Contrasting Effects of Cattle Manure Applications and Root-Induced Changes on Heavy Metal Dynamics in the Rhizosphere of Soybean in an Acidic Haplic Fluvisol: A Chronological Pot Experiment

Phytoremediation of heavy metals in soil and water: An eco-friendly, sustainable and multidisciplinary approach

Rapid industrialization, increased waste production and surge in agricultural activities, mining, contaminated irrigation water and industrial effluents contribute to the contamination of water resources due to heavy metal (HM) accumulation. Humans employ HM-contaminated resources to produce food, which eventually accumulates in the food chain. Decontamination of these valuable resources, as well as avoidance of additional contamination has long been needed to avoid detrimental health impacts. Phytoremediation is a realistic and promising strategy for heavy metal removal from polluted areas, based on the employment of hyper-accumulator plant species that are extremely tolerant to HMs present in the environment/soil. Green plants are used to remove, decompose, or detoxify hazardous metals in this technique. For soil decontamination, five types of phytoremediation methods have been used viz. phytostabilization, phytodegradation, rhizofiltration, phytoextraction and phytovolatilization. Traditional phytoremediation methods, on the other hand, have significant limits in terms of large-scale application, thus biotechnological efforts to modify plants for HM phytoremediation ways are being explored to improve the efficacy of plants as HM decontamination candidates. It is relatively a new technology that is widely regarded as economic, efficient and unique besides being environment friendly. New metal hyperaccumulators with high efficiency are being explored and employed for their use in phytoremediation and phytomining. Therefore, this review comprehensively discusses different strategies and biotechnological approaches for the removal of various HM containments from the environment, with emphasis on the advancements and implications of phytoremediation, along with their applications in cleaning up various toxic pollutants. Moreover, sources, effects of HMs and factors affecting phytoremediation of HMs metals have also been discussed.

Insights into the molecular transformation in the dissolved organic compounds of agro-waste-hydrochars by microbial-aging using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Hydrochars-based dissolved organic matters (DOM) are easily available to organisms and thus have important influence on the biota once applying hydrochars as environment amendment. Thus, positive modifications on molecular composition of DOM is indispensable before hydrochars application. In this study, the impacts of microbial-aging by anaerobic fermentation on DOM of agro-waste-hydrochars was systematically assessed. Results revealed that microbial-aging caused lower DOM release but higher DOM molecular diversity. Moreover, microbial-aging resulted in the production of more biodegradable compounds, including lipids and proteins, and reduced the aromaticity of DOM. The highly oxygenated molecules (O/C > 0.6) were shifted into lower-order ones in the hydrochars-based DOM, suggesting the transformation of hydrophilic compounds into hydrophobic ones. Additionally, microbial-aging promoted the degradation of phenols by 99.0-98.9%, phenolic acids 37.8-73.5%, and polycyclic aromatic hydrocarbons by 83.4-90.4% in hydrochar-based DOM. Overall, this study demonstrates that microbial-aging changes the molecular characteristics of hydrochars-based DOM in a positive manner.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Minerals loaded with oxygen nanobubbles mitigate arsenic translocation from paddy soils to rice

Inhibiting reductive transformation of arsenic (As) in flooded paddy soils is fundamentally important for mitigating As transfer into the food chain. In this study, oxygen-nanobubble-loaded-zeolites (ZON) and -vermiculites (VON) were tested as a novel approach for supplying oxygen to paddy soils to inhibit As influx into rice. The dynamic physio- and bio-chemical variations in the rhizosphere and bulk soil were profiled in a rhizobox experiment. Upon adding ZON and VON, the redox potential and dissolved oxygen consistently increased throughout the cultivation period. The improved redox environment inhibited As(III) release into porewater and increased As(V) adsorbed on crystalline Fe (hydr)oxides, following the reduction of arsC and arrA gene abundances and enhancement of the aioA gene. Moreover, adding ZON and VON promoted root iron plaque formation, which increased As retention on iron plaque. Both ZON and VON treatments mitigated As translocation from soil to rice, meanwhile increasing root and shoot biomass. ZON was superior to VON in repressing As transfer and promoting rice growth due to its higher oxygen loading capacity. This study provides a novel and environment-friendly material to both mitigate the As translocation from paddy soil to rice and improve rice growth.

Sewage sludge-derived hydrochar that inhibits ammonia volatilization, improves soil nitrogen retention and rice nitrogen utilization

Hydrothermal carbonization (HTC) is a promising technique for treating sewage sludge. In this study, three sewage sludge-derived hydrochars produced with water (SSHW), 1 wt% magnesium citrate (SSHM) solution, and 1 wt% magnesium citrate mixed with 1 wt% sulfuric acid (SSHMS) solution were applied to columns of packed paddy soil. We evaluated the effects of these differently modified sewage sludge-hydrochars on ammonia volatilization, soil nitrogen (N) retention and rice growth. Results showed that compared to the control, SSHMS reduced the cumulative ammonia volatilization determined after three split application of N-fertilizer. SSHM and SSHMS both reduced the yield-scale ammonia volatilization by 20.3% and 41.2% respectively. Moreover, the addition of three sewage sludge-derived hydrochars increased soil ammonium-N retention after the first supplementary fertilization; however, after the second supplementary fertilization, only SSHMS addition significantly increased soil ammonium-N retention. Of the three hydrochars tested, SSHMS has the strongest effects on soil ammonium-N retention and inhibition of ammonium-N loss in floodwater. This was attributed to increased ammonium sorption driven by SSHMS's lower surface pH and porous diameter, larger adsorption porous volume and higher abundance of carboxyl functional groups. Additionally, the increased soil N retention increased grain N content and yield. Our results provide a novel method to valorize sewage sludge into a valuable fertilizer that if applied to paddy soil it can inhibit ammonia volatilization, N loss in floodwater, and promote N use efficiency by rice, with positive implications for sustainable rice production.

