65% of its N at the end of decomposition cycle. Nitrogen in surface rice residue increased throughout the decomposition cycle due to microbiological immobilization. The change in P with time (DGD) was small and weakly defined by the exponential function. Nutrient dynamics of rice residue indicate that it is not a potential source of N and P for wheat over short-term. The change in K concentration was well described by exponential function (R ² 0.73–0.86). Potassium in rice residue decreased at a fast rate during the initial 20 days (400 DGD) and >80% was released by 40 days (680 DGD).     

Nitrogen and residue management effects on agronomic productivity and nitrogen use efficiency in rice–wheat system in Indian Punjab

Development of a sustainable and environment friendly crop production system depends on identifying effective strategies for the management of tillage and postharvest crop residues. Three-year (2004–2007) field study was initiated on two soil types to evaluate the effect of straw management (burning, incorporation and surface mulch) and tillage (conventional tillage and zero tillage) before sowing wheat and four nitrogen rates (0, 90, 120 and 150 kg N ha⁻¹) on crop yields, N use efficiency, and soil fertility in the northwestern India. Effect of tillage and straw management on nitrogen transformation in soils was investigated in a laboratory incubation study. In sandy loam, grain yield of wheat with straw mulch-zero-till (ZT) was 7% higher compared to when residues were burnt-ZT but it was similar to straw burnt-conventional till (CT), averaged across 3 years. In silt loam, grain yield of wheat with straw mulch-ZT was 4.4% higher compared to straw incorporated-CT, but it was similar to straw burnt-CT. Response to N application was generally observed up to 150 kg N ha⁻¹ except in 2004–2005 on sandy loam where N response was observed up to 120 kg N ha⁻¹, irrespective of straw and tillage treatments. In sandy loam, RE was lower (49%) for straw burnt-ZT than in other treatments (54–56%). In silt loam, RE was higher in straw mulch-ZT compared with straw incorporation-CT (65 vs. 58%). In sandy loam, AE was higher in straw burnt-CT and straw mulch-ZT compared with the other treatments (19.2 vs. 16.9 kg grain kg⁻¹ N applied). In silt loam, AE was lower in straw incorporation-CT than in other treatments (16.0 vs. 17.6 kg grain kg⁻¹ N applied). Rice yield and N uptake were not influenced by straw and tillage management treatments applied to the preceding wheat. Recycling of rice residue (incorporation and surface mulch) compared with straw burning increased soil organic carbon and the availability of soil P and K. There was more carbon sequestration in rice straw mulch with zero tillage (25%) than in straw incorporation with conventional tillage (17%). Soil N mineralization at 45 days after incubation was 15–25% higher in straw retention plots compared with on straw burnt plots.     

Fate of labeled urea-15N as basal and topdressing applications in an irrigated wheat�Cmaize rotation system in North China Plain: I winter wheat

A field micro-plot experiment for winter wheat was conducted in an irrigated winter wheat (Triticum aestivum)-summer maize (Zea mays L.) rotation system in Mazhuang, Xinji of Hebei province in the North China Plain, using the ¹⁵N isotope method to determine the effects of N application (rates and timing), and irrigation frequency on urea-¹⁵N fate, residual-N and N recovery efficiency (NRE) of wheat. The experiment was conducted under two irrigation treatments (I2 and I3, representing for two and three irrigations, respectively), at three N rates (150, 210, and 270, kg ha^?1), divided between two ¹⁵N-labeled applications of basal-¹⁵N (90 kg ha^?1) and topdress-¹⁵N (60, 120, and 180, kg N ha^?1, respectively). The total N uptake by wheat (ranging from 186 to 238 kg ha^?1) and the fertilizer-derived N (Ndff, about 34?C55%) were measured. The Ndff from labeled basal-¹⁵N and from labeled topdress-¹⁵N were about 15?C22% and 16?C40%, respectively. The NRE (measured either as recovery in grain or as the total N recovery in the plant) was higher with I3 (39?C41 or 47?C49%) than with I2 (35?C40 or 42?C47%), showing maximum NRE in grain of about 40% both at N210 with I2 and at N150 with I3 treatment. The NRE by the first wheat crop (in grain or the total N recovery in plant) was higher with labeled topdress-¹⁵N (39?C48 or 45?C56%) as compared to that with labeled basal-¹⁵N (30?C37 or 36?C45%), while the unaccounted N losses were lower with labeled basal-¹⁵N (14?C22%) relative to labeled topdress-¹⁵N (14?C35%). Higher residual N in soils was found with labeled basal-¹⁵N (41?C51%), as compared to labeled topdress-¹⁵N (18?C35%). Residual N in the 0- to 150-cm soil depth ranged from 26 to 44% while the unaccounted N losses ranged from 14 to 30%. Recovery of residual N by the 2nd and 3rd crops in the rotation was 5?C10% in the maize crop and a further 1.7?C3.5% in the subsequent wheat crop. The accumulated N recovery and the unaccounted N losses in continuous wheat?Cmaize?Cwheat rotations derived from labeled topdress-¹⁵N were 54?C64% and 16?C37%, respectively while they were 47?C53% and 16?C28%, respectively from labeled basal-¹⁵N. This study also suggested that an N rate of 210 kg ha^?1 (with a ratio of basal-N to topdress-N of 1:1.3) with two irrigation applications could optimize wheat grain yields and NRE, under the water limited conditions in North China Plain.     

