Extraction methods and availability of micronutrients for wheat under a no-till system with a surface application of lime

ABSTRACT: Micronutrient availability can be affected by the increase of the soil pH due to surface liming. A field trial was carried out on a loamy, kaolinitic, thermic Typic Hapludox at Ponta Grossa, Paraná State, Brazil. The main objective was to evaluate the effects of surface liming and re-liming on the availability of micronutrients [copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)] for wheat (Triticum aestivum L.) cropped under a no-till system. A randomized complete block design was used in a split-plot arrangement. The main plots received surface lime applications (2, 4, and 6 Mg ha) in July 1993. In the subplots, surface lime (3 Mg ha) was applied again in June 2000. In 2003, before the wheat sowing, soil samples were taken at 0-5, 510, and 10-20 cm layers. Soil cationic micronutrients concentrations using different extractants (DTPA-TEA, Mehlich-1, HCl, and Mehlich-3) and solution/soil ratios were determined. Application of lime increased soil pH at 0-5, 5-10, and 10-20 cm. The increase in soil pH by liming did not affect soil organic carbon content. The Mehlich-3 solution had a greater capacity in extracting soil micronutrients. Increasing solution/soil ratio of the DTPA-TEA, Mehlich-1, and HCl solutions generally increased the extraction of Cu, Fe, Mn, and Zn. Liming and re-liming caused a decrease in Mn concentration in the wheat leaves. Leaf concentrations of Cu, Fe and Zn were not affected by liming treatments. The solutions of DTPA-TEA, Mehlich-1, HCl, and Mehlich-3 were ineffective to predict the soil cationic micronutrients availability for a wheat crop after surface application of lime.


