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The poultry industry is one of the largest and fastest growing agro-based industries in the world. This can be attributed to an increasing demand for poultry meat and egg products. However, a major problem facing the poultry industry is the large-scale accumulation of wastes including manure and litter which may pose disposal and pollution problems unless environmentally and economically sustainable management technologies are evolved. Most of the litter produced by the poultry industry is currently applied to agricultural land as a source of nutrients and soil amendment. However environmental pollution, resulting from nutrient and contaminant leaching can occur when poultry litter is applied under soil and climatic conditions that do not favour agronomic utilisation of the manure-borne nutrients. This review examines the composition of poultry litter in relation to nutrient content and environmental contaminants, its value as a nutrient source, soil amendment, animal feed and fuel source, and cost-effective innovative technologies for improving its value. Poultry litter provides a major source of nitrogen, phosphorus and trace elements for crop production and is effective in improving physical and biological fertility, indicating that land application remains as the main option for the utilisation of this valuable resource. The alternative use of poultry litter; as an animal feed and fuel source, is limited by contaminants, and high moisture content, respectively. The review proposes best management practices to mitigate environmental consequences associated with air and water quality parameters that are impacted by land application in order to maintain the continued productivity, profitability, and sustainability of the poultry industry.

The effect of pH-increases due to Ca(OH)2 and KOH addition on the adsorption of cadmium (Cd) was examined in two soils which varied in their variable-charge components. The effect of Ca(OH)2 on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg-1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. The addition of Ca(OH)2 and KOH increased the soil pH, thereby increasing the adsorption of Cd, the effect being more pronounced in the soil dominated by variable charge components. There was a greater increase in Cd2+ adsorption in the KOH-treated than the Ca(OH)2-treated soil, which is attributed to the greater competition of Ca2+ for adsorption. Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Although addition of Ca(OH)2 effectively reduced Cd phytotoxicity, Cd uptake increased at the highest level, probably due to decreased Cd2+ adsorption resulting from increased Ca2+ competition. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of Ca(OH)2 decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fractions in soil. Since there was no direct evidence for CdCO3 or Cd(OH)2 precipitation in the variable charge soil used for the plant growth experiment, alleviation of phytotoxicity can be attributed primarily to immobilization of Cd by enhanced pH-induced increases in negative charge.

We examined the effect of biosolid compost on the adsorption and complexation of cadmium (Cd) in two soils (Egmont and Manawatu) which varied in their organic matter content. The effect of biosolid compost on the uptake of Cd from the Manawatu soil, treated with various levels of Cd (0–10 mg Cd kg-1 soil), was also examined using mustard (Brassica juncea L.) plants. The transformation of Cd in soil was evaluated by a chemical fractionation scheme. Addition of biosolid compost increased negative charge in soil. The effect of biosolid compost on Cd adsorption varied between the soils, with a large portion of the sorbed Cd remaining in solution as an organic complex. Increasing addition of Cd increased Cd concentration in plants, resulting in decreased plant growth at high levels of Cd (i.e., phytotoxicity). Addition of biosolid compost was effective in reducing the phytotoxicity of Cd as indicated by the decrease in the concentration of NH4OAC extractable-Cd and soil solution-Cd. The solid-phase fractionation study indicated that the addition of biosolid compost decreased the concentration of the soluble and exchangeable Cd fraction but increased the concentration of organic-bound Cd fraction in soil. Alleviation of Cd phytotoxicity by biosolid compost can be attributed primarily to complexation of Cd by the organic matter in the biosolid compost.

