Scientific registration n o : 2249 Symposium n o : 12 Presentation : poster

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1 Scientific registration n o : 2249 Symposium n o : 12 Presentation : poster Effect of cation-anion balance of plants on soil acidification by nitrogen fertilizers Influence du rapport cations-anions de la plante sur l'acidificacion du sol à partir des engrais azotés KIEHL Jorge de Castro(1), CAMACHO Juan Carlos Quevedo(2), PROCHNOW Luís Ignácio(1) (1) ESALQ/USP, Piracicaba, SP, Brazil, CEP (2) ESALQ/USP- Graduate student INTRODUCTION Nitrogen fertilizers, depending upon its chemical composition, may leave in the soil an acid, neutral or alkaline residue. The oxidation of NH 4 + to NO 3 - produces a considerable amount of protons H + and represents the main acidification process in the soil. It has been exhaustively demonstrated that ammonium sulfate is the most acidifying nitrogen fertilizer. Ammonium nitrate, having only half of its nitrogen in the ammonium form, is less acidifying than sulfate. Calcium nitrate, in opposition to ammonium fertilizers, has an alkaline reaction in the soil. Studies performed by Pierre (PIERRE, 1928a and 1928b and PIERRE et al., 1970) gave rise to the concept of theoretical acidity of nitrogen fertilizers. PIERRE et al. (1971) found that ammonium fertilizers have the capacity to produce an acidity equivalent to 3.5 kg of sulfuric acid per quilogram of N applied in the soil, which would require 3.57 kg of CaCO 3 to neutralize one quilogram of N in the ammonium form. This theoretical acidity, defined by the authors as the maximum amount of acidity that can be developed from complete nitrification of the ammonium-n where no crop is grown, usually does not correspond to the actual acidity observed in the field because of the influence of several factors (PIERRE et al., 1970). Therefore, it is a non-sense to consider the acidification or alkalization capacity of nitrogen fertilizers as constant values (which are usually referred to as acidity or alkalinity indexes), as can be found in fertilizer handbooks and many text-books (WALLACE, 1994). PIERRE (1928a and 1928b) found that only about half of the theoretical acidity was developed in the soil when four crop species were grown in succession in the greenhouse. He suggested that plants uptake about half their N with an equivalent amount of H + ions, or that there was an exchange of bicarbonates (HCO 3 - ) from the roots equivalent to about half of the nitrate-n absorbed. WALLACE (1994) states that only half of the applied nitrogen will generate acidity in the soil, since the other half is absorbed by the crop. So, any nitrogen provided in excess of crop need will acidify the soil at the full theoretical rate less any leached from the soil as nitric acid, less any volatilized as ammonia, less any nitrate that is denitrified, and less any nitrogen immobilized by microorganisms. The process of generating acidity or alkalinity is caused by the unequal uptake of nutrient cations (NH Ca 2+ + Mg 2+ + K + + Na + ) and nutrient anions (NO Cl - + SO H 2 PO 4 - ) by 1

2 plant roots, expressed in terms of charge equivalence. According to HAYNES (1990), in order to maintain electroneutrality at the soil-root interface, H + or OH - ions are excreted by the roots in quantities that are stoichiometrically equal to the respective excess cation or anion uptake. As far as the ionic imbalance is not too large, the plant will maintain the electroneutrality by synthesizing organic anions such as malate and citrate (HAYNES & GOH, 1978). When NH 4 + is the main form of nitrogen absorbed, the amount of absorbed cations may exceed that of anions, leading to soil acidification. As RAIJ & DIEST (1979) and BEKELE et al. (1983) pointed out, some plants like buckwheat (Fagopyrum