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Hexavalent chromium [Cr (VI)], extensively used in different industries, is one of the most toxic heavy metals. The Cr (VI) reducing bacteria could be helpful in decreasing its toxic effects. The present study was conducted to evaluate the potential of Cr (VI) reducing bacteria to improve growth and yield of okra ( Hibiscus esculentus L.) in Cr-contaminated soils. Most of the selected bacterial isolates significantly increased the growth and yield of okra. Maximum response was observed in the plants inoculated with the isolate K12 where plant height, root length, fruit weight and number of fruits per plant increased up to 77.5 percent, 72.6 percent, 1.4 fold and 2.9 fold, respectively. Moreover, inoculation with bacteria caused significant decrease in Cr (VI) concentration in soil and plant parts across all treatments. The maximum decrease of 69.6, 56.1 and 40.0 percent in Cr (VI) concentrations in soil, plant vegetative parts and plant reproductive parts, respectively, was observed in the treatment inoculated with the strain K12. Based on amplification, sequencing and analysis of 16S rDNA sequence, the strain K12 was found belonging to genus Brucella and was designated as Brucella sp. K12. These findings suggest that the strain K12 may serve as a potential bioresource to improve crop production in Cr-contaminated soils. [ABSTRACT FROM AUTHOR]

Plant-associated bacteria can have beneficial effects on the growth and health of their host. However, the role of plant growth promoting bacteria (PGPR), under metal stress, has not been widely investigated. The present study investigated the possible mandatory role of plant growth promoting rhizobacteria in protecting plants from zinc (Zn) toxicity. The exposure of maize plants to 50 µM zinc inhibited biomass production, decreased chlorophyll, total soluble protein and strongly increased accumulation of Zn in both root and shoot. Similarly, Zn enhanced hydrogen peroxide, electrolyte leakage and lipid peroxidation as indicated by malondaldehyde accumulation. Pre-soaking with novel Zn tolerant bacterial strain Proteus mirabilis (ZK1) isolated zinc (Zn) contaminated soil, alleviated the negative effect of Zn on growth and led to a decrease in oxidative injuries caused by Zn. Furthermore, strain ZK1 significantly enhanced the activities of catalase, guaiacol peroxidase, superoxide dismutase and ascorbic acid but lowered the Proline accumulation in Zn stressed plants. The results suggested that the inoculation of Zea mays plants with P. mirabilis during an earlier growth period could be related to its plant growth promoting activities and avoidance of cumulative damage upon exposure to Zn, thus reducing the negative consequences of oxidative stress caused by heavy metal toxicity. [ABSTRACT FROM AUTHOR]

In semi-arid regions, post-restoration vegetation recovery on abandoned agricultural lands often fails due to inherently low organic matter content and poor soil fertility conditions, including phosphorus (P). As such, amending these soils with controlled release P fertilizer, especially with suitable P solubilizing bacteria (PSB) may promote plant growth and productivity by stimulating biological P fertility. To this aim, a pot study was performed to evaluate the agronomic potential of maize and soil biological P pools, using encapsulated (ENRP) and non-encapsulated (NRP) nano-rock phosphate as the P fertilizer source, on reclaimed agricultural soil in the presence and absence of PSB inoculant. The experiment was setup following a 3 × 2 factorial arrangement with four replicates. Without PSB, NRP treatment showed marginal positive effects on plant growth, P nutrition and P use efficiency (PUE) compared to control treatment. Although larger gains with NRP treatment were more noticeable under PSB inoculation, ENRP was the most convenient slow-release P fertilizer, increasing plant growth, P nutrition and grain yield compared to all treatments. Importantly, PSB inoculation with ENRP resulted in significantly higher increase in soil CaCl 2 -P (8.91 mg P kg soil−1), citrate-P (26.98 mg P kg soil−1), enzyme-P (18.98 mg P kg soil−1), resin-P (11.41 mg P kg soil−1), and microbial-P (18.94 mg P kg soil−1), when compared to all treatment combinations. Although a decrease in soil HCl-P content was observed with both types of P fertilizer, significant differences were found only with PSB inoculation. A significant increase in soil biological P pools could be due to the higher specific area and crystalline structure of nano materials, providing increased number of active sites for PSB activity in the presence of biobased encapsulated shell. Furthermore, the increase in PSB abundance, higher root carboxylate secretions, and decreased rhizosphere pH in response to nano-structured P fertilizer, implies greater extension of rhizosphere promoting greater P mobilization and/or solubilization, particularly under PSB inoculated conditions. We conclude that cropping potential of abandoned agricultural lands can be enhanced by the use of nano-rock phosphate in combination with PSB inoculant, establishing a favorable micro-environment for higher plant growth and biochemical P fertility • A pot study was performed on recently reclaimed abandoned agricultural soil. • ENRP fertilizer is a highly promising controlled release P fertilizer than NRP. • ENRP-PSB fertilization efficiently establishes maize and improves productivity. • Nano-P fertilization extends rhizosphere and improves soil P biological pools. • ENRP-PSB is sustainable approach to enhance productive capacity of degraded soils. [ABSTRACT FROM AUTHOR]

