Effects Of Livestock Manure Application On Soil Heavy Metal Accumulation And Phytoavailability

In view of the heavy metal residues in livestock and poultry manure in intensive farms and the heavy metal pollution in agricultural soils after application, the heavy metal contents of 172 livestock and poultry excrement samples were collected from 104 intensive livestock farms in part of cities of China. Long term experiment of manure application affecting the content and availability of heavy metals in soil and pot experiment of heavy metal phytoavailability equivalent in livestock and poultry manure were conducted. The objectives were to investigate the effects of the types, application amount, and application years of livestock and poultry on the heavy metals accumulation, speciation, and phytoavailability in soil. Observations will provide scientific basis for the proper evaluation of environmental risk and the formulation of agricultural using standard of livestock. The main results were observed as follows:1. The contents of Cu, Zn, Cd, As, Ni, Cr, Pb, and Hg in intensive livestock and poultry mannre varied greatly from trace amounts to percentages, with averages of 377.51, 1610.40, 0.72, 7.79, 9.77, 22.51, 6.31, and 0.06 mg·kg-1, respectively. The contents of Cu, Zn, Cd, and As in pig manure were significantly higher than poultry manure and cow dung. Compared with the standards of guidelines for the safe use of animal manure in vegetable field, Zn, Cu, and As contents in 74.6%, 78.7% and 9.8% of pig manure samples exceeded the standards. The similar result was also observed in poultry manure. The contents of Cd, Ni, Cr, Pb, and Hg were all lower than the limits. In addition, the water soluble Cu and As contents accounted for 30.47% and 12.19% of the total, respectively, while for EDTA-extractable Zn and Cu which were 35.96% and 48.93%, respectively. The high mobility of Cu, Zn, and As in pig manure might enhance the risk of its release into environment.2. The results from the field experiment of 5 years indicated that different kinds of livestock and poultry manure application differentially affected heavy metals accumulation in soils. Pig manure application significantly increased the contents of Cu, Zn, and Cd by 108.16%, 74.82%, and 41.17% in soil compared with CK, respectively. Chicken manure application significantly increased the Zn content by 18.04% in the soil. While no significant effect was observed after cow dung applicaiton. The effect of manure application on soil heavy metal accumulation was in the order of pig manure> poultry manure > cow dung. These rusults indicated that the environmental risk of pig manure appliation was greater than others.3. Long-term feitilizing experiment of over 14 years was used to investigate the effects of different manure application rate and fertilization years on heavy metal contents in soils. The results showed that the contents of Cu, Zn, and Cd were significantly and positively correlated with the amounts of pig manure application, with the correlation coefficient(R2) exceeding 0.9. During the passed 14 years, the application of pig manure per ton averagely contributed to the accumulation of 1.59 mg Cu, 2.15 mg Zn, and 0.014 mg Cd in soils per kilogram, Therefor, manure application is one of the important factors influencing the accumulation of heavy metals in soils. The contents of Cu or Zn were significantly and positively correlated with the fertilization years of pig manure. The annual accumulation rates of Cu in soils after pig manure application in different amounts(10, 30, and 50 t·hm-2) were 1.30, 2.47, and 4.96 mg·kg-1·a-1, respectively. While the annual accumulation rates of Zn in soils reached 1.30, 2.47, and 4.96 mg·kg-1·a-1, respectively. When the fertilization amount in soil reached 50 t·hm-2, the Cu content in soil exceeded the limit in the National Level II(6.5<p H<7.5, Cu≤100 mg·kg-1) of Soil Environmental Quality Standard after 14-year application.4. The types, application amount, and application years had significant effect on the content of available heavy metals in soil. Application of pig manure and poultry manure significantly increased the available contents(EDTA extractable) and the percentages of Cu and Zn in soil. While no influence was found after cow dung application. The effects of livestock and poultry manure application on soil heavy metal availability was in the order of pig manure> poultry manure > cow dung. The available contents of heavy metals in soil were significantly and positively correlated with the application amounts of livestock. The available contents of heavy metals increased with the increase of application amount and years. When the fertilization amount in soil was 50 t·hm-2, the percentage of the contents of the EDTA extractable of Cu and Zn exceeded 50% of the total, while the percentage for Cd exceeding 70%, after 14-year application of pig manure. Compared with the CK, the contents of available Cu, Zn, and Cd increased by 9.5, 33.5, and 4.4 times, respectively.5. The result of the pot experiment showed that the addition of pig manure not only promoted the accumulation of Cu and Zn in soil, but also increased plant absorption of Cu and Zn. Cu and Zn uptake by plant increased with the application amount. Compared pig manure with the equivalent water-soluble heavy metallic salt, the availability of Cu or Zn in pig manure was different with that in heavy metallic salt. Zn availability in pig manure was lower than that in heavy metallic salt. When the application amount of pig manure reached 5%~20%, the coefficients of Zn phytoavailability ranged 20.76%~67.9%, The phytoavailability coefficients of Zn in pig manure applied soils was in the order of irrigated desert soil>aquic brown soil>red soil.When the application amount of pig manure was 5%, Cu phytoavailability(48.95%~89.71%) was lower than that in water-soluble copper salt. When the application amount of pig manure was above 10%, Cu phytoavailability(77%~220%) in most of pig manure was higher than that in water-soluble copper salt. The phytoavailability coefficients of Cu in pig manure applied soils was in the order of irrigated desert soil> red soil> aquic brown soil.