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Semi-arid and arid regions are characterised by drying and rewetting events, salinity as well as fires. The effects of these stresses on soils are all related to water availability and have been investigated extensively, but usually separately. To study the impact of two stresses, we combined a short heating event as it may occur in fast-moving grass fires with either a drying and rewetting event or salinity. In the first experiment, soils were incubated air-dried, moderately dried and constantly moist for 7 days after which the dried soils were rewetted. Seven days after rewetting, half of the replicates from each treatment were heated and then maintained at 60 °C for 30 min followed by rapid rewetting, while others remained unheated. The rewetting flush of respiration after 7 days was greater in previously air-dried soil than moderately dry soil. Regardless of prior water treatment, heating followed by rewetting compared to constantly moist soil increased respiration threefold and available N and P by about 20–30%. In the second experiment, NaCl was added to non-saline soil (EC1:5 0.01 dS m−1) to achieve EC1:5 1 and 4 ds m−1 (referred to as NS, S1 and S4). Soils were incubated moist for 1 month and then amended with pea residue at 10 g kg−1. Five days after residue amendment, half of the replicates from each treatment were heated and rewetted as in the first experiment. In unheated S4 compared to NS, cumulative respiration was 30% lower and available N and P were threefold and 30% higher. Heating reduced cumulative respiration by 10% in NS and S1 but by 30% in S4. Compared to unheated treatments, available N in heated NS was up to tenfold higher, but in S4, heating increased available N only up to threefold. In all salinity treatments, heating increased available P by about 15%. It can be concluded that the impact of short-term heating on nutrient availability and soil respiration was not affected by prior drying and rewetting. High salinity on the other hand reduced the impact on nutrient availability whereas the effect on soil respiration was exacerbated compared to non-saline soil.

The effects of heating and rewetting on nutrient availability and microbial activity have been studied extensively. But little is known about the effect of prior presence of plants on the impact of heating on microbial activity and nutrient availability. Soil was planted with wheat or left unplanted. After 4 weeks, roots were removed from the planted soil, washed and then added to half of the previously planted and unplanted soil at 1 g dw kg−1. Half of the replicates from each treatment were heated and then maintained at 60 °C for 30 min followed by rapid rewetting, while others remained unheated. Soil respiration was measured continuously for 14 days. Soils were sampled 2, 7 and 14 days after rewetting for available N and P pools. Heating and rewetting induced a flush of respiration and increased available N and P. The heating-induced increase in initial respiration rate, cumulative respiration and available N and P was greater in previously planted soil than unplanted soil. Heating increased available N and P three- to fivefold in planted soil, but only twofold in unplanted soil. Plant-induced changes in nutrient availability increase the impact of heating on soil respiration and nutrient availability.

Heating of soil influences soil respiration and nutrient availability. But little is known about how amendment type (plant residue or inorganic N and P fertilisers) or time between amendment application and heating influence the effect of heating. A sandy clay loam was incubated moist (165 g kg−1) for 8 days and amended with the same amount of total N and P as pea residue (C/N 59, C/P 435) or inorganic N and P (0.4 g N kg−1 and 0.06 g P kg−1) either 8 or 1 day before heating. On day 8, soils were heated to 60 °C for 30 min followed by rewetting to 165 g water kg−1 and moist incubation for 7 days. Soil was sampled before heating (day 8), 2 and 7 days after heating (day 10 and 15). Heating only reduced respiration when residue was added 1 day before heating. Heating had no effect on available N in fertiliser treatments. Heating increased available N on day 10 in the unamended soil by about 20% and in residue treatments about 10-fold, particularly when residue was added 1 day before heating. Heating increased phosphatase P two- to fivefold only on day 10; it increased citrate P on days 10 and 15 up to threefold compared with a greater effect in the residue treatments. Heating of soil had a greater effect of on respiration and available N and P pools after residue addition than unamended soil or soil amended with inorganic fertiliser.