Plant Growth Promoting Rhizobacteria (PGPR), whose role is still underestimated, plays an important (or perhaps essential) role in improving plant growth. The comprehensive understanding of bacterial plant growth promoting mechanism helps to get sustainable agriculture production under biotic and abiotic stresses. In the present study, plant growth promoting (PGP) bacterial strain Pseudomonas aeruginosa having maximum inhibitory concentration of 1500mgkg−1 against Zn was isolated from arable land, irrigated with industrial effluent and evaluated to determine it bioremediation potential. The study was mainly focused on plant biomass production, nutrient uptake and oxidative stress tolerance in relation to the activities of antioxidative enzymes and the content of non-enzymatic antioxidants. The oxidative stress tolerance was measured by estimating the MDA accumulation as well as H2O2 production in wheat plants under Zn (1000mgkg−1) stress and inoculation of soil with Zn resistant Pseudomonas aeruginosa. Zn in rooting medium reduced the plant growth, leaf photosynthetic pigments as well as uptake of N and P. However, content of MDA and H2O2 increased at higher concentration of Zn. Inoculation of P. aeruginosa improved the uptake of P and N in wheat plants with an increase in leaf chlorophyll, total soluble protein and plant biomass production. Analysis of plant root and shoot disclosed that Zn concentration was significantly lowered in P. aeruginosa inoculated zinc stressed plants as compare to the plants grown under Zn stress only. The amelioration of adverse effects of Zn stress on biomass production due to P. aeruginosa inoculation was related with enhanced antioxidative enzyme activities (SOD, POD and CAT), and the contents of non-enzymatic components such as ascorbic acid and total phenolics (TPC) as compare to Zn-treated plants. The up-gradation in antioxidative defense mechanism, resulted a reduction in H2O2 and MDA content due to the scavenging of ROS by antioxidants. It was concluded that P. aeruginosa is an ideal candidate for bioremediation and wheat growth promotion against Zn-induced oxidative stress by improving the availability of necessary nutrient, eliciting antioxidant defense system as well as by lowering the Zn metal uptake. [Copyright &y& Elsevier]

Silicon (Si) is generally considered as a benefic element for higher plants, especially for those grown under abiotic stressed environments. Current study is carried out in a hydroponic experiment to analyze the effect of Si application on barley growth, photosynthesis and ultra-structure under chromium (Cr) stress. The treatments consisted of three Si (0, 1 and 2mM) and two Cr (0 and 100μM) levels. The results showed that Si application at both levels enhanced plant growth relative to the control, and alleviated Cr toxicity, reflected by significant increase in growth and photosynthetic parameters, such as SPAD value, net photosynthetic rate (P n ), cellular CO2 concentration (C i ), stomatal conductance (G s ) and transpiration rate (T r ), and chlorophyll fluorescence efficiency (Fv/Fm), with 2mM Si having greater effect than 1mM Si. Cr stress caused ultra-structural disorders in leaves, such as uneven swelling of chloroplast, increased amount of plastoglobuli, disintegrated and disappeared thylakoid membranes, increased size and number of starch granules in leaves, and root ultra-structural modification, including increased vacuolar size, presence of Cr metal in cell walls and vacuoles, disruption and disappearance of nucleus. Exogenous Si alleviated these ultra-structural disorders both in roots and leaves. Apparently, Si and Cr behaved antagonistically, indicating that Si could be a candidate for Cr detoxification in crops under Cr-contaminated soil. [Copyright &y& Elsevier]

