Scientific registration number: 2207 Symposium number: 25 Presentation : poster Validation of annual and total cumulative loading limits stipulated by USEPA for Zn on oxisol. Validation des apports-limites en Zn annuels et totaux cumulés, recommandés par l'usepa. Application à un Oxisol brésilien ABREU Cleide Aparecida de (1), BERTON Ronaldo Severiano (1), KOEKKOEK Edwin Peter Josef (2) (1) Instituto Agronômico, Caixa Postal 28, 13001-970, Campinas, SP, Brazil (2) Dijkgraaf 4-12A, 6708 PG Wageningen, The Netherlands INTRODUCTION Land application of biosolids such as sewage sludge is restricted by several countries according to their heavy metal content, in order to prevent their accumulation in soil and their entry in the food chain. However, loading limits of heavy metals stipulated by the environmental agencies are criticised by not considering individual soil characteristics. Hence, it was observed that twenty annual applications of Cu and Zn sulfates supplied up to 415 kg ha -1 of Cu and 897 kg ha -1 of Zn, and neither corn grain nor silage yields were influenced by metal additions on a clay soil at ph > 6.5. On the other hand, the additions of Zn as sewage sludge at a level allowed by USEPA showed Zn toxicity symptoms to peanuts. In field experiments, it was also noted a decrease in yield of dicotyledon plants when soil received Zn, Ni and Cu as sewage sludge on an amount well below USEPA s loading limit. Since there is no legal requirements in Brazil for disposal of biosolids on agricultural land, it is possible that the Brazilian environmental agencies may adopt the USEPA requirements as an initial attempt to regulate metal accumulation in soil, until local research be able to modify them. Hence, the knowledge of individual soil characteristics that governs heavy metal availability to plants is essential to evaluate the environment impact caused by waste additions to the agricultural soil. Several authors have shown that the ph and the amount of iron oxides were the most important characteristics on Zn and Cd immobilization in soil, and also that soil samples with ph < 5.6 had more exchangeable Zn than samples with ph > 5.6. It was also noted that liming a soil to reach ph 6.8 decreased exchangeable Zn and its concentration in soybeans plant tops. Soil metal extractors are being developed to predict metal availability to plants. The DTPA extraction has shown to be more effective than HNO 3, NH 4 OAc and HCl for available Zn to grass. Mehlich-1, DTPA and HCl are the most extractors used in Brazil 1
for predicting Zn availability in soils with low to high levels of this element. However, research data for soils that received excessive amounts of Zn are scarce. The objective of this study was to evaluate the effect of adding increasing amounts of Zn, with two of then simulating the amounts stipulated by USEPA for annual and total cumulative loading of Zn applied as sewage sludge, on Zn availability to lettuce grown on a soil that received or not lime to increase base saturation to 90%, and on the performance of two soil extractors used for predicting Zn availability to plants when soil received an excessive amount of this element. MATERIAL AND METHODS Soil samples from the 0-20 cm layer of Hapludox soil were air dried and passed through a 5 mm sieve. Analysis for soil fertility showed: ph (CaCl 2 ) 4.8; organic matter 27g kg -1 ; resin P 18 mg dm -3 ; resin K, Ca, Mg and Al + H (ph smp ) 1, 4, 25, 16 and 43 mmol c dm -3, respectively, and soil density 1.1 g cm -3. Greenhouse studies were conducted in 2.5 dm 3 plastic pots. Two kilograms samples of Hapludox soil, (CaO 32%, MgO 18%) or not to reach 90% base saturation (ph CaCl 2 6.0), received 0; 3.5; 15.7; 70; 313 and 1400 mg kg -1 of Zn as ZnSO 4.7H 2 O, on a 2x6 factorial experiment in four randomized blocks, where rate 70 mg kg -1 was equivalent to the maximum annual loading limit (140 kg ha -1 ) and rate 1400 mg kg -1 was equivalent to the total cumulative loading limit (2800 kg ha -1 ) allowed by USEPA for Zn inputs from sewage sludge application. Water was supplied by cloth strings which connected the soil at the bottom of the pot to a reservoir underneath. All treatments received 245 mg kg -1 P as ordinary superphosphate, 290 mg kg -1 K as KCl, 100 mg kg -1 N as (NH 4 ) 2 SO 4, 200 mg kg -1 N as Mg (NO 3 ) 2.6H 2 O and 100 mg kg -1 N as Ca(NO 3 ) 2 and 1 ml of a micronutrient solution which added 0.75 mg of B (H 3 BO 3 ), 0.5 mg of Mn (MnSO 4.H 2 O), 0.25 mg of Cu (CuSO 4.5H 2 O) 0.2 mg of Mo (Na 2 MO 4.2H 2 O) to the soil. Twenty days after seedling transplant (one plant per pot), lettuce plant tops (Marisa cultivar) were harvested, placed in paper bags and dried at 65 o C in a ventilated oven for three days. Dry material was weighed and ground to pass a 0.25 mm sieve. Zn tissue concentration was determined by weighing 0.5 g of plant material and adding 1 ml HNO 3 and 2 ml H 2 O 2. The mixture was heated in a microwave oven and extracts were analysed for Zn concentrations on an Jobin Yvon simultaneous ICP-AES, Model JY50P. Soil samples of each pot were