Effects of tillage systems on soil organic carbon dynamics, structural stability and crop yields in irrigated wheat (Triticum aestivum L.)–cotton (Gossypium hirsutum L.) rotation in semi-arid Zimbabwe

In southern Africa, tillage research has focused on rainfed smallholder cropping systems, while literature on high-input irrigated cropping systems is limited. We evaluated the effects of conventional (CT), minimum (MT) and no-till (NT) tillage systems on soil organic carbon (SOC), bulk density, water-stable aggregates (WSA), mean weighted diameter (MWD) and crop yields in an irrigated wheat–cotton rotation. Soil data were monitored in the first and final year, while yields were monitored seasonally. Average bulk densities (1.5–1.7 Mg m⁻³) were similar among tillage systems, but often exceeded the critical limit (1.60 Mg m⁻³) for optimum root growth. Conversion from CT to MT and NT failed to ameliorate the high bulk densities associated with the alluvial soil. SOC (g kg⁻¹) at 0–15 cm was higher (P < 0.05) under MT (3.9–5.8) and NT (4.2–5.6) than CT (2.9–3.3). Corresponding horizon SOC stocks (Mg C ha⁻¹) for the tillage treatments were; 9.3–13.9 (MT), 9.3–13.5 (NT) and 7.3–7.7 (CT). In the final year, significant (P < 0.05) tillage effects on SOC stocks were also observed at 15–30 cm. Cumulative SOC stocks (Mg C ha⁻¹) in the 0–60 cm profile were higher (P < 0.05) under MT (32.8–39.9) and NT (32.9–41.6) than CT (27.8–30.9). On average, MT and NT sequestered between 0.55 and 0.78 Mg C ha⁻¹ year⁻¹ at 0–30 cm depth, but a net decline (0.13 Mg C ha⁻¹ year⁻¹) was observed under CT. At 0–30 cm, MT and NT had higher (P < 0.05) MWD (0.19–0.23 mm) and WSA (2.3–3.5%) than CT (MWD: 0.1–0.12 mm, WSA: ≈1.0%). Both MWD and WSA were significantly (P < 0.05) correlated to SOC. Seasonal yields showed significant (P < 0.05) tillage effects, but 6-year mean yields (t ha⁻¹) were similar (CT: 4.49, MT: 4.33, NT: 4.32 for wheat; CT: 3.30, MT: 2.82, NT: 2.83 for cotton). Overall, MT and NT improved soil structural stability and carbon sequestration, while impacts on crop productivity were limited. Therefore, MT and NT are more sustainable tillage systems for the semi-arid regions than conventional tillage. S. Chakanetsa—Deceased.     

Nitrogen and sulphur fertilization effect on leaching losses, nutrient balance and plant quality in a wheat–rapeseed rotation under a humid Mediterranean climate

An unbalanced S and/or N fertilization may have low N and S use efficiency together with substantial negative implications for yield, nutrient losses and plant quality parameters. The effect of N and S fertilization and their interactions on N?CS balances, on N?CS losses and on some plant quality parameters were investigated in a field experiment with a wheat (Triticum aestivum L.)?Crapeseed (Brassica napus L.)?Cwheat rotation (2005?C2008). The study was conducted under humid Mediterranean climatic conditions on a potentially S deficient soil. The effects of N (0, 140, 180, 220?kg?N?ha^?1 in wheat; 0, 100, 140, 180, 220?N?ha^?1 in rapeseed) combined with S fertilizer rates (0, 16 and 32?kg S ha^?1 in wheat and 0, 30, 60?kg S ha^?1 in rapeseed) were studied. Nitrogen fertilization increased yield by 55?% in wheat and 60?% in rapeseed, N concentration in grain and straw and S concentration in the grain of wheat. However, it led to a reduction in the S concentration of straw and the oil content of the rapeseed seed. The S application did not increase yield but had a positive effect on S concentration in the wheat straw. Glucosinolate concentration, a potentially toxic secondary metabolite in rapeseed, was not influenced by N or S applications. Nitrate leaching tended to increase with N application while sulphate leaching decreased. A net N and S mineralization was observed in each growing season, except for the first year in which a net S immobilization was observed. To make N fertilizer recommendations, the N mineralization from the previous crop residues should be taken into account. For S fertilizer recommendations, N supply is the most important item both from a qualitative point of view (N/S ratio in wheat grain) and an environmental point of view (S leaching).     