Greenhouse gaseous (GHGs) emissions from cropland soils are one of the major contributors to global warming. However, the extent and pattern of these climatic breakdowns are usally determined by the management practices in-place. The use of biochar on cropland soils holds a great promise for increasing the overall crop productivity. Nevertheless, biochar application to agricultural soils has grown in popularity as a strategy to off-set the negative feedback associated with agriculture GHGs emissions, i.e., CO 2 (carbon dioxide), CH 4 (methane), and N 2 O (nitrous oxide). Despite increasing efforts to uncover the potential of biochar to mitigate the farmland GHGs effects, there has been little synthesis of how different types of biochar affect GHGs fluxes from cropland soils under varied experimental conditions. Here, we presented a meta-analysis of the interactions between biochar and GHGs emissions across global cropland soils, with field experiments showing the strongest GHG mitigation potential, i.e. CO 2 (RR = −0.108) and CH 4 (RR = −0.399). The biochar pyrolysis temperature, feedstock, C: N ratio, and pH were also found to be important factors influencing GHGs emissions. A prominent reduction in N 2 O (RR = −0.13) and CH 4 (RR = −1.035) emissions was observed in neutral soils (pH = 6.6–7.3), whereas acidic soils (pH ≤ 6.5) accounted for the strongest mitigation effect on CO 2 compared to N 2 O and CH 4 emissions. We also found that a biochar application rate of 30 t ha−1 was best for mitigating GHGs emissions while achieving optimal crop yield. According to our meta-analysis, maize crop receiving biochar amendment showed a significant mitigation potential for CO 2 , N 2 O, and CH 4 emissions. On the other hand, the use of biochar had shown significant impact on the global warming potential (GWP) of total GHGs emissions. The current data synthesis takes the lead in analyzing emissions status and mitigation potential for three of the most common GHGs from cropland soils and demonstrates that biochar application can significantly reduce the emissions budget from agriculture. [Display omitted] • Biochar effects on croplands GHGs emissions were evaluated using a meta-analysis of 45 research articles. • Effectiveness of biochar to mitigate GHGs emissions from croplands relied on soil types and biochar properties. • Maize was the most responsive crop to biochar related GHGs mitigation. • Maximum mitigation effects were observed when biochar was applied at the rate of 30 t ha−1. [ABSTRACT FROM AUTHOR]

