As it is the final soil cover which supports vegetation establishment and ecosystem development, the quality of the soil material and the thickness of the soil cover are of fundamental importance in affecting rehabilitation success. However, as good soil is usually not available or expensive, the soil used is usually derived from ex situ substandard soil or subsoil that is nutrient deficient and poor structured. There are inevitable problems of soil compaction and waterlogging for clay soil, drought when coarse soil is used, as well as infertility, which can be amended by conventional measures such as plowing, organic matter amendment, nurse species planting, and fertilizer application.
Many old landfills have experienced revegetation failure to various extents as a result of leachate seepage, landfill gas evolution, poor soil management, and minimal aftercare. Landfill gas is the major cause, among other constraints such as low fertility, high soil temperature, drought, and toxicity from leachate contamination. Unless it is vented to the atmosphere or extracted for energy production, it will displace oxygen and suffocate plant roots, which usually results in the death of vegetation, and gas production can last for 75 years after the deposition of wastes. Even for an engineered site, gas problems may still exist if an impermeable layer is not formed for the entire site or the soil cap is cracked by uneven subsidence of the site. Landfill gas creates a reducing soil condition which severely impairs microbial processes such as decomposition and symbiotic nitrogen fixation; this together with elevated soil temperature of over 40 ° C is detrimental to plants, and plant growth is impeded under the adverse impact of these landfill-associated factors. Localized pollution hot spots reduce plant coverage and result in patchy greenness. A thin soil cover will exacerbate the problem of gas and leachate contamination.
The revegetation success of closed landfills depends heavily upon the quality of the soil cover material, adaptation of the planted vegetation to the landfill environment, and aftercare management strategy. In containment and entombment landfills, contamination by landfill gas and leachate is usually greatly alleviated, though not necessarily eliminated. However, the final soil cover may remain stressful for plant growth, and there is also concern that the containment design may elevate nutrient and water stresses on these landfills. Thin soil cover, poor soil quality, and unfavorable landfill conditions will result in poor vegetation growth, especially in the initial phase of ecosystem development.
It is important for rehabilitated landfills to develop a functional soil-plant system, as shortage of nutrients, in particular nitrogen, is common in most imported soils for use as the final top layer on completed landfills. This can be achieved by the addition of chemical fertilizers at the onset of postclosure revegetation works. However, as repeated application is costly, revegetated sites are usually left to nature for the accumulation of nutrients needed for the establishment of self-perpetuating nutrient cycle. This has to be achieved to allow good vegetation growth, the establishment of a fully functional soil-plant system, and ecosystem development. Plant growth during the early phase of ecosystem rehabilitation is usually limited by the rate of nutrient turnover, and the use of poor soil material as the final cover will inevitably result in rehabilitated sites that are neither productive nor sustainable. There is a paucity of information on the nutrient fluxes and compartmentation in landfill cover soils, and there is only partial idea of nutrient mobilization and immobilization as a function of soil status, and stage of soil development and vegetation succession. Shortage of mineral nutrients could be either due to a lack of sufficient nutrient capital or a failure in mineralization processes. Therefore, litterfall, litter quality, mineralization rate, and the level of biological activity are important determinants of landfill soil quality. Slow decomposition rate implies that nutrients are trapped in organic matter and are not available to nutrient transformation.
Nutrients such as nitrogen and phosphorus accumulate in landfill soil as the ecosystem develops, and their levels have a positive correlation with vegetation establishment. In abandoned landfills, without much aftercare, litter from invaded vegetation is the primary source of organic matter and nutrients in the absence of biological fixation. However, there is a lack of information regarding the nitrogen capital of landfill soils. Nitrogen is supplied from fertilizer application, decomposition, biological fixation, and rainfall. It is susceptible to immobilization on the youngest sites, and the primary production of newly established grassy vegetation cannot rely on decomposition, even though the rate is comparatively high for a sustainable nitrogen turnover. Total amount of nitrogen mineralized on more mature woodlands is high, but it is unclear as to how much nitrogen accumulated in the soil is sufficient to create a self-perpetuating ecosystem on closed landfills.
Within the soil, the microflora, fauna, and the abiotic components are all important and interrelated compartments of the landfill ecosystem. Former landfills support diverse soil and litter fauna which have an active role in the detritus food web. They comprise of high diversity and populations of saprohagous arthropods and macroin-vertebrates such as isopods, millipedes, and centipedes that are tolerant of the landfill environment. Springtails and mites are abundant in landfills with gas problems. Earthworms are also adaptive to landfill conditions and have been inoculated to landfills for soil amelioration, but natural colonization and soil improvement appear to be slow, and it takes 3-14 years for earthworm species to invade landfills. Low accumulation of organic matter and patchy coverage of vegetation can hinder the recruitment as well as the mobility of earthworms in landfills.
The best soil cover on landfills should support diverse communities of soil microflora and invertebrates which play crucial roles in organic matter decomposition and nutrient cycling. Active populations of microorganisms and invertebrates will improve the physicochemical status of the soil, which in turn encourage the colonization of plants to support more diverse animal species, thus forming a community of a greater structural complexity and functional stability. This is important not only for the success of revegetation but also the successional development afterwards. In the long run, this will facilitate autogenic change which is the result of the recruitment of late-successional species and the development of ecosystem processes on these man-made habitats.
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