Heavy metal occurrence and risk assessment in dairy feeds and manures from the typical intensive dairy farms in China

Modern farming practice features extensive overuse of additives in animal feed. Subsequent use of manure as a fertilizer has resulted in significant heavy metal accumulation in agricultural soil, which is particularly apparent in areas of intensive farming. Here, samples of dairy feed, manure, water, and soil were collected from four intensive dairy farms in China and analyzed to assess selected heavy metal concentrations (Cu, Zn, Cr, Ni, Pb, and Cd). Results revealed that all feed samples contained the selected heavy metals, attesting to the wide use of additives during intensive dairy farming. The average Cr and Pb concentrations were 6.1 to 17.1 times greater than their recommended guidelines. Overall, average heavy metal concentrations in manure decreased in the following order: Zn > Cu > Cr > Ni > Pb > Cd. Using data obtained from the sequential extraction procedure, proposed by the Community Bureau of Reference (BCR), metal bioavailability also decreased according to the following order: Pb (69.4%) > Cr (63.7%) > Ni (60.8%) > Cu (53.4%) > Zn (50.0%) > Cd (34.5%). Heavy metal levels in sampled wastewater were also relatively high; however, surface and well water levels were relatively low. Although use of manure in dairy farming has not resulted in serious pollution until now, Zn, Cu, and Cd are all known to pose significant risk to soil quality. Finally, principal component analysis (PCA) results indicated that heavy metal levels in soil originated predominantly from parent soil materials and were then enhanced by anthropogenic sources.

A Reduced Phosphorus Application Rate Using a Mycorrhizal Plant as the Preceding Crop Maintains Soybean Seeds' Nutritional Quality

We tested whether introducing an arbuscular mycorrhizal fungi (AMF)-host plant with a reduced P application rate could maintain soybean seeds' nutrient quality. The dynamic variation of 14 nutrients was analyzed in source and sink organs during the seed-filling stage. The AMF-host and non-AMF-host plants, sunflower and mustard, were grown as preceding crops (PCs). Soybeans, the succeeding crops, were planted with three different phosphorus levels, namely, 0, 50, and 150 kg P2O5 ha-1. The results showed that the AMF-host PC with a reduced P application rate maintained the seed's yield and nutrients quality. During the seed-filling stage, the AMF-host PC with a reduced P application rate increased the uptake of most nutrients compared to the non-AMF-host PC, and improved the remobilization efficiency of all nutrients except Mn, Fe, and Se, compared to the optimal P application rate. These results could help improve the utilization efficiency of P fertilizers and protect soybeans' nutritional value.

Total and available metal concentrations in soils from six long-term fertilization sites across China

Approximately 19% of agricultural soils in China are contaminated by heavy metals. However, the effects of agricultural management practices on soil contamination are not well understood. Taking advantage of six long-term (23-34 years) field sites across China, this study examined the effects of different agricultural fertilization treatments, including control (no fertilization), inorganic nitrogen, phosphorus and potassium fertilization (NPK), manure fertilization (M), and NPK plus manure fertilization (NPKM), on the total and available metal concentrations in soils. The results showed that after 23-34 years of fertilization, the M and NPKM treatments significantly increased the total concentration of cadmium (Cd), copper (Cu), and zinc (Zn) in soils compared with the concentrations measured for the control and NPK treatments. In contrast, the fertilization treatments had almost no influence on soil lead (Pb) and nickel (Ni) concentrations. The results of analysis via diffusive gradients in thin films demonstrated that long-term sheep or cattle manure fertilization increased the available metals, especially Cd, Cu, and Zn, but long-term swine manure application decreased the available metals, except for Cu and Zn, in soils. Further analysis revealed that the manure source, soil pH level, and biogeochemical properties of metals affected the availability of Cd, Cu, Pb, Zn, and Ni in soils. Collectively, organic fertilizers had the potential to reduce metal uptake by crops, but caution should be taken to reduce metal concentrations in manure.

The complexation of rhizosphere and nonrhizosphere soil organic matter with chromium: Using elemental analysis combined with FTIR spectroscopy

Complexation is a main mechanism controlling the reactions between soil organic matter (SOM) and heavy metals, which still have not been fully understood up to date. The objective of this study was to compare the SOM composition of nonrhizosphere and rhizosphere in low Cr treatment with that in high Cr treatment and to find out how metal concentrations affect the complexation with SOM. The results revealed that both the hydroxyl and the carboxyl were significantly different under different Cr treatment groups. For nonrhizosphere samples, the high Cr treatment tended to have less hydroxyl contents and more structural changes on hydroxyl (3389-3381 cm^-1) than the low Cr treatment (3389-3388 cm^-1), while in the rhizosphere samples the reverse happened. The gap of the different Cr treated band area in the rhizosphere samples (44 a.u of the gap) was greatly smaller than that in the nonrhizosphere samples (576 a.u of the gap). In both the rhizosphere and nonrhizosphere samples, the high Cr treatment showed greater structural changes on carboxylic acids (11, 12 a.u changes based on the control) than the low Cr treatment (4, 6 a.u). The unsaturated carboxylic acids could account for downward frequency shift and the contents in the nonrhizosphere samples were slightly greater than that in the rhizosphere samples. This study used elemental analysis combined with FTIR spectroscopy to explore the effects of metal concentrations on the complexation of Cr with SOM and the composition of SOM. These findings give a way to understanding part of the complexation mechanisms between the metal and SOM.