Little is known about the impact of short-term heating to 50–100 °C as it may occur in fast-moving low-intensity fires on microbial activity and nutrient availability. Heating of soils usually induces drying, but it is unclear if the effect of heating is only due to this water loss or if other factors are also important. Two experiments were carried out where soils were heated to 60 °C. The experiments included constantly moist controls and air-dried soils which were dried at 30 °C to the same water content as the heated soils. After heating/air-drying, the soils were rapidly rewet and incubated for 2 weeks at a constant water content optimal for microbial activity. The aim of the first experiment was to assess the effect of water content before heating and heat duration on soil respiration and nutrient availability. Soil was incubated for 14 days at 33, 82, and 165 g water kg−1 (referred to as W33, W82, and W165, corresponding to 10, 30, and 50% maximum water-holding capacity) where 165 g water kg−1 is optimal for microbial activity in this soil. The soils were then heated to 60 °C and maintained at this temperature for 30 or 90 min. Heat duration had little effect on the measured properties. In heated soils, cumulative respiration after rewetting was about threefold higher than in the constantly moist control. Two days after heating, available N in heated soils was twofold higher than in the constantly moist control and 0.3 to twofold higher than the corresponding air-dried soils. Two weeks after heating, available N differed little between the constantly moist control and heated soils that were at W33 and W82 before heating, but it was about twofold higher in heated soil that was at W165 before heating. Available P 2 days after heating was highest in heated soils, but 2 weeks after heating, available P was lower in heated soils than the constantly moist control. In the air-dried controls which were dried at 30 °C to the same water content as in heated soils prior to rewetting, cumulative respiration, available N and P after rewetting differed little from the constantly moist soil. The aim of the second experiment was to determine the effect of soil nutrient content on soil respiration and nutrient availability after heating. Two soils differing in organic matter, total N, and total P content were used either separately or as mixes with different proportions of the soils. Soils were heated and maintained at 60 °C for 30 min. Before and after heating/air-drying, the soils were maintained at optimal water content (180 g water kg−1). Two and 7 days after heating, available N was 10–30% higher in heated soils than the constantly moist control and air-dried soils. It can be concluded that the effect of short-term heating followed by rewetting on soil respiration and available N and P is not only due to soil drying, but possibly also heating-induced changes in soil organic matter composition and availability as well as soil P sorption capacity.

With increasing global demand for both food and energy, the importance of microorganism based soil and plant health management has been widely considered. Nitrogen-fixing microorganisms, together with the contribution from plants and soil, can fulfill the nitrogen requirements of plants in different ecosystems. Such nutrient requirements are met either completely or partially, while maintaining soil sustainability. Symbiotic nitrogen fixation, associative and free-living nitrogen-fixing microorganisms can convert atmospheric nitrogen into ammonia, providing nitrogen to the plant while enhancing plant yield. Moreover, nitrogen fixation can indirectly affect the soil nitrogen pool and enrich microbial diversity in the soil to support soil health. These diverse nitrogen-fixing microorganisms have various mechanisms for fixing nitrogen, which contribute favorably towards the sustainability of plant and soil health.

Rapid surge of interest for carbon nanotube (CNT) in the last decade has made it an imperative member of nanomaterial family. Because of the distinctive physicochemical properties, CNTs are widely used in a number of scientific applications including plant sciences. This review mainly describes the role of CNT in plant sciences. Contradictory effects of CNT on plants physiology are reported. CNT can act as plant growth inducer causing enhanced plant dry biomass and root/shoot lengths. At the same time, CNT can cause negative effects on plants by forming reactive oxygen species in plant tissues, consequently leading to cell death. Enhanced seed germination with CNT is related to the water uptake process. CNT can be positioned as micro-tubes inside the plant body to enhance the water uptake efficiency. Due to its ability to act as a slow-release fertilizer and plant growth promoter, CNT is transpiring as a novel nano-carbon fertilizer in the field of agricultural sciences. On the other hand, accumulation of CNT in soil can cause deleterious effects on soil microbial diversity, composition and population. It can further modify the balance between plant-toxic metals in soil, thereby enhancing the translocation of heavy metal(loids) into the plant system. The research gaps that need careful attention have been identified in this review. Refereed/Peer-reviewed