The widespread use of chromium (Cr) has a deleterious impact on the environment. A number of pathways, both biotic and abiotic in character, determine the fate and speciation of Cr in soils. Chromium exists in two predominant species in the environment: trivalent [(Cr(III)] and hexavalent [Cr(VI)]. Of these two forms, Cr(III) is nontoxic and is strongly bound to soil particles, whereas Cr(VI) is more toxic and soluble and readily leaches into groundwater. The toxicity of Cr(VI) can be mitigated by reducing it to Cr(III) species. The objective of this study was to examine the effect of organic carbon sources on the reduction, microbial respiration, and phytoavailability of Cr(VI) in soils. Organic carbon sources, such as black carbon (BC) and biochar, were tested for their potential in reducing Cr(VI) in acidic and alkaline contaminated soils. An alkaline soil was selected to monitor the phytotoxicity of Cr(VI) in sunflower plant. Our results showed that using BC resulted in greater reduction of Cr(VI) in soils compared with biochar. This is attributed to the differences in dissolved organic carbon and functional groups that provide electrons for the reduction of Cr(VI). When increasing levels of Cr were added to soils, both microbial respiration and plant growth decreased. The application of BC was more effective than biochar in increasing the microbial population and in mitigating the phytotoxicity of Cr(VI). The net benefit of BC emerged as an increase in plant biomass and a decrease in Cr concentration in plant tissue. Consequently, it was concluded that BC is a potential reducing amendment in mitigating Cr(VI) toxicity in soil and plants.

The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil Ñ for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive ³²P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.

\"Depending on the degree of intensification of livestock feeding, animal manures have turned from a precious resource into a waste product.\" ( Eck and Stewart, 1995 ) Intensive confined livestock and poultry production systems generate large quantities of manure by-products, which have the potential for being recycled on arable land. Protecting the quality of the environment is a major consideration when developing management practices to effectively use manure by-products as a nutrient resource and soil conditioner in agricultural production system. To date, most of the environmental problems associated with land application of manure by-products have centered on the contamination of groundwater and/or surface water with two major nutrients, nitrogen (N) and phosphorus (P). With increasing use of trace elements (metal is substituted for trace elements for brevity throughout the text) as nutritional supplement in the form of feed additive in intensive animal production industries, manure application has emerged as an important source of certain metals (e.g., As, Cu, and Zn) input in soils. Unlike application of sewage sludge, where application rate is limited based on allowable metal loadings, regulations governing livestock and poultry manure by-products are generally based on total N and/or P loading. Both sewage sludge and manure by-products are applied on land to primarily benefit from their N and/or P content but without regard to metals in the latter. The danger lies in accumulation of manure-borne metals since they virtually don't degrade with the potential of eventually becoming phytotoxic. In order to reduce the risk of offsite contamination, it is prudent to propose that land application guidelines for manure by-products be developed that consider their total composition rather than just only specific component (i.e., N and/or P). The present review aims to examine the impact of increased usage of certain metals, especially As, Cu, and Zn, in livestock and poultry production on the quality of manure by-products in relation to their distribution in soils and their subsequent bioavailability to plants. The review first discusses the various sources, concentration, and distribution of these metals in manure by-products. The beneficial effects of manure addition to overcome the deficiency of these metals in soils and the detrimental effects of manure-borne metals on plant growth and microbial functions are also examined. The practical implications of manure-borne metals on environmental contamination are discussed in relation to management guidelines for the safe and beneficial use of manure by-products in agricultural soils.

In this study, seven organic amendments (biosolid compost, farm yard manure, fish manure, horse manure, spent mushroom, pig manure, and poultry manure) were investigated for their effects on the reduction of hexavalent chromium [chromate, Cr(VI)] in a mineral soil (Manawatu sandy soil) low in organic matter content. Addition of organic amendments enhanced the rate of reduction of Cr(VI) to Cr(III) in the soil. At the same level of total organic carbon addition, there was a significant difference in the extent of Cr(VI) reduction among the soils treated with organic amendments. There was, however, a significant positive linear relationship between the extent of Cr(VI) reduction and the amount of dissolved organic carbon in the soil. The effect of biosolid compost on the uptake of Cr(VI) from the soil, treated with various levels of Cr(VI) (0–1200 mg Cr kg−1 soil), was examined with mustard (Brassica juncea L.) plants. Increasing addition of Cr(VI) increased Cr concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of the biosolid compost was effective in reducing the phytotoxicity of Cr(VI). The redistribution of Cr(VI) in various soil components was evaluated by a sequential fractionation scheme. In the unamended soil, the concentration of Cr was higher in the organic-bound, oxide-bound, and residual fractions than in the soluble and exchangeable fractions. Addition of organic amendments also decreased the concentration of the soluble and exchangeable fractions but especially increased the organic-bound fraction in soil.

