Since the inception of industrial revolution, metal refining plants using pyrometallurgical processes have generated the prodigious emissions of lead （Pb） and cadmium （Cd）. As the core target of such pollutants, a large number of soils are nowadays contaminated over widespread areas, posing a great threat to public health worldwide. Unlike organic pollutants, Pb and Cd do not undergo chemical or microbial breakdown and stay likely in site for longer duration after their release. Immobilization is an in-situ remediation technique that uses cost-effective soil amendments to reduce Pb and Cd availability in the contaminated soils. The Pb and Cd contamination in the soil environment is reviewed with focus on source enrichment, speciation and associated health risks, and immobilization options using various soil amendments. Commonly applied and emerging cost-effective soil amendments for Pb and Cd immobilization include phosphate compounds, liming, animal manure, biosolids, metal oxides, and biochar. These immobilizing agents could reduce the transfer of metal pollutants or residues to food web （plant uptake and leaching to subsurface water） and their long-term sustainability in heavy metal fixation needs further assessment.
Heavy metal contamination of agricultural soils poses risks and hazards to humans. The remediation of heavy metal-polluted soils has become a hot topic in environmental science and engineering. In this review, the application of clay minerals for the remediation of heavy metal-polluted agricultural soils is summarized, in terms of their remediation effects and mechanisms, influencing factors, and future focus. Typical clay minerals, natural sepiolite, palygorskite, and bentonite, have been widely utilized for the in-situ immobilization of heavy metals in soils, especially Cd-polluted paddy soils and wastewater-irrigated farmland soils. Clay minerals are able to increase soil pH, decrease the chemical-extractable fractions and bioavailability of heavy metals in soils, and reduce the heavy metal contents in edible parts of plants. The immobilization effects have been confirmed in field-scale demonstrations and pot trials. Clay minerals can improve the environmental quality of soils and alleviate the hazards of heavy metals to plants. As main factors affecting the immobilization effects, the pH and water condition of soils have drawn academic attention. The remediation mechanisms mainly include liming, precipitation, and sorption effects. However, the molecular mechanisms of microscopic immobilization are unclear. F~ture studies should focus on the long-term stability and improvement of clay minerals in order to obtain a better remediation effect.
Hyperaccumulators concentrate trace metals and heavy metals in their shoots when grown in metal-contaminated soils and these trace metal-loaded plants may be removed by harvesting the fields. Studies exploring the beneficial role of these hyperaccumulators to clean up the environment have led to the development of phytoextraction. The success of phytoextraction depends upon the high biomass of plant species and bioavailability of metals for plant uptake. The phytoavailability of metals is influenced by soil- associated factors, such as pH, redox potential, cation exchange capacity, soil type, and soil texture, and by plant-associated factors, such as root exudates and root rhizosphere processes （microorganisms）. Efficiency of phytoextraction can be improved by advanced agronomic practices including soil and crop management by application of genetic engineering to enhance the metal tolerance, shoot translocation, accumulation, and sequestration and by application of chelate treatments to enhance metal bioavailability. Application of microorganisms including bacteria and mycorrhiza may facilitate the phytoextraction application at commercially large scale.
Global warming, as a result of an increase in the mean temperature of the planet, might lead to catastrophic events for humanity. This temperature increase is mainly the result of an increase in the atmospheric greenhouse gases （GHG） concentration. Water vapor, carbon dioxide （CO2）, methane （CH4） and nitrous oxide （N20） are the most important GHG, and human activities, such as industry, livestock and agriculture, contribute to the production of these gases. Methane, at an atmospheric concentration of 1.7 gmol tool-1 currently, is responsible for 16% of the global warming due to its relatively high global warming potential. Soils play an important role in the CH4 cycle as methanotrophy （oxidation of CH4） and methanogenesis （production of CH4） take place in them. Understanding methanogenesis and methanotrophy is essential to establish new agriculture techniques and industrial processes that contribute to a better balance of GHG. The current knowledge of methanogenesis and methanotrophy in soils, anaerobic CH4 oxidation and methanotrophy in extreme environments is also discussed.