esculentum) have an exceptional capacity to extract Ca and Mg from the soil and will accumulate more cations than anions even if NO 3 - is the primary source of nitrogen. Many other workers have studied the effect of cation and anion uptake by plants on the soil reaction (CUNNINGHAM, 1964a, 1964b and 1964c; KIRKBY, 1968; RILEY & BARBER, 1969; PIERRE & BANWART, 1973; JARVIS & ROBSON, 1983; RÖMHELD, 1986; RAIJ et al., 1988 and PETERSEN & BÖTTGER, 1991). RILEY & BARBER (1969) observed an accumulation of HCO 3 - and an increase in ph in the root-soil interface of soybean as compared to the original soil. Since the magnitude of these alterations was related to the NO 3 - level of the soil solution, the effect was attributed to a greater uptake of anions than cations by the plant so that HCO 3 - was released to maintain electrical neutrality. JARVIS & ROBSON (1983) observed that changes in soil ph were strongly related to cation/anion balance, with increasing acidity resulting from the decreasing ash alkalinity of shoots of plants supplied with NH 4 +. Studying Italian rye-grass, CUNNINGHAM (1964a) found that the sum of cations (Na + K + Ca + Mg) in the plant ranged widely and depended on the sum of anions (N + P + Cl + S), all values expressed in miliequivalents/100g. The of cations was positively correlated with of anions and with %N in the grass. CUNNINGHAM (1964c) measured or calculated the ratios of of cations: of anions (R) of sixty two crops and pastures herbs and found to be negatively correlated with %N in the plants. Some authors (PIERRE et al., 1970; PIERRE & BANWART, 1973 and GIJSMAN, 1990b) prefer to consider the difference between the of cations and the of anions instead of their ratio, the result being named excess base or EB. To evaluate the quantitative effect of crops on the acidity produced in soils from nitrification of NH 4 NO 3 fertilizer, PIERRE et al. (1970) and PIERRE & BANWART (1973) considered the excess base/n ratio (EB/N) of the plant, which is obtained by dividing the EB by the amount of N in the plant, expressed in equivalents as well. The EB/N ratio should be a better criterion to evaluate the effect of plants on soil reaction since the excess of cations contributes to increase the acidifying effect of the fertilizer, whereas the amount of N accumulated will have an opposite effect. PIERRE et al. (1970) found that the acidity developed from NH 4 NO 3 in uncropped soil was almost equal to the theoretical value. When oats was grown, the increase in acidity was 27% lower than the theoretical value, but when buckwheat was cultivated it was 87% higher. These deviations from the expected value were quantitatively explained by the EB/N ratios in the plants. These authors suggested that crops having an EB/N less than one should decrease the acidity produced in the soil by ammonium fertilizers, whereas those with a ratio greater than one should increase it. Later, PIERRE & BANWART (1973) determined the EB and EB/N ratios of various crop species and found strong evidences that this hypothesis was true. It can be seen from this review that the EB/N ratio should allow a better evaluation of the effect of plants on the reaction of soils treated with nitrogen fertilizers, since some plant species tend to accumulate more N in their tissues than others. In this way, the latest studies of Pierre and coworkers should be highlighted. However, in their experiments most of the nitrogen was absorbed as 2