Ion retention in variable-charge soils can be enhanced by the presence of certain ions with opposite charge, thereby influencing the movement of these ions through the soil profile. Studies examining these interactions are still incipient, however, especially regarding its modeling. We present results from batch and miscible displacement experiments describing Ca2+ and SO₄²⁻movement in a variable-charge soil from New Zealand. Evidence was found for ion-pair adsorption (IPA) of both Ca2+and SO₄²⁻ The results were modeled using the convection-dispersion equation (CDE), coupled with two different mathematical approaches proposed to account for IPA. The first approach related IPA to the single soil adsorption capacity, which is governed by particle-surface phenomena. For the second approach, IPA was related solely to the soil solution concentration. Both these approaches described the adsorption data from the batch experiment reasonably well, as well as the breakthrough curves from the miscible displacement experiments. The first approach showed better overall agreement. Significant differences were found, however, when the adsorption parameters were identified by fitting models to data from either batch or miscible displacement experiments. Although more studies are needed to better understand IPA, our results showed that the extent of IPA can be large and it should not be ignored when predicting SO₄²⁻ and Ca2+movement in variable-charge soils.

In areas that remain unaffected by industrial pollution soil acidification is mainly caused by the release of protons (H⁺) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. In this review the processes of H⁺ ions release during N cycling and its effect on soil acidification are examined. The major processes leading to acidification during N cycling in soils are: (i) the imbalance of cation over anion uptake in the rhizosphere of plants either actively fixing N₂ gas or taking up $NH_4^ + $ ions as the major source of N, (ii) the net nitrification of N derived from fixation or from $NH_4^ + $ and R-NH₂ based fertilizers, and (iii) the removal of plant and animal products containing N derived from the process described in (i) and losses of NO₃-N by leaching when the Í input form is N₂, NH4 or R-$NH_4^ + $. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a \"localized\" effect and can be balanced by the release of hydroxyl (OH⁻) ions during subsequent plant decomposition. Nitrification of fixed N₂ or $NH_4^ + $ and R-NH₂ based fertilizers, and loss of N from the soil either by removal of products or by leaching of NO₃-N with a companion basic cation, lead to 'permanent' acidification.

Measuring pH of soil samples (at four to five depths down to 300 mm) collected three times from a long-term (16 years) field trial involving annual application of six forms of phosphate fertilizers at the rate of 30 kg P ha−1 yr−1 showed that soil acidity in all treatments, including the untreated control, increased with time. The rates of acidification (pH unit yr−1 during the first 10 years) in the topsoil (0–75 mm depth) were in the order, diammonium phosphate (0.038)>control, single superphosphate>Jordan partially acidulated phosphate rock (JPAPR)>North Carolina partially acidulated phosphate rock (NCPAPR), Jordan phosphate rock (JPR)>North Carolina phosphate rock (NCPR) (0.010). Of the 480 kg P ha−1 applied over the 16 year period, 71 and 57% of P from NCPR and JPR dissolved. The theoretical liming values derived from the dissolution of NCPR and JPR were 1698 and 1303 kg CaCO3 ha−1 respectively. Liming values of the two PRs calculated from the increase in soil pH over control treatment (ΔpH) down to 300 mm soil depth were 640 and 414 kg CaCO3 ha−1 for NCPR and JPR, respectively. The lower liming values estimated from the ΔpH method is probably due to proton transfer resulting from the secondary reactions of dissolved fertilizer phosphate with soil constituents, the unaccounted liming effect of the PRs below 300 mm soil depth and the lower soil pH buffering capacities measured from a short-term pH titration method used in the estimation of the liming values. The results of this long-term field study showed that continuous use of certain phosphate rocks (PRs) can significantly slow down the rate of acidification in pastoral soils.