Effect of zinc-enriched urea on productivity,zinc uptake and efficiency of an aromatic rice–wheat cropping system

Zinc deficiency is prevalent worldwide and is a barrier in achieving yield targets in crops. It is also now recognized as a leading risk factor for disease in humans in developing countries. Generally, soil application of 5–17 kg Zn ha⁻¹ y⁻¹ (25–85 kg zinc sulphate heptahydrate ha⁻¹ y⁻¹) or more is recommended for rice. However, in the developing rice-growing countries of Asia, zinc sulphate of desired quality is not readily available and is also quite expensive, and the farmers generally fail to apply Zn, resulting in crop yield loss in rice. Availability of zinc-enriched urea (ZEU) makes possible not only the availability of quality zinc, but also assures its application. Therefore, field experiments were conducted for two consecutive years at the research farm of Indian Agricultural Research Institute, New Delhi, India, during rainy (rice) and winter (wheat) seasons of 2004–2006 on a sandy clay-loam soil to study the effect of various concentrations of zinc enrichment of urea on productivity, zinc concentrations, its uptake and use indices of aromatic rice–wheat cropping system. Eight treatments comprising prilled urea (PU) and 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5% zinc-enriched urea, replicated three times, were compared in a randomized block design. The enrichment of PU was done through zinc oxide containing 80% zinc. The results of this study revealed that the zinc-enriched urea (ZEU) had a significant effect on growth, yield attributes and yields of aromatic rice. Highest values for all these attributes and yields were recorded at the highest enrichment (3.5%) of the PU with zinc. The highest zinc concentration and uptake in rice grain and straw were also significantly higher with the highest level (3.5%) of zinc enrichment. The highest total zinc uptake recorded was 1,168 and 1,353 g ha⁻¹, during 2004 and 2005, respectively, with 3.5% ZEU. However, a major increase in grain yield of rice was recorded up to 1.0% zinc enrichment. The residual effect of zinc-enriched urea on succeeding wheat yield and zinc uptake was significant only at a higher level of zinc-enriched urea and only in the second year of study. Overall, 1.0% zinc-enriched urea recorded significantly higher productivity and zinc uptake over PU in the rice–wheat cropping system and is recommended for Delhi and adjoining areas. The recommendation is also made keeping in view the fact that with increased levels of zinc enrichment of urea, the partial factor productivity, agronomic efficiency, apparent recovery and physiological efficiency of applied zinc in a rice–wheat system decreased significantly. Considering all the economic parameters (benefit, benefit:cost ratio, IR gained IR⁻¹ invested in zinc), 1.0% ZEU proved the most economic source for aromatic rice–wheat cropping system and therefore is recommended for rice–wheat cropping system in Delhi and adjoining areas of north India.     

Electroless nickel–phosphorus deposition on carbon steel CK-75 and study of the effects of some parameters on properties of the deposits

Electroless nickel–phosphorus (Ni–P) deposition provides coatings with high hardness and excellent resistance to wear and abrasion. In this study, autocatalytic deposition of Ni–P alloy has been carried out on steel CK-75 sheets from bath containing nickel sulfate hexahydrate, sodium hypophosphite hydrate, thiourea, lactic acid, and sodium acetate. The effects of lactic acid concentration, pH and temperature on deposition rate, composition of deposits, and hardness have been studied. Also the changes in the hardness and structure of deposits by heat-treatment were studied by X-ray diffraction and scanning electron microscopy methods. It is shown that deposits crystallized after heat-treatment at 400°C for 1 h and crystallization to Ni and Ni₃P was observed.     