The increase in usage of heavy metals in different industrial activities causes their existence in effluents. Excessive concentrations of these heavy metals pollute soil and water. Heavy metals cause toxicities and other harmful effects not only in humans and animals but also in plants and soil microorganisms. Heavy metals disrupt many biochemical and physiological activities in bacteria, including growth, development, enzyme and hormone production. Indole acetic acid (IAA) is one of the most important hormones in plants, which is secreted by both bacteria and plants. The present study assessed the effects of Ni, Pb and Cu on the growth of Bradyrhizobium japonicum and IAA production. B. japonicum were grown in yeast mannitol broth in a rotary shaker at 100 rpm for 18 h for each variant. Ni, Pb and Cu with concentrations ranging from 2.5–40 mg/L were added to B. japonicum cultures and turbidity was measured at 600 nm. Effects of Ni, Pb and Cu on IAA production were determined by measuring the IAA content with Salkowski method. The metal uptake by B. japonicum was determined in the range of 5–50 mg/L. A lettuce seedling assay was conducted to observe the effect of B. japonicum on plant growth under heavy metal stress. Lettuce seedlings inoculated with the bacterium were spiked with 5–500 mg/L of Ni, Pb and Cu. Shoot and root length, and plant dry biomass were measured after 5 days of germination. Bacterial growth was reduced with the increase of Ni and Cu concentrations. However, there was no significant growth effect with Pb in the considered range. The IAA production also followed the same pattern with Pb and Ni, but in the presence of Cu, IAA content was slightly increased and reached an equilibrium. The growth retardations observed in lettuce in the presence of Pb, Ni and Cu were in the decreasing order of Ni > Cu > Pb. Bacterial inoculation reduced the heavy metal stress and increased the shoot and root lengths of lettuce seedlings. FTIR spectrum of the bacterial biomass showed that amine and nitro groups are responsible in metal sorption process. The Pb-treated bacterial biomass showed a considerable difference in the FTIR spectrum compared to other treatments. According to our results, B. japonicum is sensitive to heavy metal stress with a retardation of growth and IAA production, though it is able to enhance the growth of lettuce seedlings under heavy metal stress.

The transformation of trace metals (TMs) in natural environmental systems has created significant concerns in recent decades. Ultramafic environments lead to potential risks to the agricultural products and, subsequently, to human health. This unique review presents geochemistry of ultramafic soils, TM fractionation (i.e. sequential and single extraction techniques), TM uptake and accumulation mechanisms of ultramafic flora, and ultramafic-associated health risks to human and agricultural crops. Ultramafic soils contain high levels of TMs (i.e. Cr, Ni, Mn, and Co) and have a low Ca:Mg ratio together with deficiencies in essential macronutrients required for the growth of crops. Even though a higher portion of TMs bind with the residual fraction of ultramafic soils, environmental changes (i.e. natural or anthropogenic) may increase the levels of TMs in the bioavailable or extractable fractions of ultramafic soils. Extremophile plants that have evolved to thrive in ultramafic soils present clear examples of evolutionary adaptations to TM resistance. The release of TMs into water sources and accumulation in food crops in and around ultramafic localities increases health risks for humans. Therefore, more focused investigations need to be implemented to understand the mechanisms related to the mobility and bioavailability of TMs in different ultramafic environments. Research gaps and directions for future studies are also discussed in this review. Lastly, we consider the importance of characterizing terrestrial ultramafic soil and its effect on crop plants in the context of multi-decadal plans by NASA and other space agencies to establish human colonies on Mars. Keywords: Soil contamination, Geochemistry, Trace elements, Bioaccumulation, Translocation, Extremophytes

In this study, we investigated the effect of biochar (BC) and fungal bacterial co-inoculation (FB) on soil enzymatic activity and immobilization of heavy metals in serpentine soil in Sri Lanka. A pot experiment was conducted with tomatoes (Lycopersicon esculentum L.) at 1, 2.5, and 5 % (w/w) BC ratios. Polyphenol oxidase, catalase and dehydrogenase activities were determined by idometric, potassium permanganate oxidisable, and spectrophotometric methods, respectively. Heavy metal concentrations were assessed by 0.01 M CaCl2 and sequential extraction methods. An increase in BC application reduced polyphenol oxidase, dehydrogenase, and catalase activity. The application of FB increased soil dehydrogenase activity, with the maximum activity found in 1 % BC700 + FB treatment. Moreover, the CaCl2 extractable metals (Ni, Mn, and Cr) in 5 % BC700 amended soil decreased by 92, 94, and 100 %, respectively, compared to the control. Sequential extraction showed that the exchangeable concentrations of Ni, Mn, and Cr decreased by 55, 70, and 80 % in 5 % BC700, respectively. Results suggest that the addition of BC to serpentine soil immobilizes heavy metals and decreases soil enzymatic activities. The addition of FB to serpentine soil improves plant growth by mitigating heavy metal toxicity and enhancing soil enzymatic activities.