Environmental pollution caused by metals, radionuclides and organic pollutants affects quality of the biosphere： soil, water and air. Currently, great efforts have been made to reduce, remove or stabilize contaminants in polluted sites. There has been increasing interest in phytoremediationthe use of plants to reduce concentration of pollutants or to render them harmless. This paper provides a brief review of recent progress in the research and practical application of phytoremediation techniques. Improvements in phytoremediation due to utilization of organic amendments, namely, agro- and industrial wastes （such as sugar beet residue, composted sewage sludge or molasses）, biochar, humic substances, plant extracts and exudates are discussed, as well as their influences on soil structure and characteristics, plants growth and bioavailability of pollutants. Both plant-assisted phytoremediation and the use of natural materials in the absence of remediating plant are believed to be cost-effective and environmentally friendly approaches for soil cleanup. However, the characterization and quantification of a range of natural materials used in phytoremediation are essential in order to implement these approaches to practice.
The expected rise in temperature and decreased precipitation owing to climate change and unabated anthropogenic activities add complexity and uncertainty to agro-industry. The impact of soil nutrient imbalance, mismanaged use of chemicals, high temperature, flood or drought, soil salinity, and heavy metal pollutions, with regard to food security, is increasingly being explored worldwide. This review describes the role of soil-plant-microbe interactions along with organic manure in solving stressed agriculture problems. Beneficial microbes associated with plants are known to stimulate plant growth and enhance plant resistance to biotic （diseases） and abiotic （salinity, drought, pollutions, etc.） stresses. The plant growth-promoting rhizobemteria （PGPR） and mycorrhizae, a key component of soil microbiota, could play vital roles in the maintenance of plant fitness and soil health under stressed environments. The application of organic manure as a soil conditioner to stressed soils along with suitable microbial strains could further enhance the plant-microbe associations and increase the crop yield. A combination of plant, stress-tolerant microbe, and organic amendment represents the tripartite association to offer a favourable environment to the proliferation of beneficial rhizosphere microbes that in turn enhance the plant growth performance in disturbed agro-ecosystem. Agriculture land use patterns with the proper exploitation of plant-microbe associations, with compatible beneficial microbial agents, could be one of the most effective strategies in the management of the concerned agriculture lands owing to climate change resilience. However, the association of such microbes with plants for stressed agriculture management still needs to be explored in greater depth.
Abandonment of agricultural land results in on- and off-site consequences for the ecosystem. In this study, 105 rainfall simulations were carried out in agriculture lands of the Mediterranean belt in Spain (vineyards in Málaga, almond orchards in Murcia, and orange and olive orchards in Valencia) and in paired abandoned lands to assess the impact of land abandonment on soil and water losses. After abandonment, soil detachment decreased drastically in the olive and orange orchards, while vineyards did not show any difference and almond orchards registered higher erosion rates after the abandonment. Terraced orchards of oranges and olives recovered a dense vegetation cover after the abandonment, while the sloping terrain of almond orchards and vineyards enhanced the development of crusts and rills and a negligible vegetation cover resulted in high erosion rates. The contrasted responses to land abandonment in Mediterranean agricultural lands suggest that land abandonment should be programmed and managed with soil erosion control strategies for some years to avoid land degradation.
Achieving both high yield and high nitrogen use efficiency （NUE） simultaneously has become a major challenge with increased global demand for food, depletion of natural resources, and deterioration of environment. As the greatest consumers of N fertilizer in the world, Chinese farmers have overused N, and there has been poor synchrony between crop N demand and N supply because of limited understanding of the N uptake-yield relationship. To address this problem, this study evaluated the total and dynamic N requirement for different yield ranges of two major crops （maize and wheat）, and suggested improvements to N management strategies. Whole-plant N aboveground uptake requirement per grain yield （Nreq） initially deceased with grain yield improvement and then stagnated, and yet most farmers still believed that more fertilizer and higher grain yield were synonymous. When maize yield increased from 〈 7.5 to 〉 12.0 Mg ha-I, Nreq decreased from 19.8 to 17.0 kg Mg-1 grain. For wheat, it decreased from 27.1 kg Mg-1 grain for grain yield 〈 4.5 Mg ha-1 to 22.7 kg Mg-1 grain for yield 〉 9.0 Mg ha-1. Meanwhile, the percentage of dry matter and N accumulation in the middle-late growing season increased significantly with grain yield, which indicated that N fertilization should be concentrated in the middle-late stage to match crop demand while farmers often applied the majority of N fertilizer either before sowing or during early growth stages. We accordingly developed an integrated soil-crop system management strategy that simultaneously increases both grain yield and NUE.