The effects of promoters on catalytic properties and deactivation–regeneration of the catalyst in the synthesis of dimethyl carbonate

The effects of different alkali metal promoters in PdCl₂-CuCl₂/activated carbon (a.c.) catalyst on the reaction performance for synthesizing dimethyl carbonate (DMC) by gas-phase oxidative carbonylation of methanol were studied. The bulk and surface properties of catalyst PdCl₂-CuCl₂-CH₃COOK/a.c. were characterized by XRD, XPS, and AAS techniques. On the basis of catalyst characterization and activity evaluation, the functions of promoters were further investigated, and the deactivation–regeneration of catalyst PdCl₂-CuCl₂-CH₃COOK/a.c. was also discussed. The results show that the space time yield (STY) of DMC on catalysts with different alkali metal promoters ranks in the following order: K>Na>Li. The main reason for catalyst deactivation is the loss of chlorine. Fortunately, during the preparation of the catalyst, the interaction between CH₃COOK and PdCl₂ or CuCl₂ that results in the formation of KCl limits the loss of chlorine. An obvious increase of the catalyst lifetime and catalytic activity is observed by treating fresh catalyst with a methanol solution of methyl chloroacetate. If deactivated catalyst is treated with a methanol solution of methyl chloroacetate in N₂ stream at 200 °C for 4 h and then treated in N₂ stream at 200 °C for 2 h, the catalytic activity can be restored effectively and the regeneration induction period can be shortened. The catalytic activity after two times of regeneration can still be restored to 93% of the fresh catalyst. The run time of this catalyst is up to 300 h.     

Preparation of a novel phosphorus–nitrogen flame retardant and its effects on the flame retardancy and physical properties of polyketone

In this study, a novel phosphorus–nitrogen flame retardant (PNFR) was synthesized, and its flame retardancy for polyketone (PK) was systematically investigated. The chemical structure of PNFR was characterized using NMR spectroscopy. The PNFR was considered to generate phosphoric acid, which catalyzed the char formation reaction of PK via condensed-phase decomposition. When 7 wt% PNFR was incorporated into PK, the limiting oxygen index value was increased from 20.2 to 25.9%. In addition, this PK-PNFR sample achieved the rating of VTM-0 because it exhibited self-extinguishable properties and did not drip when fire was removed from it. As the PNFR content increased, while the initial decomposition temperature decreased, the amount of residual char considerably increased. Thus, the PNFR was thought to impart flame-retarding ability to PK via superior condensed-phase mechanism. Additionally, the decrease in melting temperature and enthalpy of fusion was observed after the addition of PNFR, which implied that the PNFR could act as plasticizer for PK.     

Tillage and residue management effects on soil carbon and CO<Subscript>2</Subscript> emission in a wheat–corn double-cropping system

The mitigation of CO₂ emission into the atmosphere is important and any information on how to implement adjustments to agricultural practices and improve soil organic matter (SOM) stock would be helpful. We studied the effect of tillage and residue management on soil carbon sequestration and CO₂ emissions in loam soil cropped in a winter wheat–corn rotation in northern China. There were five treatments: mouldboard ploughing, rotary tillage and no-tillage with chopped residues (MC, RC and NC), additional no-tillage with whole residue (NW) and mouldboard ploughing without residue (CK). After 5 years of each tillage system, MC and RC had higher annual CO₂ efflux from soil. The CO₂ effluxes were correlated with the ratio of dissolved organic carbon to soil microbial biomass (DOC/MBC) among treatments. This effect may be due to less immobilization of soil carbon by microorganisms under long-time intensive tillage. Although both MBC and DOC showed seasonal variability, when averaged across the sampling period only MBC discriminated between treatments. After 5 years of tillage, all treatments except CK increased SOM (0.16–0.99 Mg C ha⁻¹ year⁻¹) at 0–30 cm depth and NC was the greatest, resulting from historical SOM depletion and large C return from recent residues. Despite the lowest CO₂ flux being from the NW treatment, lower input residue from decreased biomass may have lowered C sequestration. To improve soil C sequestration in rotations, the input of residue and the CO₂ emission should be balanced by adopting appropriate tillage and residue management.     

Influence of ceramic phase content and its morphology on mechanical properties of MgO–C refractories under high temperature nitriding

To evaluate the influences of ceramic phase content and its morphology on the mechanical properties of MgO–C refractories, pre-prepared Si powder-phenolic resin loaded with Fe₂O₃ was introduced to prepare MgO–C refractories via catalytic nitridation. The effects of nitriding temperature and Fe₂O₃ content on the phase composition, microstructure evolution and properties of MgO–C refractories were studied and compared. The results show that the increase of the nitriding temperature was conducive to the in-situ formation of the ceramic phases, and a new phase of Mg₂SiO₄ was formed at temperatures ≥1450 °C. Both the increase in nitriding temperature and the addition of catalyst could inhibit the growth of α-Si₃N₄ to promote the formation of β-Si₃N₄ and MgSiN₂. In addition, the formation of excessive ceramic phases caused samples after nitriding to expand violently and form more porous, thereby reducing the physical properties of MgO–C refractories.     

