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WASTE MANAGEMENT

Copper and Zinc Soil Accumulation and Plant Concentration in Irrigated Maize Fertilized with Liquid Swine Manure

^a Centre Universitat de Lleida (UdL)-IRTA, Av. Rovira Roure, 191, Lleida, 25198, Spain
^b Secció d'Avaluació de Recursos Agraris, Dep. d'Agricultura, Alimentació i Acció Rural, Generalitat de Catalunya, Av. Rovira Roure, 191, Lleida 25198, Spain

^* Corresponding author (pilar.berenguer{at}pvcf.udl.cat).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Fertilization of crops with liquid swine (Sus scrofa domesticus) manure (LSM) is a common practice throughout the world. In the Ebro Valley (northeast Spain) intensive swine production is very important and generates high quantities of LSM. Fertilizing maize (Zea mays L.) with LSM is a common waste disposal option. Nevertheless, continuous LSM application could have negative effects as heavy metal soil contamination could lead to plant toxicity. We assessed the effects of applying 29 and 51 m³ LSM ha^–1 yr^–1 to a field of maize during 6 yr. We measured the accumulation of total and extractable (EDTA) Cu and Zn in the soil and the concentration of these nutrients in maize plants and grain. During the 6 yr of the experiment a total of 6.6 to 11.9 kg Cu ha^–1 and 12.8 to 22.5 kg Zn ha^–1 (29 and 51 m³ LSM ha^–1 yr^–1, respectively) were applied to the soil. Total Cu and Zn soil concentrations increased by 32 and 11%, respectively, after 6 yr of LSM application. Extractable Cu and Zn soil concentrations increased more than 60% after 6 yr of consecutive LSM applications. It would take at least two to three centuries of regular LSM application to reach phytotoxic soil concentrations for Cu and Zn. Maize grain yields were about 13 to 14 Mg ha^–1 over the 6 yr period, which also seems to confirm the absence of phytotoxicity. Copper and Zn concentrations in whole maize plants and grain during the last 2 yr of the experiment were lower than threshold values for animal and human ingestion (30 mg Cu kg^–1 and 500–1300 mg Zn kg^–1).

Abbreviations: AIR, annual input rate • ASAR, annual soil accumulation rate • DM, dry matter • EU, European Union • LSM, liquid swine manure

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication September 25, 2007.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
MAIZE AND SWINE PRODUCTION are important economic activities in different areas of the world. The Ebro Valley produces more than 99,000 ha of irrigated maize (Ministerio de Agricultura, Pesca y Alimentación, 2006) and 7.8 million head of swine (Ministerio de Agricultura, Pesca y Alimentación, 2006). The large amount of swine excrement produced in this region is applied to agricultural soils with a double objective: to fertilize maize and recycle waste material. Liquid swine manure may improve soil fertility, adding organic matter and nutrients to soil. The main element applied with LSM is N. Other macronutrients, such as P, K, and Mg, are present in noticeable amounts, while micronutrients, including Cu and Zn, are present in relatively small amounts (Sánchez and González, 2005). However, excessive LSM application could have negative effects on the environment, causing leaching of N (Martinez and Peu, 2000; Berenguer et al., 2008) or soil contamination due to the accumulation of heavy metals (L'Herroux et al., 1997; Diez et al., 2001).

Copper and Zn are essential nutrients needed for plant growth and development, playing a significant role in several physiological processes (Chang and Page, 2000; Kabata-Pendias and Pendias, 2001). Even so, these metals may become phytotoxic and cause metabolic disorders at high soil concentrations (Kabata-Pendias and Pendias, 2001; Alva et al., 2000; Chang and Page, 2000). Copper and Zn are frequently added to livestock diets as supplements or growth factors and have a positive influence on livestock growth and reproduction (Li et al., 2005). Nevertheless, animals assimilate them poorly, so up to 90% of the Cu and Zn ingested are excreted in feces and urine (Coppenet et al., 1993; Monge, 1997; Aldrich et al., 2002; Nicholson et al., 2003). Consequently, Cu and Zn are incorporated into soil when crops are fertilized with manure from animals whose diets have been supplemented with these metals (L'Herroux et al., 1997; Chang and Page, 2000; Nicholson et al., 2003).