Introduction
Surface application of lime without incorporation has been the usual practice to control soil acidity in notill (NT) systems (Caires et al., 2005).However, surface liming can decrease the cationic micronutrients availability as a consequence of increasing soil pH at the most superficial layers of the soil (Caires & Da Fonseca, 2000;Godsey et al., 2007).
Studies related to bioavailability of cationic micronutrients in Brazilian soils have been carried out in pots under greenhouse conditions or in field trials under conventional tillage systems (Camargo et al., 1982;Bataglia & Raij, 1989;Abreu et al., 1994Abreu et al., , 1996Abreu et al., , 1998Abreu et al., , 2002;;Rodrigues et al. 2001;Nascimento et al., 2006).In no-till systems, soil organic matter and cationic micronutrients concentrations have been higher up to 10 cm depth (Zanão Júnior et al., 2007).Because the surface-applied Sci.Agric.(Piracicaba, Braz.), v.67, n.1, p.60-70, January/February 2010 lime under NT systems has been more effective in increasing pH near the soil surface (Caires et al., 2005), more information is necessary concerning the effects of liming on cationic micronutrient bioavailability in this cropping system.In addition, well-known extractants, i.e. chelating and/or acid solutions reported in Norvell (1984) and Abreu et al. (2007), must be more studied under NT systems to identify the more appropriated soil micronutrient extractant.
From the economic point of view wheat is, one of the most important winter crops in Southern Brazil.Therefore, the wheat crop was chosen to be used in this study to evaluate the effects of surface liming and re-liming in a long term NT system on availability of cationic micronutrients (Cu, Fe, Mn, and Zn).Different extractants and soil/solution ratios were evaluated, as well as the micronutrients concentrations in wheat leaves to determine the most appropriated soil extractants for available cationic micronutrients in this crop production system.
A randomized complete block design, with three replications in a split-plot arrangement was used.The main plots (8.0 m × 6.3 m) treatment consisted of a surface dolomitic lime broadcasting at the rates of 0, 2, 4, and 6 Mg ha -1 in July 1993.The lime rates were calculated to raise the base saturation in the topsoil (0-20 cm) to 50, 70 and 90% base saturation.The dolomitic lime used contained 176 g kg -1 Ca, 136 g kg -1 Mg, and 84% effective calcium carbonate equivalent (ECCE).Approximately, seven years later (in June 2000) the main plots were subdivided in two subplots (4.0 m × 6.3 m) to study the influence of surface re-liming (196 g kg -1 Ca, 130 g kg -1 Mg, and 90% ECCE) at the rates of 0 and 3 Mg ha -1 .The reliming highest rate was calculated to raise the topsoil base saturation to 65% (Caires et al., 2000) regarding the treatment on which were applied 4 Mg ha -1 of lime in July 1993 [pH 0.01 mol L -1 CaCl 2 of 4.6; cation exchange capacity (CEC) at pH 7.0 of 110.8 mmol c dm -3 ; and 41% of base saturation].
The experimental area has been fertilized with N, P, and K, without addition of cationic micronutrients.Soybean [Glycine max (L.) Merr.] or corn (Zea mays L.) in the spring-summer season, and wheat or triticale (× Triticosecale) or black oat (Avena strigosa Schreb.) in the autumn-winter season has been cultivated.More details about the historic of cropping and fertilization and the effects of surface application of lime in the soil-plant system was reported elsewhere (Caires et al. 2005).
Wheat, cv.CD 104 (moderately susceptible to the soil exchangeable Al 3+ ) was sown in June 2003, with 17 cm between rows and 140 kg of seed per hectare, for a population between 250 and 300 plants m -2 .Fertilizers were applied by top dressing at the rates of 80 kg ha -1 N and 35 kg ha -1 K, as ammonium nitrate and potassium chloride, respectively.
Wheat plants flowered fully 84 days after emergence, and the crop maturation occurred 126 days after emerging.Air temperature was adequate for wheat growing and rainfall was on a considerable amount before sowing (55 mm) and before the plant flowering stage (56 mm).However, there was an extended water deficit during the vegetative development stage.Rainfall was 434 mm during the wheat crop cycle.More details about the wheat crop management are in Caires et al. (2006b).
During the flowering period of the wheat crop, leaves were sampled from 30 plants (flag leaf standard) of each subplot (Bataglia et al., 1978).These samples were washed in de-ionized water, dried in a forced-air oven at 60ºC until constant mass was achieved, and ground in a Wiley type mill to pass a 0.75 mm screen.After nitric-perchloric acid digestion of the plant tissues, Cu, Fe, Mn, and Zn concentrations were determined by atomic absorption spectrophotometry, according to Malavolta et al. (1997).
All soil extracting solution suspensions were shaken on a horizontal-circular shaking machine at 220 rpm.After this step, all suspensions were filtered through Whatman #42 filter paper and the concentration of micronutrients were measured using a PerkinElmer Optima 3000 XL simultaneous inductively coupled plasma optical emission spectrometry (ICP-OES), under routine operating conditions, at the 324.754 nm, 259.940 nm, 257.610 nm, and 213.856 nm atomic lines for Cu, Fe, Mn, and Zn, respectively.
Soil and plant data were submitted to variance and regression analyses.Regression equations were adjusted to the obtained data according to lime rates (0, 2, 4, and 6 Mg ha -1 ), adopting the magnitude of coefficients of determination (p < 0.05) as the criteria of choice.The effects of re-liming at 0 and 3 Mg ha -1 were compared by the Tukey test (p = 0.05).When a no significant interaction of the liming versus the re-liming treatment was observed, the effects of treatments were compared by using their means.Simple linear correlation analyses (Pearson correlation, p < 0.05) were obtained for soil pH and cationic micronutrients extracted by different procedures.All statistical analyses were performed using the SAS program, version 8.02 (SAS Institute, 1999).

Soil organic carbon and soil reaction
No interactions were observed between the surface liming rates (0, 2, 4, and 6 Mg ha -1 ) and surface re-liming  (0 and 3 Mg ha -1 ) for SOC and soil pH.Surface liming and re-liming did not cause changes in the SOC concentrations.The mean concentrations of SOC at 0-5, 5-10 and 10-20 cm layers were 25.1, 19.2 and 17.7 g dm -3 , respectively.Caires et al. (2006a) reported similar results for other Oxisol (clayey, kaolinitic, thermic Rhodic Hapludox) under a NT system.
The availability of micronutrients to the crops is controlled by many soil factors such as pH, soil organic matter, temperature, and moisture (Fageria et al., 2002).In this study, because the wheat crop presented adequate growth and yield (Caires et al., 2006b) and SOC concentrations were unchanged due to liming treatments, soil pH was assumed to be the major factor on determining the bioavailability of Cu, Fe, Mn, and Zn.