Atomistic thermodynamics study of the adsorption and the effects of water–gas shift reactants on Cu catalysts under reaction conditions

Density-functional theory (DFT) calculations were performed to determine the structure and stability of oxygen, carbon monoxide and sulfur adsorption on Cu(111), (100) and (110) surfaces that are in equilibrium with a water–gas shift (WGS) reactive environment of H₂, H₂S, H₂O and CO. An atomistic thermodynamic framework based on DFT was used for describing the phase behaviors of the adsorbates on different Cu facets. Phase diagrams of each possible adsorbate on each surface were constructed as a function of the corresponding chemical potential which showed sulfur poisoning occurs even at ppm levels of H₂S in the environment at low temperatures. Under reaction conditions relevant to WGS at low temperature, CO and S adsorbed surface structures were found to be more stable then the clean catalyst surfaces. At high temperatures and high hydrogen pressures, a poisoned surface can be regenerated back to a clean surface. The shapes of a Cu nanoparticle in the WGS reaction conditions under various sulfur chemical potentials were determined using the Wulff construction. We found that the crystal shape changes significantly from one dominated by (111) and (100) facets at very low sulfur chemical potentials to a shape dominated by (110) facets at higher sulfur chemical potentials, suggesting that reactive site distributions may change under reaction conditions.     

Simulation and experimental investigation of preferred β-Sialon growth and its effects on thermo-mechanical properties of Al2O3–C refractories

β-Sialon bonded Al₂O₃–C refractories possess high strength and superior thermal shock performance. In this study, the growth of preferred β-Sialon (Si₃Al₃O₃N₅) and its effects on thermo-mechanical properties of Al₂O₃–C refractories were investigated via simulations and experiments. The results indicate that the additive Fe₂O₃ contributed to the formation of β-Sialon and helped its column structure become plate-like. Transmission electron microscopy confirmed that the (101) crystal plane was a growth plane of plate-like β-Sialon. The growth mechanism of β-Sialon was suggested by density functional theory; calculation results revealed that the key step for the formation and growth of β-Sialon was the adsorption of the gaseous molecule Al₂O on the Si₃N₄ (101) crystal plane. It was found that the existence of Fe atoms could significantly reduce the adsorption energy. Additionally, Al₂O₃–C refractories containing plate-like β-Sialon possessed a high cold modulus of rupture and crushing strength, which increased by 40% and 15%, respectively, compared with the specimens containing column β-Sialon. It was also found that the formation of plate-like β-Sialon resulted in significantly better thermal shock resistance for the Al₂O₃–C refractory specimens, and the residual strength loss ratio of the sintered specimens was only 4% after five thermal shock cycles.     

Preparation of dispersible nanosilica surface-capped by hexamethyl disilazane via an in situ surface-modification method and investigation of its effects on the mechanical properties of styrene–butadiene/butadiene rubber

Silica has been established as one of the most promising materials in green tires. The filler–rubber interactions can increase the comprehensive performance of rubber composites. In this study, sodium silicate was used as the silicon source and hexamethyl disilazane (HMDS; molecular formula: C₆H₁₉NSi₂) was used as a modifier to synthesize dispersible silica (DNS) via an in situ surface-modification method. The effects of the HMDS-capped silica on the properties of rubber–matrix composites made of styrene–butadiene rubber (SBR) and high-cis-polybutadiene rubber (BR9000 or BR) were investigated with Zeosil 1165MP (Z1165-MP; a commercial highly dispersible silica produced by Rhodia for the production of green tires in the rubber industry) as a reference. The results show that the SBR–BR–DNS composite was before the SBR–BR–Z1165-MP composite in increasing the tear strength and elongation at break and reducing the compression heat buildup. On the basis of the resulting properties, the reinforcing behaviors in the rubber–matrix composites were analyzed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47763.     

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO₃ coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al₂O₃–MgO castables. The effects of Ca²⁺/Cit^3? mole ratio in precursor solution on coating characteristics of CaCO₃ coated corundum aggregates as well as the effects of CaCO₃ coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO₃ coating is increased and the size of CaCO₃ particles in coatings decreases first and then increases as Ca²⁺/Cit^3? mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA₆) layer at aggregate/matrix interface after sintering at 1600 °C. The Al₂O₃–MgO castables exhibit the best thermal shock resistance when Ca²⁺/Cit^3? mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA₆ layer with thickness of 10 μm, which is in-situ formed through reaction between Al₂O₃ and CaO derived from CaCO₃ coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al₂O₃–MgO castables.