Several studies have shown that soil acts as a long-term sink for Cu and Zn when it is fertilized with manure and that this increases risk of soil pollution (L'Herroux et al., 1997; Chang and Page, 2000; Nicholson et al., 2003). For this reason, the European Union (EU) introduced a series of directives to set threshold values for heavy metals in agricultural soils and thereby reduce the risk of environmental problems. Thus, the maximum amounts of Cu and Zn that can be added to agricultural land in a given year are 12 and 30 kg ha^–1 yr^–1, respectively (European Union, 1986; Boletín Oficial del Estado, 1990). Under our conditions (basic calcareous soils), soil concentrations of more than 210 mg kg^–1 for Cu and 450 mg kg^–1 for Zn could cause soil contamination and phytotoxicity (Boletín Oficial del Estado, 1990). Moreover, when animals are fed on diets with concentrations of more than 500 mg Zn kg^–1 and 25 to 30 mg Cu kg^–1, they may suffer a series of health problems (Gardiner et al., 1995; National Research Council, 1980).

Availability of Cu and Zn to plants depends on soil pH. Readily soluble forms of Cu and Zn, which are the most phytotoxic ones, increase as soil pH decreases (Alva et al., 2000, Martínez et al., 2002). On our basic soils, LSM could be applied with a lower risk of phytotoxicity than in acid soils, although the evolution of soil pH should always be considered.

At present, one of the most important problems affecting intensive agriculture in different parts of Europe is water pollution due to N leaching. In livestock farming areas, N leaching is mainly caused by overfertilization with manure (Sisquella et al., 2004). For this reason, an EU nitrate directive (European Union, 1991) limited the quantity of N that could be applied to soil. Some areas of the Ebro Valley, as well as many others in Europe, are considered nitrate vulnerable areas and no more than 180 to 210 kg N ha^–1 should derive from organic materials additions (Diari Oficial de la Generalitat de Catalunya, 1998, 2004). In the case of LSM, this amount is equivalent to approximately 30 to 60 m³ LSM ha^–1 yr^–1. Due to the great intensification of swine production, these areas are growing and becoming more important in the Ebro Valley. Liquid swine manure rates permitted by the EU nitrate directive (European Union, 1991) prevail over rates permitted by the EU directive related to Cu and Zn applications (European Union, 1986) because they are more restrictive.

Based on LSM applications rates permitted by EU nitrate directive (European Union, 1991), we conducted a 6-yr field experiment with irrigated maize. The main objective was to evaluate potential risk of soil contamination and Cu and Zn phytotoxicity as a result of LSM fertilization.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Experimental Design
The study was conducted over a 6-yr period (2002–2007), at the Gimenells Research Station (northeast Spain; 41°65' N, 0°39'E) on a Petrocalcic Calcixerept soil (Soil Survey Staff, 1998). There were no problems with drainage or salinity, though the soil had a petrocalcic horizon at a depth of 80 to 100 cm that limited root penetration. Table 1 summarizes the main soil characteristics at the beginning of the experiment.

Measurements
Liquid swine manure nutrient composition was determined throughout the 6 yr of the experiment. After LSM samples were collected from each LSM tank, an ash extraction was performed with 1 M HCl and diluted with distilled water. Nutrient composition was determined by Atomic Absorption Spectrophotometry (PerkinElmer 2100, Norwalk, CT).

After 2006 and 2007 maize harvest, we analyzed total and ethylenediaminetetraacetic acid (EDTA) extractable Cu and Zn in soil. Soil samples were taken from depths of 0 to 30 cm because soil retains most of Cu and Zn in its 0 to 20 cm layer when they are applied with LSM (Martinez and Peu, 2000). Three soil cores in 2006 and four soil cores in 2007 were mixed to obtain a final soil sample. Total (extrated with aqua regia; Ministry of Agriculture, Fisheries and Food, 1986), and Na₂–EDTA extractable (Ministry of Agriculture, Fisheries and Food, 1986) Cu and Zn were determined by Atomic Absorption Spectrophotometry (PerkinElmer 2100, Norwalk, CT). Soil pH (water, 1:2.5; Porta et al., 1986), and organic matter (Walkey-Black method; Alison, 1965) were also determined after 2006 and 2007 maize harvests.

From 2002 to 2007, maize grain yield (14% moisture) was measured at physiological maturity by harvesting two central rows (1.4 by 15 m) from each plot. The rest of the plants were removed from the field with a commercial combine. Afterward, maize stover was also removed with a stover-harvesting machine.