also taken for Zn availability indices using Mehlich 3, HCl and DTPA methods. Analysis of variance was used to determine significant differences due to treatments, whereas means within treatments with or without lime were compared using the Duncan`s test at the 0.05 probability level. Linear and curvelinear regressions were conducted to evaluate significant differences for the rates applied. RESULTS AND DISCUSSION Lettuce seedlings did not present any growth at the rate of 1400 mg kg -1 Zn and died 10 days after transplanting. At the rate of 313 mg kg -1 Zn in the un soil, leaves showed acute symptoms of Zn toxicity (cupped leaves and no head formation), with oldest leaves yellowing, wilting and dying. Lettuce dry matter yield was not affected until rate 313 mg kg -1 Zn in the soil and significantly decreased with the Zn amounts added to the un soil (Figure 1), indicating that an increase in soil ph is necessary in order to maintain crop yield when an excess of Zn is added to this soil. 2
Zinc concentration in lettuce leaves increased significantly with the rates applied to the soil (Figure 2). Liming the soil to 90% base saturation, which increased soil ph (CaCl 2 ) to 6.0, significantly decreased Zn availability to plants when compared to the un treatment. Lime efficiency in decreasing Zn concentration ranged from 40% for control treatment up to 80% for rate 313 mg kg -1. Several authors also have found a decrease in Zn availability as soil ph increased, which was probably due to Zn precipitation as ZnOH. According to the Brazilian law, maximum amount of Zn allowed in fresh foods for direct consumption is 50 mg kg -1. Hence, considering that lettuce was made of 95% water, it can be observed from linear regressions that Zn additions to the soil, as ZnSO 4 7H 2 O, could not be greater than 77 mg kg -1 and 401 mg kg -1 for the un and soil, respectively. Since maximum annual loading limit allowed by USEPA is 70 mg kg -1 (140 kg ha -1 ), results for Zn absorption showed that the amount annually allowed by USEPA is valid for Zn additions to the soil under investigation, with or without lime, at least for the first Zn application. However, liming the soil to ph (CaCl 2 ) 6.0 is recommended in this rate in order to avoid a decrease in yield. As expected, Zn extracted by DTPA and HCl methods increased with the rates of Zn applied to the and un soil (Figure 3a and 3b). Also, both extractors were able to show the decrease in soil Zn availability as result of lime addition. DTPA method extracted higher amounts of Zn added to the soil than HCl method. Therefore, for each mg kg -1 Zn added to the un soil, DTPA method retrieved 64% against 40% from HCl, while the amounts recovered in the soil were 43% and 30% for DTPA and HCl methods, respectively. Significantly linear correlation coefficients (r 2 =0.78** and 0.71**) were obtained from relating the amounts of Zn extracted by DTPA and HCl methods and the concentration in plants, indicating that both methods could be used for predicting Zn availability to plants even when high amounts of this element was added to soil. CONCLUSIONS One single application of 1400 mg kg -1 Zn as ZnSO 4 7H 2 O, which simulated total cumulative loading limit allowed by USEPA (2800 kg ha -1 ), was lethal to lettuce for both and un soil. Liming soil to ph 6.0 was efficient in preventing Zn toxicity and a decrease in dry matter yield at the rate of 313 mg kg -1. Zn concentration in lettuce leaves was not higher than permitted by Brazilian law for rate 70 mg kg -1, which simulated maximum annual loading limit allowed by USEPA, indicating that limit proposed by American Environmental Agency was valid for Zn additions to the soil under investigation, with or without lime, at least for the first Zn application. Zn extracted by DTPA and HCl methods correlated significantly with Zn concentration in lettuce leaves for both and un soil, suggesting that these two methods could be used for predicting Zn availability to plants even when high amounts of this element is added to soil. Key words: pollutants, legislation, soil analysis, zinc, lettuce Mots clés : polluants, législation, analyses de sol, zinc, oxisol, laitue 3
12.0 Dry matter yield, g pot -1 10.0 8.0 6.0 4.0 2.0 0.0 un y = 8.756-0.0024x Zn-soi l R 2 = 0.74 NS y = 9.033-0.0234 x Zn-soi l R 2 = 0.95** Figure 1: Lettuce dry matter yield grown in oxisol or not to reach 90% base saturation and treated with increasing rates of zinc. 10.0 Zn-plant, mg kg -1 8.0 6.0 4.0 2.0 lny= 4.55 + 0.039x - 0,00009x 2 R 2 = 0.988** lny= 4.05 + 0.021x - 0,00004x 2 R 2 = 0.996** un Figure 2: Zn leaf concentration in lettuce grown in oxisol or not to reach 90% base saturation and treated with increasing rates of zinc 4
250 (a) 140 (b) 200 y = 2.26 + 0.46x + 0.0006x 2 120 y = 0.75 + 0.40x R 2 = 0.999** Zn - soil, mg dm -3 150 100 R 2 = 1.00** y = 1.0 + 0.43x R 2 = 0.995** Zn soil, mg dm -3 100 80 60 y = 2.16 + 0.31x R 2 = 0.999** 40 50 un 20 un 0 0 Figure 3: Relationship between zinc rates in soil amended or not with lime and zinc extracted by DTPA (a) and HCl (b) solutions 5