This study evaluated the efficacy of a microbial biofilm in removing Ni ions in aqueous media. The biofilm was developed incorporating a garden soil fungus with a bacterium isolated from Ni-rich serpentinite soil. The biofilm was characterized using microscopy, scanning electron microscopy, Fourier transform infrared (FTIR) investigations, and Boehm and potentiometric titrations. Ni removal was determined using batch experiments as a function of pH, Ni concentration, and time. The adsorption isotherm assay was conducted with varying Ni concentrations from 25 to 500 mg/L for 4 days. Isotherm and kinetic modeling were applied to the experimental data to understand the mechanisms of Ni removal. The zero point charge at pH 4.5 indicated the pH values greater than 4.5 is favorable for Ni adsorption. Acidic nature of the biofilm was reflected from Boehm titration data showing higher number of acidic groups than basic groups. With the increase in initial Ni concentration, the uptake increased from 3.43 t...

Unfortunately, in the original publication of the article, Prof. Yong Sik Ok’s affiliation was incorrectly published. The author’s affiliation is as follows.

Heavy metal contamination in soils can influence plants and animals, often leading to toxicosis. Heavy metals can impact various biochemical processes in plants, including enzyme and antioxidant production, protein mobilization and photosynthesis. Hydrolyzing enzymes play a major role in seed germination. Enzymes such as acid phosphatases, proteases and α-amylases are known to facilitate both seed germination and seedling growth via mobilizing nutrients in the endosperm. In the presence of heavy metals, starch is immobilized and nutrient sources become limited. Moreover, a reduction in proteolytic enzyme activity and an increase in protein and amino acid content can be observed under heavy metal stress. Proline, is an amino acid which is essential for cellular metabolism. Numerous studies have shown an increase in proline content under oxidative stress in higher plants. Furthermore, heat shock protein production has also been observed under heavy metal stress. The chloroplast small heat shock proteins (Hsp) reduce photosynthesis damage, rather than repair or help to recover from heavy metal-induced damage. Heavy metals are destructive substances for photosynthesis. They are involved in destabilizing enzymes, oxidizing photosystem II (PS II) and disrupting the electron transport chain and mineral metabolism. Although the physiological effects of Cd have been investigated thoroughly, other metals such as As, Cr, Hg, Cu and Pb have received relatively little attention. Among agricultural plants, rice has been studied extensively; additional studies are needed to characterize toxicities of different heavy metals on other crops. This review summarizes the current state of our understanding of the effects of heavy metal stress on seed germination and seedling development and highlights informational gaps and areas for future research.

Soils may be exposed to short heating events for example in fast-moving grass fires which can reoccur if not all fuel was burnt in the first heating. Little is known about the effect of a second heating on microbial activity and nutrient availability. The aim of this study was to determine the effect of one and two heating events on soil respiration and nutrient availability. The soil was incubated moist (165 g kg−1) for 7 days. On day 8, soils were left unheated (CM) or heated and then maintained for 30 min at 60 °C followed by rapid rewetting to 165 g kg−1. Heated treatments included heated once (H8) or heated a second time 4, 8 and 16 days after the first heating event (H8-12, H8-16 and H8-24). Soil respiration was measured from day 8–26, soils were sampled on day 10, 14, 18 and 26 for available N and P pools. Heating and rewetting induced a flush of respiration. Compared to unheated soil cumulative respiration was about 10 % higher in H8 and 20% higher in - H8-12 and H8-16, but 30% higher in H8-24. From two days after heating, available N and P were about 25% higher in heated soils than the unheated control. The second heating induced a further increase in available N and P compared to the first heating. Microbial biomass P was generally 25–50% lower in heated soils than the control and was lower after the second than the first heating. Citrate P was 30–50% higher in heated soils than the control, irrespective of number of heating events. HCl P which was the largest P pool was not affected by heating. It can be concluded that a short heating event followed by rewetting increases N and P availability with a further increase after the second heating, particularly when the interval between heating events is long. The increase in nutrient availability may aid plant recovery after heating, but could also increase nutrient loss by leaching.