Polycyclic aromatic hydrocarbons (PAHs) are mainly produced by combustion processes and consist of a number of toxic compounds. They are always emitted as a mixture and have become a major type of pollutants in urban areas. The degree of soil contamination by PAHs is of special concern in areas immediately in proximity to cities with heavy traffic, factories, older buildings, and infrastructure. The accumulation of soil PAHs is also affected by non–anthropogenic factors, such as climate, vegetation, and soil property. This paper reviews three typical source identification techniques, including diagnostic ratios, positive matrix factorization, and principle components analysis. The advantages or disadvantages of these techniques are analyzed. It is recommended that multiple identification techniques be used to determine the sources in order to minimize the weaknesses inherent in each method and thereby to strengthen the conclusions for PAH source identification.
Northern peatlands store a large amount of carbon and play a significant role in the global carbon cycle. Owing to the presence of waterlogged and anaerobic conditions, peatlands are typically a source of methane （CH4）, a very potent greenhouse gas. This paper reviews the key mechanisms of peatland CH4 production, consumption and transport and the major environmental and biotic controls on peatland CH4 emissions. The advantages and disadvantages of micrometeorological and chamber methods in measuring CH4 fluxes from northern peatlands are also discussed. The magnitude of CH4 flux varies considerably among peatland types （bogs and fens） and microtopographic locations （hummocks and hollows）. Some anthropogenic activities including forestry, peat harvesting and industrial emission of sulphur dioxide can cause a reduction in CH4 release from northern peatlands. Further research should be conducted to investigate the influence of plant growth forms on CH4 flux from northern peatlands, determine the water table threshold at which plant production in peatlands enhances CH4 release, and quantify peatland CH4 exchange at plant community level with a higher temporal resolution using automatic chambers.
Overuse of antibiotics has become a serious ecological problem worldwide. There is growing concern that antibiotics are losing their effectiveness due to an increased antibiotic resistance in bacteria. During the last twenty years, consumption of antibiotics has increased rapidly in China, which has been cited as one of the world＇s worst abusers of antibiotics. This review summarizes the current state of antibiotic contamination in China＇s three major rivers （the Yangtze River, Yellow River, and Pearl River） and illustrates the occurrence and fate of antibiotics in conventional municipal wastewater treatment plants （VC-VVTPs）. The analytical data indicate that traditional W-WTPs cannot completely remove these concerned pharmaceuticals, as seen in the large difference between the distribution coefficient （Kd） and the uneven removal efficiency of various types of antibiotics. Although constructed wetlands （CWs） offer a potential way to remove these antibiotics from water supplies, knowledge of their mechanisms is limited. There are four main factors affecting the performance of CWs used for the treatment of antibiotics in water supplies, the types and configurations of CWs, hydraulic load rates, substrates, and plants and microorganisms. Further researches focusing on these factors are needed to improve the removal efficiency of antibiotics in CWs.
Cadmium （Cd） contamination has posed an increasing challenge to environmental quality and food security. In recent years, phytoremediation has been particularly scrutinized because it is cost-effective and environmentally friendly, especially the use of metal-hyperaccumulating plants to extract or mine heavy metals from polluted soils. Under Cd stress, responses of hyperaccumulator and non-hyperaccumulator plants differ in morphological responses and physiological processes such as photosynthesis and respiration, uptake, transport, and assimilation of minerals and nitrogen, and water uptake and transport, which contribute to their ability to accumulate and detoxify Cd. This review aims to provide a brief overview of the recent progresses in the differential responses of hyperaccumulator and non-accumulator plants to Cd toxicity in terms of growth and physiological processes. Such information might be useful in developing phytoremediation technology for contaminated soils.
Soil capacity to support life and to produce economic goods and services is strongly linked to the maintenance of good soil physical quality (SPQ). In this study, the SPQ of citrus orchards was assessed under three different soil managements, namely no-tillage using herbicides, tillage under chemical farming, and no-tillage under organic farming. Commonly used indicators, such as soil bulk density, organic carbon content, and structural stability index, were considered in conjunction with capacitive indicators estimated by the Beerkan estimation of soil transfer parameter (BEST) method. The measurements taken at the L'Alcoleja Experimental Station in Spain yielded optimal values for soil bulk density and organic carbon content in 100% and 70% of cases for organic farming. The values of structural stability index indicated that the soil was stable in 90% of cases. Differences between the soil management practices were particularly clear in terms of plant-available water capacity and saturated hydraulic conductivity. Under organic farming, the soil had the greatest ability to store and provide water to plant roots, and to quickly drain excess water and facilitate root proliferation. Management practices adopted under organic farming (such as vegetation cover between the trees, chipping after pruning, and spreading the chips on the soil surface) improved the SPQ. Conversely, the conventional management strategies unequivocally led to soil degradation owing to the loss of organic matter, soil compaction, and reduced structural stability. The results in this study show that organic farming has a clear positive impact on the SPQ, suggesting that tillage and herbicide treatments should be avoided.