Given the significance of nitrogen (N) as the most constraining nutrient in agro-ecosystems, it is crucial to develop an updated model for N fertilizers management to achieve higher crop yields while minimizing the negative impacts on the environment. Coated urea is touted as one of the most important controlled-release N fertilizers used in agriculture to reduce cropland emissions and improve nitrogen use efficiency (NUE) for optimal crop yields. The sustainability of coated urea depends on the trade-offs between crop productivity, NUE and greenhouse gas emissions (CO 2 , CH 4 and N 2 O); however, role of various agro-edaphic factors in influencing these trade-offs remains unclear. To determine the effects of soil properties, climatic conditions, experimental conditions, and type of coated urea on greenhouse gas emissions, NH 3 losses, crop productivity, and NUE, we conducted a meta-analysis using data from 76 peer-reviewed studies. Our results showed that the application of coated urea under field conditions contributed to a greater reduction in N 2 O emissions (−48.67%) and higher NUE (58.72%), but crop yields were not significant. Across different climate regions, subtropical monsoon climate showed a perceptible mitigation for CO 2 , CH 4 and NH 3 (−78.38%; −83.33%; −27.46%), while temperate climate reduced N 2 O emissions by −70.36%. For different crops, only rice demonstrated reduction in CO 2 , CH 4 , N 2 O and NH 3 losses. On the other hand, our findings revealed a mitigating trade-off between CO 2 and CH 4 emissions on medium-textured soils and N 2 O emissions on fine-textured soils. A significant reduction in N 2 O and NH 3 losses was evident when coated urea was applied to soils with a pH > 5.5. Interestingly, application of coated urea to soils with higher C/N ratios increased NH 3 losses but showed a noticeable N 2 O reduction. We found that polymer-coated urea reduced CH 4 and N 2 O emissions and NH 3 losses at the expense of higher CO 2 emissions. Moreover, application of a lower dose of coated urea (0–100 kg N ha−1) enhanced CO 2 and CH 4 mitigation, while N 2 O mitigation increased linearly with increasing dose of coated urea. Most importantly, our results showed that the application of coated urea leads to a large mismatch between NUE, crop yields and greenhouse gas mitigation. By and large, the application of coated urea did not correspond with higher crop yields despite significant reduction in the emissions and improved NUE. Overall, these results suggest that site-specific agro-edaphic conditions should be considered when applying coated urea to reduce these emissions and N volatilization losses for increasing NUE and crop yields. • Coated urea led to greater NUE by reducing N 2 O emissions under field conditions. • Rice unequivocally exhibited significant GHGs mitigation and reduced NH 3 losses. • Coated urea reduced CO 2 -CH 4 emissions on medium and N 2 O on fine-textured soils. • Crop yields did not match the improved NUE and GHGs mitigation effects of coated urea. • Efficacy of coated urea fertilizers could depend on the given agro-edaphic conditions. [ABSTRACT FROM AUTHOR]

In semi-arid regions, soil phosphorus (P) dynamics in cereal-legume intercropping are not yet fully elucidated, particularly in relation to integrated application of fertilizers. To this aim, we investigate the effects of different fertilizers on various P fractions in relation to the rhizosphere-microbial processes in a cowpea/maize intercropping system. Field experiments were conducted during two consecutive years (2016–2017) in a split-plot design by establishing cowpea/maize alone or intercropped onto the main plot, while the sub-plot was treated with four types of fertilization, i.e. no fertilizer addition (control), organic amendment (compost), mineral fertilizers (NPK) and multi-nutrient enriched compost (NPKEC). Our results showed that NPKEC fertilizer increased NaHCO 3 -P i by 69% in maize, 62% in cowpea and 93% in intercropped plots compared to control plots. Similarly, a significant increase in the NaHCO 3 -P o fraction was also recorded with NPKEC treatment in all cropping systems. In case of moderately labile P, NPKEC fertilizer caused the highest increase of NaOH-P o (12.87 ± 0.50 mg P kg−1 soil) and NaOH-P i (22.29 ± 0.83 mg P kg−1 soil) fractions in intercropped plots. Except for intercropping, NPK application caused an increase in the non-available P fraction (HCl-P i), while the use of NPKEC decreased the HCl-P i concentration in all cropping systems, suggesting stronger merits both for intercropping and NPKEC. Surprisingly, maize exhibited substantially higher phosphatases activity compared to cowpea in monoculture amended with compost, implying distinct crop strategies for adaptation under low P conditions. Based on the multi-factor analysis, the close association of NaHCO 3 –P with P solubilizing bacteria, root carboxylates and pH indicated that rhizosphere processes are the strongest predictors of immediately available P. Since alkaline phosphatase (ALP) is a P-degrading enzyme of microbial origin, rhizosphere related ALP association may have originated from root-associated microflora promoting P mobilization. Furthermore, the strong association of microbial biomass P (MBP) and acid phosphates (ACP) with NaOH–P fraction indicated moderately available P cycle in soil was mainly driven by microbial-related processes. Factor analysis map and two-way ANOVA confirmed that fertilization regime had a stronger effect on all tested variables compared to cropping system. Altogether, our results suggest that a combination of microbial-rhizosphere processes controls the dynamics of P fertility in semi-arid soils. In the broader context of improving soil P fertility, it is highly recommended the use of environmentally sustainable sources of fertilizer, such as NPKEC, which can enhance the competitive performance of legume-cereal intercropping under semi-arid agroecosystems. Image 1 • Multi-nutrient compost (NPKEC) was tested for P fertility under cowpea-maize cropping. • In all cropping systems, NPKEC increased labile inorganic P ~62–93% than control. • Intercropping with NPKEC reduced non-available inorganic P by 29% over NPK fertilizers. • Type of fertilization steers intercropping related rhizosphere-microbial P processes. • Multi-nutrient compost enhanced competitiveness of intercrops in semi-arid agroecosystem. [ABSTRACT FROM AUTHOR]