Due to industrialization, excessive use of pesticides and fertilizer and improper waste management practices cause heavy metal accumulation in both soil and water. Due to the nondegradable and persistent nature, heavy metals can be accumulated in soils for hundreds of years. They enter the bodies of plants and animals and thereby cause negative health impacts to the environment. Even though the soil heavy metal remediation is a must, it is not an easy task to achieve. Among many physical and chemical methods, phytoremediation plays an important role, due to its efficient and convenient nature. Rhizophere microbes play an important role in phytoremediation. Since, rhizosphere is the immediate vicinity of the root, the chemical and physical changes in that environment can easily effect heavy metal uptake by the plant. By siderophore production, acidification, releasing plant growth promoters, reducing the plant stress conditions and through redox changes rhizosphere enhances the phytoreomediation processes. However, plants can bioconcentrate (phytoextraction) and also bioimmobilize the toxic heavy metals through rhizospheric processes. This chapter summaries the role of rhizospheric organisms for facilitation of heavy metal uptake, the different mechanisms of enhancing the availability of heavy metals in the rhizosphere, the genetic diversity, and the microbial genera that involve in these processes.

Hydrocarbon contamination of soil is one of the major environmental problems today due to activities related to the petrochemical industry. Mechanical and chemical remediation and restoration to remove hydrocarbons from contaminated sites have limited effectiveness and costly. Bioremediation is the promising technology for the treatment of these contaminated sites since it is cost-effective and will lead to complete mineralization. Fungi and bacteria have been considered as highly effective in oil degradation. Several bacteria are even known to feed exclusively on hydrocarbons; Arthrobacter, Burkholderia, Mycobacterium, Pseudomonas, Sphingomonas, and Rhodococcus. Fungal genera, namely, Amorphoteca, Neosartorya, Talaromyces, and Graphium are proved to be the potential organisms for hydrocarbon degradation. Although laboratory experiments have indicated that the bacteria can ubiquitously degrade oil constituents, to date there is little convincing evidence that bioaugmentation (addition of more bacteria) significantly enhances the extent of oil biodegradation in soil. The potential benefits of using genetically modified bacteria represent a research frontier with significant results. However, many concerns are often raising due to the effectiveness of indigenous species, limited understanding of various phytoremediation mechanisms, including the regulation of enzyme systems that degrade pollutants. Thus, this chapter presents an updated overview of petroleum hydrocarbon degradation by microorganisms focusing biosurfactants and their mechanisms.

This study assesses the effect of N-fixing bacteria and biochar synergism on plant growth and development of Vigna mungo under heavy metal stress (HM). Heavy metal stress is a worldwide problem, which causes critical effects on plant life due to oxidative stress. Application of biochar is a recent biological remediation technique, which often leads to an immobilization of heavy metals in soil. . Synergism of bacteria and biochar is a novel aspect to enhance plant growth under heavy metal stress. Woody biochar a byproduct of a dendro power industry was added as 1, 2.5 and 5% amounts combination with Bradyrhizobium japonicum, where mung seedlings were planted in serpentine soil rich in Ni, Mn, Cr and Co. Pot experiments were conducted for 12 weeks. The plant height, heavy metal uptake by plants, soil bioavailable heavy metal contents, soil N and P and microbial biomass carbon (MBC) were measured. The plant growth was enhanced with biochar amendment but a retardation was observed with high biochar application (5%). The soil N and P increased with the increase of biochar addition percentage while soil MBC showed reductions at 5% biochar amendment. Both soil bioavailable fractions of HM and up take of HMs by plants were gradually reduced with increase in biochar content. Based on the results, 2.5% biochar synergism with bacteria was the best for plant growth and soil nutrition status. Despite the synergism, available N was negatively correlated with the decrease of bioavailable metal percentage in soil whereas it was conversely for P.

