Aluminum toxicity in no tillage system in Southern Brazil Toxicité aluminique en système de non-culture dans le sud du Brésil

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1 Scientific registration n o : 2213 Symposium n o : 13B Presentation: poster Aluminum toxicity in no tillage system in Southern Brazil Toxicité aluminique en système de non-culture dans le sud du Brésil ANGHINONI Ibanor, SALET Roberto Luiz Departamento de Solos, Faculdade de Agronomia/UFRGS. Caixa Postal 776 CEP Porto Alegre RS, Brazil. INTRODUCTION The cropped area under no tillage system is growing very fast in Southtern Brazil, especially in the last five years. The area has almost doubled every year in the southermost state, Rio Grande do Sul, 0.3 million ha in the 1992/93 growing season and about 2.0 million ha in 1996/97. It is expected that 60% (4.0 million ha) of the agricultural area of this state and 40% (20.0 million ha) of the agricultural area of the country will be under no tillage by the year The knowledge and technologies generated by research to supply data for the Soil Testing Program for lime and fertilizer recommendation has increased much more slowly than is required. One of the major constraints is the lacking of information on aluminum toxicity, a very important problem in acidic soils. As opposed to that observed in conventional tillage, fairly high yields hare been obtained for corn (> 6.0 t ha -1 ), soybean (> 3.0 t ha -1 ) and wheat (> 2.7 t ha -1 ) either in crop fields or experimental areas under no tillage (> 5 years) with low ph soils (< 5.0), high exchangeable aluminum (> 1.5 cmol c dm -3 ) and high lime requirement (> 7.0 t ha -1 ). These results led to the hypothesis that aluminum toxicity to plants could be lower in the no tillage system due to: a) lower ionic activity in soil solution because of the higher ionic concentration produced by surface fertilizer and lime application and b) crop residue decomposition (nutrient cycling) and/or soluble organic ligands, such as fulvic or low molecular weight acids, with high ionic stability constants of their Al complexes, continuously being formed by such surface crop residue decomposition. This research was conducted to verify, first, if the aluminum toxicity is really lower in no tillage systems and, secondly, if so, which mechanism, increase of ionic strength or aluminum complexation by organic ligand in the soil solution, is the most important. MATERIAIS AND METHODS A series of two experiments were conducted in the greenhouse to study the chemistry of aluminum in soil solution from no tillage system and its effect on soybean (Glycine max. L. Merril) root growth. In the first experiment, soybean (cv BR-16, sensitive to aluminum) seedlings were grown in soil solution from established no tillage and conventional tillage field plots with no lime application for the last nine years. The experiment was located in the Wheat National Research Center/EMBRAPA, in Passo Fundo, in an Oxisol with water ph of 5.2. Soil solution was extracted from 0-8 cm surface layer by the displacement procedure in columns (Adams, 1974), filtered and placed in small tubes (40 cm 3 of solution). Four aluminum levels (0, 0.075, 0.15 and 0.30 mmol dm -3 ) were then added to the soil solutions and soybean seedlings were cultivated in the tubes for four days. The solution ph was maintained at ph 4.6 in a continuously aerated system. An additional treatment of CaCl mol dm -3 was used. 1

2 The second experiment was carried out to determine whether high ionic strength or complexation with organic ligand compounds in soil solution is more important to reduce aluminum toxicity. Seedlings of the same soybean cultivar were grown for four days in solutions with organic compounds and/or inorganic salts, to give the same composition and strength as used in the previous experiment, and treated with the same aluminum levels. Inorganic salts were used to prepare the solutions. The soluble organic carbon was obtained by incubation of surface no till crop residues for eight months in washed sand, and extracted in the pressure plate at 0.1 Mpa. The solution was prepared to give the same soluble organic content as in the first experiment. During the plant growth period, the solutions in the tubes were monitored to maintain the original soluble carbon level, electrical conductivity and ph (4.6). A complete random design, with three replicates, was used in both experiments. Soil solution was analysed for calcium, magnesium, aluminum (atomic absorption spectroscopy), potassium, sodium (flame photometry), ammonium, nitrate (semi-micro Kjeldahl distilation), sulfate (turbidimetry), phosphorus, soluble organic carbon (colorimetry) and aluminum speciation estimates (Soilsoln - Wolt, 1989). Root length was determined by Tennant s method (1975). RESULTS AND DISCUSSION First Experiment Characterization of soil solution: Concentration of cations and anions in soil solution was higher in the no tillage system (Table 1) and electrical conductivity was almost double (0.42 ds m -1 ) as compared with conventional tillage (0.22 ds m -1 ). The application of fertilizer in the top soil and the decomposition of residues at the surface of the soil increase nutrient content in the solid phase of the top soil (Salet, 1994). This effect is not evident in conventional tillage because of nutrient redistribution in the soil profile by tillage. Soluble organic carbon was almost double in the soil solution of the no tillage system. This is a result of the higher organic matter content and higher decomposition of surface residues in top soil in the system (Salet, 1994). Total soluble aluminum content was also higher in the no tillage system (0.15 mmol dm -3 ) as compared with the conventional one (0.08 mmol dm -3 ). Higher organic ligand content in that system may complex aluminum in the solid phase of the soil, and release it into the liquid phase. Biotest: Total root length (primary and secondary roots) was very similar (about 40 cm) in the control solution (0.02 mmol dm -3 CaCl 2, no aluminum) and in the original aluminum content of no (0.15 mmol dm -3 ) and conventional (0.08 mmol dm -1 ) tillage. These results show no rhizotoxicity effects with twice the total soluble aluminum in the no tillage system as compared with the conventional system (Figure 1). By increasing the aluminum concentration in soil solution in this system, total root length was decreased to half (20 cm) with the first aluminum level (0.15 mmol dm -3 ) and continued to decrease (13 cm) with 0.23 mmol dm -3 of aluminum. However, root growth was not affected by the aluminum concentrations in the no tillage system. In this system, root length only decreased when total soluble aluminum was 0.30 mmol dm -3. The biotest results indicate that the soil solution of the no tillage system has a higher capacity to reduce aluminum toxicity; thus aluminum is less toxic in the no tillage system as compared to the conventional one. Variation in organic carbon and electrical conductivity: The composition of the soil solution can present high variability in space and time in the field. Thus, the results obtained in the biotest cannot be repeated in soil solutions sampled at different places and plant growth stages. Soil solution was therefore taken in different field experiments (four) and crops. Soil organic carbon and electrical conductivity, two important factors that affect aluminum toxicity, were always higher in the soil solutions of the no tillage treatments (Table 2). The samples were taken at the initial growth of soybean (Nov./Dec.) and of winter - wheat, oat and barley (July) crops in Southern Brazil. The initial period of growth is the most sensitive to aluminum toxicity for most agricultural species. The higher values for electrical conductivity and soluble organic carbon found in the field indicate that the biotest results are valid for well established no tillage systems. 2

