Abstract
The presence of elevated concentrations of heavy metals (Cu and Zn) in pig slurry and, particularly, in the solid fraction obtained after solid-liquid separation is a limiting factor for their use in agricultural soils. These metals are further concentrated if compost is produced from solid pig slurry. This paper studies the influence of the composting of the separated solid phase of pig slurry on the concentrations and solubility of Cu and Zn, and their distribution in the different particle size fractions, as well as evaluating their potential toxic effects on seed germination and seedling growth. Two composts were prepared with the solid fraction of pig slurry in a piglets and sows farm, using two different bulking agents (cereal straw and cotton gin waste). The concentrations of Cu and Zn in the mature compost were higher due to organic matter degradation; however, their solubility decreased from 0.72 and 1.76 % in the solid fraction of pig slurry to 0.18 and 0.30 % of total Cu and Zn, respectively, in the compost prepared with cotton gin waste. Zinc was concentrated in the smallest particle size fraction, while the Cu concentration was highest in the largest particles, and associated to the organic matter/humic fraction. The elimination of the smallest particle size fraction would not reduce significantly the total heavy metal concentration of the composts. Nevertheless, the low solubility of both metals in the composts avoided any significant toxic effect on seed germination and also in the growth test when compost was present at low rates.
Highlights
The intensive and large-scale development of pig farming has led to the concentration of pig slurry production in small areas that offer limited land for its disposal and use in agriculture (FAO, 2009; Sáez et al, 2017)
The composts studied in the experiment were prepared with a mixture of the solid fraction (SF) of pig slurry obtained from a piglets and sows farm located in southeast Spain and two different bulking materials: cereal straw and cotton gin waste
This was probably because the pig slurry studied here was mainly produced by piglets, and this slurry has a higher concentration of Zn than that from fattening pigs (Moral et al, 2008), since Zn is provided as a feed additive (ZnO) to piglets in order to avoid digestion problems and to improve nutrients assimilation (Carlson et al, 1999; Woodworth et al, 1999)
Summary
The intensive and large-scale development of pig farming has led to the concentration of pig slurry production in small areas that offer limited land for its disposal and use in agriculture (FAO, 2009; Sáez et al, 2017). The application of pig slurry to agricultural soil is limited to 170 kg N ha−1 in areas vulnerable to nitrate pollution (CEE, 1991). These facts have led both farmers and researchers to seek alternative uses and management options for pig slurry, in order to avoid the need to transport, or the excessive accumulation of this material in the farm, with the consequent environmental concern that this may pose (Burton and Turner, 2003). The use of compost rich in heavy metals would affect soil, and surface and groundwater and the surrounding environment, and may end in the introduction of the contaminants into the food chain (Smith, 2009)
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