Pulses, an essential component of the Sri Lankan diet, have high levels of phytic acid (PA) which chelate essential mineral nutrients limiting their bioavailability. Three locally grown pulses; mung beans, black gram and soya bean were investigated to determine the effects of popular processing methods on their Zn, Fe and Ca bioavailability as affected mainly by phytic acid (PA). Processed mung beans ( Vigna radiata var . MI 6) and black gram ( Vigna mungo var. MI 1) had very high levels of PA. In mung beans, with increasing germination time, a significant decrease (P=0.05) in PA from 2517 to 1998 mg per 100g (dry weight basis) was observed but PA:Zn molar ratio did not improve. Natural fermentation of black gram, did not affect PA level effectively, but levels decreased when inoculated with a lactic acid bacterium or with Pantoea agglomerans . In soya bean ( Glycine max var. Pb ), significant (P=0.05) reduction in PA was observed in tempeh (487 mg/100 g dry weight). Overall, in soya products, the Zn bioavailability has increased from low to moderate levels in both tofu and tempeh. The molar ratio of PA:Fe was lower in all soya products. Although, PA:Zn molar ratio has decreased in germinated mung beans, the Ca x PA:Zn ratio exceeded 0.5 indicating that Ca influences Zn bioavailability DOI: http://dx.doi.org/10.4038/cjsbs.v41i1.4534 Ceylon Journal of Science ( Bio. Sci. ) 41 (1): 19-26, 2012

Serpentine soils derived from ultramafic rocks containing high contents of Ni, Cr, Mn, and Co. However, Ni is the most predominant and bioavailable heavy metal in serpentine soil. Due to high heavy metal content, high Mg, low nutrient contents (N, P, Ca), and low organic matter in this particular extreme environments give an inhospitable environment for plant growth. Therefore, these extreme conditions have led for the formation of serpentine specialized flora, which could thrive under these unfavorable conditions. Hence, serpentine sites are distributed throughout the world as patches leading to serpentine endemics. The genes of these specialized plants do not show an interchange, thereby forming a specialized genetic pool. However, some of the serpentine areas are utilized in agriculture and it has caused accumulation of toxic heavy metals in edible plant parts. Consumption of heavy metal-containing plants causes bioaccumulation in herbivores and humans. Moreover, these sites may cause groundwater contaminations again causing bioaccumulation in living organisms. The plants living in serpentine areas show exclusive mechanisms confined to them for both heavy metal uptake and storage. The rhizosphere acidification is one of the predominant factors for heavy metal uptake. ZIP family proteins are a group of transporter proteins, which are involved in membrane transport of Ni serpentine flora whereas via histidine, nicotianamine, and yellow strip L like family is important in Ni transport through the xylem. Moreover, cation diffusion factor and organic acids mediate vacuolar sequestration of Ni. Though numerous studies are carried on heavy metals such as Cd and As, no sufficient research is carried out on membrane Ni transporters. Hence, more research is necessary to manipulate Ni hyperaccumulators in biotechnological applications.

Unfortunately, in the original publication of the article, Prof. Yong Sik Ok’s affiliation was incorrectly published. The author’s affiliation is as follows.

This study was conducted to understand the role of bacterial–fungal interactions on heavy metal uptake by Zea mays plants. A pot experiment was conducted for 90 days with Z. mays in serpentine soil inoculated with a Gram-negative bacterium, fungus (Aspergilllus sp.) and both microbes to determine the effects of inoculation on nickel, manganese, chromium and cobalt concentrations in plant tissue and soil. Soil nutrients and soil enzyme activities were measured to determine the effect of inoculations on soil quality. Inoculation of microorganisms increased shoot and root biomass, and the maximum biomass was in the bacterial–fungal inoculation. This could be due to the solubilisation of phosphate and production of indole acetic acid. Although the combination treatment contributed to an increase in heavy metal uptake in Z. mays plants, the lowest translocation was observed in the combination treatment. Moreover, the soil available nitrogen, available phosphorous and total organic carbon content were increased with the microbial inoculation. Similarly, the soil dehydrogenase activity was higher as a result of microbial inoculation, whereas the highest dehydrogenase activity was reported in the combination inoculation. This study confirms the synergistic effect of bacterial–fungal inoculation as a soil-quality enhancer and as a plant-growth promoter in the presence of heavy metals.