3 nitrate, and no information is available about the validity of the EB/N ratio when ammonium is the major form of nitrogen entering the plant. This study had the objective of determining the balance of cations and anions in three plant species treated with different forms of nitrogen (nitrate, ammonium + nitrate and ammonium) and to evaluate the influence of this balance on soil acidification. MATERIALS AND METHODS The greenhouse experiment was carried out in pots filled with 4 kg soil samples collected from the Ap horizon of a dystrophic Dusky-Red Latosol (Typic Haplorthox) of the State of São Paulo, Brazil. Some properties of the soil were: clay = 77.5%; ph in 0.01 mol L -1 CaCl 2 = 3.65; O.M = 22.5 g kg -1 ; sum of bases = 8.0 mmol c kg -1 ; CEC = 9.2 mmol c kg -1 ; base saturation = 8,5%. Treatments consisted of four cultivation methods (no crop, maize, rice and buckwheat), three nitrogen fertilizers (calcium nitrate, CN; ammonium nitrate, AN; and ammonium sulfate, AS) and three nitrogen rates (0, 800 e mg of N per pot). The soil sample was limed with a mixture of CaCO 3 and MgCO 3 (in a ratio of 4:1) in order to raise the ph in 0.01 mol L -1 CaCl 2 to 5.6 (equivalent to a base saturation of approximately 74%), moistened to 60% of the WHC and incubated for 15 days. The amount of lime was determined from a neutralization curve previously prepared. All pots received a basal fertilization which included P, K, Cu, Fe, Zn, Mn, Mo and B. Nitrogen was applied as a solution with a pipette, in amounts and forms according to the respective treatments. Soil samples were thoroughly mixed and remoistened. Four hybrid maize (Zea mays L.) plants with medium tolerance to soil acidity, eight rice (Oryza sativa L.) plants and six buckwheat (Fagopyrum esculentum Moench) plants were cultivated for 24, 32 and 40 days, respectively. Plants were cut near the soil surface and the tops oven-dried at 65 0 C, weighted, grinded and subjected to determination of total N, P, K, Ca, Mg, S, Na and Cl. Excess base (EB) in the plant tops was calculated by the equation EB = of cations (Ca 2+ + Mg 2+ + K + + Na + ) - of anions (H 2 PO SO Cl - ), all values expressed in mmol c kg -1. Excess base/n ratio was also calculated by dividing the excess base by the amount of N in the plant, expressed in mmol c kg -1. The ph in water and in 0.01 mol L -1 CaCl 2 of the soil samples were determined. RESULTS AND DISCUSSION In absence of plants, CN caused only a small reduction in soil ph, probably due to the salt effect (Table 1). AN and AS, however, reduced more drastically both ph in water and in CaCl 2, especially AS. When maize and rice were cultivated, CN increased soil ph, certainly because the intense absorption of NO 3 - was accompanied by the release of basic anions (OH - and HCO 3 - ) by the roots to preserve electroneutrality in the system, as was pointed out by PIERRE (1928a and 1928b), BEKELE et al. (1983), RÖMHELD (1986), and WALLACE (1994). In the buckwheat cultivated soil CN had an opposite effect, as the ph showed a slight decrease; as commented by RAIJ & DIEST (1979) and BEKELE et al. (1983), some plants like buckwheat (Fagopyrum esculentum) extract higher amounts of Ca and Mg from the soil and 3

4 accumulate more cations than anions, even when NO 3 - is the primary source of nitrogen. When the ammonium-containing fertilizers (AN and AS) were applied, the ph values decreased up to 1.31 unit in all cultivated and non-cultivated pots, an evidence, according to RAIJ et al. (1988), of a strong absorption of a cation, NH 4 +, together with the elimination of H + by the roots to equilibrate the charges in the plant. Table 1. Effect of cultivation and nitrogen fertilizers on the ph H2O (and ph CaCl2 between parenthesis) values of a dystrophic Dusky-Red Latosol (1) Nitrogen Rate of N, mg kg -1 Crop fertilizer (2) Maize CN 5.84 (5.25) 6.00a (5.40a) 5.95a (5.55a) AN 5.84 (5.25) 5.64b (5.10b) 5.34b (4.93b) AS 5.84 (5.25) 5.10c (4.60c) 4.72c (4.25c) Rice CN 5.86 (5.28) 6.20a (5.51a) 6.30a (5.96a) AN 5.86 (5.28) 5.62b (5.13b) 5.26b (4.85b) AS 5.86 (5.28) 4.95c (4.51c) 4.55c (4.27c) Buckwheat CN 5.83 (5.36) 5.73a (5.28a) 5.62a (5.43a) AN 5.83 (5.36) 5.46b (5.11b) 5.10b (4.83b) AS 5.83 (5.36) 4.98c (4.72c) 4.63c (4.45c) No crop CN 5.61 (5.35) 5.42a (5.30a) 5.35a (5.25a) AN 5.61 (5.35) 4.79b (4.70b) 4.81b (4.77b) AS 5.61 (5.35) 4.51c (4.35c) 4.89b (4.73b) (1) For each crop and nitrogen rate, means were separated by Tukey test, P=0.05 (2) CN, calcium nitrate; AN, ammonium nitrate; AS, ammonium sulfate EB values were higher in plants treated with CN than in plants receiving AN or AS, showing that nitrate stimulated the absorption of the cations Ca 2+, Mg 2+, K + and Na + and reduced the absorption of the anions H 2 PO - 2-4, SO 4 and Cl - (Figure 1). Increasing the application rate of nitrogen as CN increased the EB of maize and buckwheat, especially the later, whereas in the form of AN and AS nitrogen reduced the EB of all crops. These results were also found by JARVIS & ROBSON (1983a and 1983b) and by GIJSMAN (1990a and 1990b). The EB in buckwheat was considerably higher than in the grasses, even when no - nitrogen was applied (Figure 1). Since NO 3 stimulates the absorption of bases, the highest differences between crops were observed when CN was applied. Considering the application rate of 200 mg kg -1, the EB increased from the values of cmol c kg -1 observed in maize or cmol c kg -1 verified in rice, to the cmol c kg -1 found in buckwheat (data not shown). These results confirm the high tendency of dicotyledons to accumulate higher amounts of cations than monocotyledons, as was also observed by PIERRE & BANWART (1973) and RAIJ et al. (1988). 4

5 The EB/N ratio in all crop species reduced as the nitrogen rate increased, indicating that the increasing amounts of absorbed N were not followed by similar increases in the excess base (Figure 2). As described for EB, plants receiving CN, especially buckwheat, showed a clear tendency to present a higher EB/N ratio than plants fertilized with the other forms of nitrogen. In other words, plants accumulated more cations in excess of anions per unit of absorbed nitrogen when treated with nitrate than with ammonium fertilizer, since NO 3 - enhances the absorption of cations and depresses that of anions. When nitrogen was not applied, or when it was applied as CN, buckwheat showed EB/N ratios higher than the grasses; however, when the ammonium form was added, the EB/N ratio of buckwheat dropped to the level of those found in grasses. Therefore, the observations of PIERRE et al. (1970) and PIERRE & BANWART (1973) that plants with EB/N ratios less than one should decrease the acidity of nitrogen fertilizers, whereas those with a ratio greater than one should increase it, showed to be valid only when the plants were fertilized exclusively with nitrate nitrogen, since the application of AN and AS resulted in low EB/N values in all crop species. 200 MAIZE EXCESS BASE, cmol c kg RICE BUCKWHEAT CN AN AS NITROGEN RATE, mg kg -1 N Figure 1. Excess base in the tops of plants cultivated in dystrophic Dusky-Red Latosol treated with different rates of nitrogen in the forms of calcium nitrate (CN), ammonium nitrate (AN) and ammonium sulfate (AS) 5

6 2,0 1,5 1,0 MAIZE CN AN AS EXCESS BASE/N, cmol c kg -1 0,5 0,0 2,0 1,5 1,0 0,5 0,0 2,5 2,0 RICE BUCKWHEAT 1,5 1,0 0,5 0, NITROGEN RATE, mg kg -1 N References Figure 2. Excess base in the tops of plants cultivated in dystrophic Dusky-Red Latosol treated with different rates of nitrogen in the forms of calcium nitrate (CN), ammonium nitrate (AN) and ammonium sulfate (AS) BEKELE, T; CINO, B.J.; EHLERT, P.A.I.; MAAS, A.A. van; DIEST, A. van. An evaluation of plant-borne factors promoting the solubilization of alkaline rock phosphates. Plant and Soil, The Hague, 75: , CUNNINGHAM, R.K. Cation-anion relationships in crop nutrition. I. Factors affecting cations in Italian rye-grass. J. Agric. Sci., Cambridge, 63:97-101, 1964a. CUNNINGHAM, R.K. Cation-anion