In 2006 and 2007, at physiological maturity, plant biomass was determined by harvesting and weighting 4 m of plants from a central row. At the same time, grain samples were taken from four ears. Three whole plants from the same row were chopped and dried (65°C during 48 h) to determine moisture content. Subsamples from the three chopped plants and from the grain were taken and analyzed to determine Cu and Zn concentrations. These elements were analyzed by inductively coupled argon plasma spectrophotometry (Polyscan 61E, Thermo Jarrell-Ash Corp., Franklin, MA), after the calcinated plant ash had been digested with nitric acid (Mills and Jones, 1996).

Calculations
Annual input rate (AIR) of Cu and Zn to soil were estimated by the ratio between the total amount of these metals applied during the experiment and the number of years of the experiment. Cu and Zn annual soil accumulation rate (ASAR) were estimated by the following expression:

ASAR (mg kg^–1) = [(final metal soil concentration in LSM fertilized plots–final metal concentration in LSM nonfertilized plots)/number of years of the experiment].

Statistical Analysis
Results were subjected to analysis of variance, applying the General Linear Model procedure of the Statistical Analysis System (SAS Institute, 1991). Means were separated by Fisher's least significant difference (LSD 0.05).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Concentrations of micro and macronutrients in the LSM used in the experiment (Table 2) were within normal ranges reported in the literature (Bernal et al., 1992; L'Herroux et al., 1997; Nicholson et al., 1999; Sánchez and González, 2005). Liquid swine manure pH varied from year to year, but was always 7.5 or higher.

Amounts of Cu and Zn applied and AIR for the experimental period are presented in Tables 3 and 4 . Nicholson et al. (2003) obtained similar values in a study conducted in the United Kingdom. They reported AIR values of 1.68 kg Cu ha^–1 yr^–1 and 2.32 kg Zn ha^–1 yr^–1, using LSM applications of 250 kg N ha^–1 (approximately 40 m³ LSM ha^–1 yr^–1). The AIR values obtained in our study were much lower (<16%) than those allowed by EU law (European Union, 1986), which established maximum soil AIR values of 12 kg Cu ha^–1 yr^–1 and 30 kg Zn ha^–1 yr^–1.

Total Copper and Zinc Soil Content
Table 6 presents soil Cu and Zn concentrations after 5 and 6 consecutive years of LSM applications. Concentrations of soil Cu and Zn in non-LSM fertilized plots were normal for our region (López-Arias and Grau-Corbí, 2004) and low-normal according to the values proposed by Mantovi et al. (2003): 15 to 40 mg Cu kg^–1 and 50 to 100 mg Zn kg^–1. This suggests that our soil initially had a low potential risk of Cu and Zn phytotoxicity. After 6 yr of LSM application, total Cu and Zn soil content increased by 32 and 11%, respectively. Other studies also found an accumulation of these metals in the soil when fertilizing with LSM (Mantovi et al., 2003; L'Herroux et al., 1997). Table 4 shows the calculated ASAR for total Cu and Zn, which were similar between LSM treatments. Unexpectedly, ASAR of total Cu and Zn were higher than AIR. This means that the amounts of total Cu and Zn recovered in the 0 to 30 cm soil layer were higher than the amounts applied with LSM. Other authors (Martinez and Peu, 2000) also reported percentages of recovery of Cu and Zn higher than 100%. This fact would emphasize the idea that the most part of the Cu and Zn applied with LSM remains in the soil.

Extractable Copper and Zinc Soil Content
Extractable (EDTA) Cu and Zn soil concentrations after 6 yr of LSM applications were low for both control and LSM treatments (Table 6). However, during this period, there was a significant enrichment of soil Cu and Zn (EDTA) in plots fertilized with LSM, as observed by other authors (Coppenet et al., 1993; Martinez and Peu, 2000). Low LSM rates increased Cu by 62% and Zn by 83%, while high LSM rates increased Cu by 131% and Zn by 106%. Extractable soil concentrations of Cu and Zn showed a similar trend in 2006 and 2007, but values of the last year were slightly lower than in 2006. The increase of soil organic matter caused by the application of LSM could have favored the immobilization of the labile Cu and Zn fractions (Canarutto et al., 1991; Ciavatta et al., 1993; Mantovi et al.,2003). This could have happened in 2007 and could explain the lowest Cu and Zn (EDTA) soil concentrations.