3 Second Experiment Estimates of aluminum species: Free aluminum (Fe 3+ ) is the most mononuclear toxic form. Due to ionic charge, Al 3+ may have a very high activity close to the negative charges of the cell wall and plasmatic membrane and causes inhibition of root growth. Any solution that can restict the content of Al 3+ species will potentially be more efficient to reduce aluminum toxicity. Aluminum speciation estimates were made at each aluminum level in the organic and inorganic solutions used in the biotest (Figure 2). The content of Al 3+ increased in both solutions with added aluminum; however, it was three times higher in the inorganic solution. The organic ligands of the organic solution were, certainly, more effective than inorganic ions to reduce Al 3+ content. Biotest: The length of primary soybean root was similar for organic, inorganic and CaCl 2 solutions with no aluminum addition (Figure 3). However, with the addition of the first aluminum level, the length of primary root was greatly decreased in the inorganic solution, while in the organic solution it only decreased in the highest aluminum level (0.30 mmol dm -3 ). Primary root length was higher in all aluminum treatments in the organic solution. The total root length was also similar in no aluminum (organic, inorganic and CaCl 2 ) solutions (Figure 4). It was also decreased, in a drastic fashion, by adding aluminum in the inorganic solution. Decrease in root length was observed in the organic solution in the two highest aluminum levels (0.15 and 0.30 mmol dm -3 ). Organic solution is, then, more effective to reduce the rhizotoxic effects of aluminum. The lower effect of the most toxic aluminum species (Al 3+ ) found for the organic solution, would explain the lower aluminum toxicity. At can be concluded that the decrease of aluminum toxicity in the no tillage system, observed in the first experiment, can be explained by the complexation of aluminum with organic ligands. BIBLIOGRAPHY ADAMS, F. Soil solution. In: CARSON, E.W. (Ed.) The Plant root and its environment. Charlottesville, University of Virginia, P SALET, R.L. Dynamics of ions in soil solution under no tillage system. Porto Alegre, Faculdade de Agronomia/UFRGS, p. (M. S. Dissertation, in Portuguese) TENNANT, D. A test of a modified line intersect method of estimating rooth length. J. of Applied Ecol., Oxford, 63: , WOLT, J.D. A program for teaching equilibria modeling of soil solution composition. J. Agron. Educ., Minnesota, 18:40-42, Keywords: aluminum, no tillage, soil solution Mots clés : aluminium, non culture, solution du sol Table 1. Chemical parameters in soil solution of different tillage systems (1) System Ca Mg K Na NH 4 Al mmol dm No till Conv. Till ph EC (2) NO 3 PO 4 SO 4 SOC (3) ds m mmol dm No till Conv. Till (1) Soil layer of 0-8 cm (2) Electrical conductivity (3) Soluble organic carbon 3

4 Table 2. Soluble organic carbon and electrical conductivity in soil (1) solution of different tillage systems and in different location and periods of the year Soil solution parameter System Experiment field location (2) OR MO CP CO Nov/96 Jul/97 Dez/92 Jan/94 Jul/94 Jul/96 EC (3) No till ds m -1 Conv.till SOC (4) No till mmol dm -3 Conv. till (1) Surface layer (0-5 cm) (2) Oxisol (3) Electrical conductivity (4) Soluble organic carbon 4

5 Figure 1. Total soybean root length cultivated in soil solution of different tillage systems and aluminum levels. * Points with different letters are significantly different at the 5% level (Tukey test). Capital letters indicate differences between the two solutions and small letters indicate differences between aluminum levels in the some solution. Figure 2. Estimates of free Al 3+ in different solutions whiter increasing aluminum levels (mean of 3 replicates). 5

6 Figure 3. Primary soybean root length cultivated in different solutions and aluminum levels. * Points with different letters are significantly different at the 5% level (Tukey test). Capital letters indicate differences between the two solutions and small letters indicate differences between aluminum levels in the some solution. Figure 4. Total soybean root length cultivated in different solutions and aluminum levels. * Points with different letters are significantly different at the 5% level (Tukey test). Capital letters indicate differences between the two solutions and small letters indicate differences between aluminum levels in the some solution. 6