Characterization of soil hydraulic properties is important to environment management; however, it is well recognized that it is laborious, time-consuming and expensive to directly measure soil hydraulic properties. This paper reviews the development of pedotransfer functions （PTFs） used as an alternative tool to estimate soil hydraulic properties during the last two decades. Modern soil survey techniques like satellite imagery/remote sensing has been used in developing PTFs. Compared to mechanistic approaches, empirical relationships between physical properties and hydraulic properties have received wide preference for predicting soil hydraulic properties. Many PTFs based on different parametric functions can be found in the literature. A number of researchers have pursued a universal function that can describe water retention characteristics of all types of soils, but no single function can be termed generic though van Cenuchten （VG） function has been the most widely adopted. Most of the reported parametric PTFs focus on estimation of VG parameters to obtain water retention curve （WRC）. A number of physical, morphological and chemical properties have been used as predictor variables in PTFs. Conventionally, regression algorithms/techniques （statistical/neurM regression） have been used for calibrating PTFs. However, there are reports of utilizing data mining techniques, e.g., pattern recognition and genetic algorithm. It is inferred that it is critical to refine the data used for calibration to improve the accuracy and reliability of the PTFs. Many statistical indices, including root mean square error （RMSE）, index of agreement （d）, maximum absolute error （ME）, mean absolute error （MAE）, coefficient of determination （r2） and correlation coefficient （r）, have been used by different researchers to evaluate and validate PTFs. It is argued that being location specific, research interest in PTFs will continue till generic PTFs are developed and validated. In future studies, improved methods will be required to extract information from the existing database.
Animal manure consists of predominantly urine and feces, but also may contain bedding materials, dropped feed, scurf, and other farming wastes. Manure is typically applied to soils as fertilizer for agricultural production. The estimated amount of manure produced in 12 major livestock-producing countries is 9 × 10^9 Mg of manure annually. Manure is rich in plant nutrients. However, manure is also considered as an environmental pollutant when it is over-applied to cropland or following runoff into surface water. Manure can also influence global climate change via emissions of methane （CH4） and nitrous oxide （N2O）. Thus, increased and updated knowledge of applied and environmental chemistry of animal manure is needed to shed light on the research and development of animal manure utilization and minimization of its adverse environmental concerns. The advances in basic and applied studies of manure major components, organic matter, phosphorus, and nitrogen, primarily related to US livestock production are summarized in this review. Detailed focus was placed on three notable challenges for future manure research： 1） soil application of animal manure, 2） manure phytate phosphorus, and 3） manure nitrogen availability. This review may contribute to the global effort in sustainable and environmentally sound agriculture by stimulating new ideas and directions in animal manure research, and promoting application of knowledge and insight derived from manure research into improved manure management strategies.
Biochar（BC）, known as the new black gold, is a stable, novel carbonaceous by-product that is synthesized through pyrolysis of biological materials in the absence of O_2.Recently, an emerging interest in the application of BC as a robust soil amendment has given rise to a broad research area in science and technology.It is considered a promising remediation option for heavy metal（HM）-contaminated soils to reduce HM bioavailability to plants.Remediation efficacy of BC depends on the porosity, composition,pyrolysis temperature, feedstock, and residence time of pyrolysis.This review article aimed to present an overview of BC use in the immobilization of HMs, i.e., Cd, As, Pb, Zn, Ni, Cu, Mn, Cr, and Sb, in contaminated soils.The remaining uncertain factors, including the specific soil HM immobilization mechanisms, long-term beneficial effects, and potential environmental risks associated with BC application are analyzed.Future research must be conducted to ensure that the management of environmental pollution is in accord with ecological sustainability and adaptation of the black gold biotechnology on a commercial basis for immobilization of HMs in contaminated soils.