relationships in crop nutrition. II. Factors affecting the ratios of sum of the cations: sum of the anions in Italian rye-grass. J. Agric. Sci., Cambridge, 63: , 1964b. CUNNINGHAM, R.K. Cation-anion relationships in crop nutrition. III. Relationships between the ratios of sum of the cations: sum of the anions and nitrogen concentrations in several plant species. J. Agric. Sci., Cambridge, 63: ,1964c. GIJSMAN, Rhizosphere ph along different root zones of Douglas-fir (Pseudotsuga menziesii), as affected by source of nitrogen. Plant and Soil, The Hague, 124: , 1990a 6

7 GIJSMAN, A.J. Nitrogen nutrition of Douglas-fir (Pseudotsuga menziesii) on strongly acid sandy soil. I. Growth, nutrient uptake and ionic balance. Plant and Soil, The Hague, 126:53-61, 1990b. HAYNES, R.J. Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere ph. Plant and Soil, The Hague, 126: , HAYNES, R.J. & GOH, K.M. Ammonium and nitrate nutrition of plants. Biol. Reviews, Cambridge, 53: , JARVIS, S.C. & ROBSON, A.D. The effects of nitrogen nutrition of plants on the development of acidity in western Australian soils. I. Effects with subterranean clover grown under leaching conditions. Austr. J. Agric. Res., East Melbourne, 34: , 1983a. JARVIS, S.C. & ROBSON, A.D. The effects of nitrogen nutrition of plants on the development of acidity in western Australian soils. II. Effects of differences in cation-anion balance between plant species grown under non-leaching conditions. Austr. J. Agric. Res., East Melbourne, 34: ,1983b. KIRKBY, E.A. Influence of ammonium and nitrate nutrition on the cation-anion balance and nitrogen and carbohydrate metabolism of white mustard plants grown in dilute nutrient solution. Soil Science, Baltimore, 105: , PETERSEN, W. & BÖTTGER, M. Contribution of organic acids to the acidification of the rhizosphere of maize seedlings. Plant and Soil, The Hague, 132: , PIERRE, W.H. Nitrogenous fertilizers and soil acidity. I: Effect of various nitrogenous fertilizers on soil reaction. J. Amer. Soc. Agron., Madison, 20: , 1928a PIERRE, W.H. Nitrogenous fertilizers and soil acidity. II: The use of fertilizers combinations, lime, and basic slag in correcting the acidity formed by various nitrogenous fertilizers. J. Amer. Soc. Agron., Madison, 20: ,1928b PIERRE, W.H. & BANWART, W.L. Excess-base and excess-base/nitrogen ratio of various crop species and parts of plants. Agron. J., Madison, 61:91-96, PIERRE, W.H.; MEISINGER, J.; BIRCHETT, J.R. Cation-anion balance in crops as facts in determining the effect of nitrogen fertilizers on soil acidity. Agron. J., Madison, 62: , PIERRE, W.H.; WEBB, J.R.; SHRADER, W.D. Quantitative effects of nitrogen fertilizers on the development and downward movement of soil acidity in relation to level of fertilization and crop removal in continuous corn cropping system. Agron. J., Madison, 63: , RAIJ, B. van & DIEST, A. van Utilization of phosphate from different sources by six plant species. Plant and Soil, The Hague, 51: , RAIJ, B. van; CANTARELLA, H.; FURLANI, P.R. Efeito na reação do solo da absorção de amônio e nitrato pelo sorgo, na presença e na ausência de gesso. R. bras. Ci. Solo, Campinas, RILEY, D. & BARBER, S.A. Bicarbonate accumulation and ph changes at the soybean (Glycine max L. Merr.) root-soil interface. Soil Sci. Soc. Amer. Proc., Madison, 33: , RÖMHELD, V. ph changes in the rhizosphere of various crop plants, in relation to the supply of plant nutrients. Potash Rev. Subj., 6, th s. WALLACE, A. Soil acidification from use of too much fertilizer. Comm. Soil Sci. Plant Anal., New York, 25(1/2):87-92, Keywords : cation-anion balance; excess base; excess base/nitrogen ratio; soil acidification; soil reaction; soil acidity; nitrogen fertilizers. Mots clés : balance ionique, alcalinité, azot, acidification du sol, engrais azotés 7