The effect of landfill leachate irrigation on different soil characteristics and plant nutrition: a review

Abstract



Introduction

The decomposition of urban solid wastes deposited in landfills generates leachate a liquid effluent rich in ammoniacal-N, Na, K, and an organic fraction. Biological treatments are efficient in removing the more easily degradable organic fractions from leachates generated in young landfills. However, as the landfill ages, the remaining organic C becomes more recalcitrant, mainly as humic acids. Thus, the treatment of leachates from aged landfills must combine physical–chemical and biological methods or more sophisticated processes like membrane technology, but at higher costs. Nevertheless, even after treatment, the leachate may not fit the parameters required by environmental agencies for release in water streams. On the other hand, in arid and semi -arid areas, water is a restrictive factor in the expansion of agriculture and increased production. In addition, in these areas, we are facing a shortage of organic matter in the soil. In this context, soil applications have become a promising way to recycle landfill leachates. Due to soil’s physical, chemical, and biological characteristics, it can retain and degrade potentially pollutant residues, whereas the organic and mineral fractions can improve their fertility and act as a source of nutrients for crops. However, the effects on soil properties and growth and yield of the plant depend on the characteristic of the residue. The dose of residue to be applied in agricultural soil is generally based on the most limiting factor, like total N, salinizing ions (e.g., Na, K, and Cl), or heavy metal contents. The pH values in leachates generated from older landfill cells are generally alkaline; these pH levels promote the precipitation of heavy metals and reduce the risk of soil contamination. Thus, a detailed characterization of the landfill leachate before its application to the soil is important to prevent potential problems to its functions and the environment. The present study is a review of previous studies on the effects of landfill leachate irrigation on the properties of soil and plant nutrition.



Materials and Methods

To carry out this study, keywords such as "Landfill leachate", "Composition of landfill leachate" and "Landfill leachate irrigation" were searched in the Web of Science, Google Scholar, ScienceDirect and SID databases. For these keywords, 205 articles were found from 1989 to 2023. After the screening, quality review and removal of repetitive and unrelated articles, 110 relevant articles were used. The main criterion for selecting articles was the effects of landfill leachate irrigation on the various properties of soil, and the nutrition of different plant species. The quality of the articles was evaluated through the SJR index, the citation, the Impact Factor and the SNIP index.



Results and Discussion

Xenobiotic and heavy metal components of leachate often induce soil contamination and modify its natural environment. Laboratory and field investigations reported that leachate permeations into the subsurface led to the production of denitrifiers and methanogens, which reduced soil’s hydraulic conductivity and caused clogging. Reduced soil hydraulic conductivity enhanced gas production, metal precipitation and formation of discontinuous microbial aggregates and continuous biofilms, which ultimately, reduce soil production capacity. Leachate percolation modifies soil physicochemical characteristics by reducing its microbial biomass due to bacterial toxicity, reduces phosphorous-fixing capacity increasing soil acidity and alters the pH from the increase of its ion content. These effects depend on the composition of disposing of waste. Unlike the suggestions that show leachate in soils as a pollution threat, other studies dispute these claims citing its role in enriching soils. In semi-arid and arid areas, leachate increased soil organic content, enhanced crusting improved soil structure, and regulated the pH of calcic soils to enhance their productive capacity. Macro and micro-elements of leachate such as Cl, Fe, Mn, N, P and Zn, improved soil fertility hence their usefulness as fertilizers. In addition, leachate carries many intermediates and products of waste degradation. There is a concern, however, that the benefits of leachate irrigation may be offset by the presence of inhibitory chemicals. Many researchs have identified some impacts such as leaf injury, yield reduction and poor survival rate.In contrast, there was research which demonstrated the beneficial effects of leachate irrigation on plants. Application with diluted or low-strength leachate may stimulate plant growth. The yield of Brassica chinensis and Brassica parachinensis was stimulated when being irrigated with diluted leachate. Moreover, irrigation with diluted, low-strength leachate enhanced the growth, survival and stomatal conductance of Acacia confusa, Leucaena leuocephala and Eucalyptus torelliana. Contradictory results are not only observed between experiments, but even between plant species within a study. These results show that, soil characteristics, species, leachate source, methods of application and their interactions all had significant influences on the outcome of leachate irrigation.



Conclusion

In general, due to the significant amounts of nitrogen and nutrients in landfill leachate, it can be used as a liquid fertilizer and alternative to irrigation water in arid areas. But before use, you should pay attention to the points for use in agricultural land to get the best performance with the least environmental damage. Before irrigation, the composition of landfill leachate should be determined to use correctly and with the least damage. Choosing appropriate plant species that have a higher tolerance to leachate is also one of the important issues in leachate irrigation. Considering the amount of heavy metals, salinity, ions such as sodium and chlorine, and the considerable amount of nitrogen forms in the leachate, it is recommended to refrain from repeated and multi-year use of raw leachate, especially in sensitive plants, unless it is used in a diluted form. Alternating irrigation is also a suitable solution, which will increase the yield while leaching away the salts and excess amounts of nitrogen. The volume of leachate irrigation should be calculated according to the specie and tolerance of the plant, the amount of nitrogen needed and the toxicity of the leachate.