Trace element-contaminated soils （TECSs） are one of the consequences of the past industrial development worldwide. Excessive exposure to trace elements （TEs） represents a permanent threat to ecosystems and humans worldwide owing to the capacity of metal（loid）s to cross the cell membranes of living organisms and of human epithelia, and their interference with cell metabolism. Quantification of TE bioavailability in soils is complicated due to the polyphasic and reactive nature of soil constituents. To unravel critical factors controlling soil TE bioavailability and to quantify the ecological toxicity of TECSs, TEs are pivotal for evaluating excessive exposure or deficiencies and controlling the ecological risks. While current knowledge.on TE bioavailability and related cumulative consequences is growing, the lack of an integrated use of this concept still hinders＇its utilization for a more holistic view of ecosystem vulnerability and risks for human health. Bioavailability is not generally included in models for decision making in the appraisal of TECS remediation options. In this review we describe the methods for determining the TE bioavailability and technological developments, gaps in current knowledge, and research needed to better understand how TE bioavailability can be controlled by sustainable TECS management altering key chemical properties, which would allow policy decisions for environmental protection and risk management.
Biochar as a soil amendment is confronted with the challenge that it must benefit soil health as it can be by no means separated from soils once it is added. The available literature even though sparse and mostly based on short-term studies has been encouraging and the trend obtained so far has raised many hopes. Biochar has been reported to positively impact an array of soil processes ranging from benefiting soil biology, controlling soil-borne pathogens, enhancing nitrogen fixation, improving soil physical and chemical properties, decreasing nitrate （NO3 ） leaching and nitrous oxide （N2O） emission to remediation of contaminated soils. However, very little biochar is still utilized as soil amendment mainly because these benefits are yet to be quantified, and also the mechanisms by which the soil health is improved are poorly understood. Due to the infancy of research regarding this subject, there are still more questions than answers. The future research efforts must focus on carrying out long-term experiments and uncover the mechanisms underlying these processes so that key concerns surrounding the use of biochar are addressed before its large scale application is recommended.
Mineral nutrients are fundamentally metals and other inorganic compounds. The life cycle of these mineral nutrients begins in soil, their primary source. Soil provides minerals to plants and through the plants the minerals go to animals and humans; animal products are also the source of mineral nutrients for humans. Plant foods contain almost all of the mineral nutrients established as essential for human nutrition. They provide much of our skeletal structure, e.g., bones and teeth. They are critical to countless body processes by serving as essential co-factors for a number of enzymes. Humans can not utilize most foods without critical minerals and enzymes responsible for digestion and absorption. Though mineral nutrients are essential nutrients, the body requires them in small, precise amounts. We require them in the form found in crops and they can be classified into three different categories： major, secondary, and micro or trace minerals. This classification is based upon their requirement rather than on their relative importance. Major minerals such as potassium （K） and phosphorus （P） are required in amounts of up to 10 g d-1. The daily requirement of secondary and micro minerals ranges from 400 to 1 500 mg d-1 and 45 ~tg d-1 to 11 mg d-1, respectively. To protect humans from mineral nutrient deficiencies, the key is to consume a variety of foods in modest quantities, such as different whole grains, low fat dairy, and different meats, vegetables and fruits. For insurance purposes, a supplement containing various mineral nutrients can be taken daily.
Thermally modified organic materials commonly known as biochar have gained popularity of being used as a soil amendment. Little information, however, is available on the role of biochar in alleviating the negative impacts of saline water on soil productivity and plant growth. This study, therefore, was conducted to investigate the effects of Conocarpus biochar （BC） and organic farm residues （FR） at different application rates of 0.0% （control）, 4.0% and 8.0% （weight/weight） on yield and quality of tomatoes grown on a sandy soil under drip irrigation with saline or non-saline water. The availability of P, K, Fe, Mn, Zn and Cu to plants was also investigated. The results demonstrated clearly that addition of BC or FR increased the vegetative growth, yield and quality parameters in all irrigation treatments. It was found that salt stress adversely affected soil productivity, as indicated by the lower vegetative growth and yield components of tomato plants. However, this suppressing effect on the vegetative growth and yield tended to decline with application of FR or BC, especially at the high application rate and in the presence of biochar. Under saline irrigation system, for instance, the total tomato yield increased over the control by 14.0%-43.3% with BC and by 3.9%-35.6% with FR. These could be attributed to enhancement effects of FR or BC on soil properties, as indicated by increases in soil organic matter content and nutrient availability. Therefore, biochar may be effectively used as a soil amendment for enhancing the productivity of salt-affected sandy soils under arid conditions.