Extractable cationic micronutrients
Soil extractable Cu, for all extraction methods, was not influenced by the interaction between lime rates (0, 2, 4, and 6 Mg ha -1 ) and re-liming (0 and 3 Mg ha -1 ).Copper concentrations decreased linearly with increasing lime application rates for DTPA-TEA solution (5:1), Mehlich-1 solution (5:1 and 10:1) and HCl solution (5:1 and 10:1) at 0-5 cm layer; and also by 0.1 mol L -1 HCl solution (5:1) at 5-10 cm layer (Figure 1).Negative correlations were observed between concentrations of extractable Cu and soil pH, mainly, at 0-5 cm layer (Table 1).Solubility of Cu 2+ is soil pH dependent and decreases 100-fold for each unit increase in pH (Fageria et al., 2002).Moreover, these authors also state that increases in soil pH above 6.0 induces hydrolysis of hydrated Cu which can lead to a stronger Cu adsorption by the clay minerals and organic matter.

Micronutrients in the wheat leaves
The 1993 liming and the 2000 re-liming treatments did not affect the Cu, Fe and Zn concentrations in the wheat leaves in 2003 (Figure 5).However, Mn concentration in the leaves was influenced by the interaction between lime rates (0, 2, 4, and 6 Mg ha -1 ) and re-liming (0 and 3 Mg ha -1 ).Increasing the surface liming rate decreased linearly Mn concentration in the wheat leaves both in the plots with or without re-liming.Surface reliming caused a decrease in Mn concentration in the wheat leaves, mainly in the plots not limed in 1993.However, these observed Mn concentrations were adequate for wheat, according to Malavolta et al. (1997).
Changes in cationic micronutrients availability at the soil most superficial layers after liming affect the mineral nutrition of plants, mainly under NT systems (Caires & Da Fonseca, 2000;Godsey et al., 2007;Caires et al., 2008).However, the cationic micronutrients concentrations extracted by the DTPA-TEA, Mehlich-1, Mehlich-3, and HCl solutions after the surface lime application under a NT system cannot represent the available amount to the crops.Our study highlight the difficulty of selecting a procedure for the extraction of these micronutrients that would be more appropriated to predict their cationic micronutrients bioavailability.This is in agreement with the results obtained in other studies in Brazilian soils (Vidal-Vázquez et al., 2005;Nascimento et al., 2006;Abreu et al., 2007).

Figure 1 -
Figure 1 -Soil copper (Cu) concentrations extracted by different procedures as affected by surface-applied lime rates, without ( ) and with ( ) surface re-liming at the rate of 3 Mg ha -1 .*p < 0.05 and **p < 0.01.

Figure 2 -
Figure 2 -Soil iron (Fe) concentrations extracted by different procedures as affected by surface-applied lime rates, without ( ) and with ( ) surface re-liming at the rate of 3 Mg ha -1 .*p < 0.05 and **p < 0.01.

Figure 3 -
Figure 3 -Soil manganese (Mn) concentrations extracted by different procedures as affected by surface-applied lime rates, without ( ) and with ( ) surface re-liming at the rate of 3 Mg ha -1 .*p < 0.05 and **p < 0.01.

Figure 4 -Figure 5 -
Figure 4 -Soil zinc (Zn) concentrations extracted by different procedures as affected by surface-applied lime rates, without ( ) and with ( ) surface re-liming at the rate of 3 Mg ha -1 .*p < 0.05.

Table 2 -
Minimal (Min), maximum (Max) and mean of cationic micronutrients concentrations in soil considering all treatments, in different layers, after liming and re-liming according to extraction methods.