The concentrations of EDTA-soluble Cu and Zn in soil represent the potentially plant-available and easily leachable fractions (Martinez and Peu, 2000). The evolution of these fractions should therefore be taken into account because they represent the risk of phytotoxicity. No Cu and Zn leaching should have been expected in our field experiment if we consider Martinez and Peu (2000) studies in which EDTA Cu and Zn were hardly leached. They conducted an experiment in acid soils (heavy metals mobility is greater in acid soils than in basic soils as ours) and they used very high LSM rates (986 m³ LSM ha^–1 yr^–1 during 5 yr).

Annual soil accumulation rates of extractable Cu (0.27 and 0.57 mg Cu kg^–1 for 29 and 51 m³ ha^–1 yr^–1, respectively) and Zn (0.73 and 0.93 mg Zn kg^–1 for 29 and 51 m³ ha^–1 yr^–1, respectively) were lower than ASAR of total Cu and Zn (Table 4). Considering rates of 29 and 51 m³ LSM ha^–1 yr^–1, the initially low Cu and Zn soil concentrations and current EDTA soil Cu and Zn ASAR values, no phytotoxicity would be expected in the foreseeable future.

Concentrations of Copper and Zinc in Maize
Grain yields and biomasses in 2006 and 2007 increased as either LSM or inorganic N rates increased (Table 7 ). Thus, there was no evidence of yield diminishment due to Cu or Zn phytotoxicity. Grain yields and biomasses from 2002 to 2005 also showed no sign of phytotoxicity (Berenguer et al., 2008).

In our experiment, the levels of Cu and Zn in either maize plants or grain never reached phytotoxic concentrations: 21 mg Cu kg^–1 (Mocquot et al., 1996) and 100 mg kg^–1 for Zn (Mantovi et al., 2003). Moreover, according to the maximum acceptable concentrations proposed by the National Research Council, (1980), there would similarly be no risk of Cu or Zn toxicity for animals (e.g., 20–30 mg Cu kg^–1 and 500–700 mg Zn kg^–1 for sheep, Ovis aries).

Plant and grain uptakes of Cu and Zn in our experiment were relatively small in comparison with the amounts applied through LSM, and confirmed observations by Chang and Page (2000), Aldrich et al. (2002) and Nicholson et al. (2003). Plant uptake of Cu represented <6% of AIR in both 2006 and 2007. On the other hand, Zn plant uptake represented 9 and 16% of AIR in 2006 for 29 and 51 m³ ha^–1, respectively. In 2007, 13% (29 m³ ha^–1) and 22% (51 m³ ha^–1) of the applied Zn was uptaken by the plants. This suggested that almost all the Cu and most of the Zn applied through LSM remained in the soil after harvest.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This study confirmed previous findings related to the accumulation of Cu and Zn over time in soils fertilized with LSM. Copper and Zn applied to soil through LSM at rates of 29 and 51 m³ ha^–1 yr^–1 produced Cu and Zn annual input rates that were <16% of the maximum levels allowed by EU Directives.

After 6 yr of applying LSM to a maize monoculture, soil concentrations of Cu and Zn increased, especially with the 51 m³ LSM ha^–1 yr^–1 rate. Given the noticeable and rapid Cu and Zn soil enrichment, studying the evolution of soil Cu and Zn content would help to plan land use strategies aimed at reducing environmental hazards associated with these elements. Nevertheless, our results suggest that neither Cu nor Zn levels will reach the threshold soil concentrations established by EU directives in the next two to three centuries if maize producers follow present legislation (30–60 m³ LSM ha^–1 yr^–1).

Maize plants did not show any signs of Cu and Zn phytotoxicity after 6 consecutive years of applying LSM. Copper and Zn concentrations in maize grain and whole plant would not suppose a risk for human or animal health. We also observed a decrease in soil pH, which could increase the threat of Cu and Zn phytotoxicity in the future. On the other hand, soil organic matter increased when applying LSM and could immobilize Cu and Zn available for plants and reduce future phytotoxicity hazards.

	ACKNOWLEDGMENTS

 
We thank the staff of the field crops laboratory of the University of Lleida and of the UdL-IRTA center for their helpful assistance. This work was funded by the Comisión Interministerial de Ciencia y Tecnología (CICYT), AGL2001-2214-C06-05. We thank the University of Lleida for funding the doctorate studies of Pilar Berenguer.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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