Wastewater is an alternative, valuable and cost effective resource for irrigation in water-scarce arid and sami-arid regions of the world including Pakistan. Soils near urban centers are cultivated for vegetable and cash crops using untreated wastewater. Current study was performed with objectives of assessing impacts of untreated textile wastewater on some soil chemical, biological and enzymatic activities. The microcosm incubation study used a clay loam soil that received 0 (distilled-water), 25, 50 and 100% wastewater concentrations and incubated for 30 and 60 days under optimum temperature and moisture conditions. Soil respiration was measured periodically throughout the experiment over 60 days. After the incubation periods of 30- and 60-d, soils were destructively analyzed for pH, electrical conductivity (EC), water extractable organic matter (WEOM), microbial biomass carbon (MBC), microbial metabolic quotient ( q CO 2 ) and dehydrogenase enzymatic activity. Results revealed that wastewater and incubation time significantly altered chemical, biological and enzymatic properties of soils. The observed large surge in soil respiration, at initial stage, was stimulated by dissolved organic matter in wastewater. Dehydrogenase activity increased significantly with increasing wastewater concentrations. Increase in q CO 2 with wastewater concentration and incubation time suggested more stress to microorganisms but also enhanced microbial activity under stress to synthesize biomass. We found significant positive (R 2 = 0.64, p < 0.001) relationship between soil respiration and MBC, however, correlation between WEOM and MBC was significant negative (R 2 = 0.18, p < 0.01) indicating a dynamic mismatch between carbon substrate, soil respiration and buildup of MBC pool. Wastewater concentration and incubation time interaction had significant (p < 0.01) effect on WEOM suggesting that WEOM accumulated over time and comparatively less utilized by microorganisms. Short- and long-term effects of untreated wastewater on soil physico-chemical and biological health should be assessed before its use for crop production. [ABSTRACT FROM AUTHOR]

Contamination of agricultural soils with trace metals present lethal consequences in terms of diverse ecological and environmental problems that entail entry of metal in food chain, soil deterioration, plant growth suppression, yield reduction and alteration in microbial community. Metal polluted soils have become a major concern for scientists around the globe. Phytoremediation involves the hyperaccumulation of metals in different plant parts. Phytoremediation of metals from polluted soils could be enhanced through inoculation with metal resistant plant growth promoting (PGP) bacteria. These PGP bacteria not only promote plant growth but also enhance metal uptake by plants. There are a number of reports in the literature where PGP bacterial inoculation improves metal accumulation in different plant parts without influencing plant growth. Therefore, there is a need to select PGP bacterial strains which possess the potential to improve plant growth as well as expedite the phytoremediation of metals. In this review, we have discussed the mechanisms possessed by PGP bacteria to promote plant growth and phytoremediation of metals. The central part of this review deals with the recent advances in microbial assisted-phytoremediation of metals. [ABSTRACT FROM AUTHOR]

Organic amendments (animal manure and biochar) to agricultural soils may enhance soil organic carbon (SOC) contents, improve soil fertility and crop productivity but also contribute to global warming through nitrous oxide (N 2 O) emission. However, the effects of organic amendments on N 2 O emissions from agricultural soils seem variable among numerous research studies and remains uncertain. Here, eighty-five publications (peer-reviewed) were selected to perform a meta-analysis study. The results of this meta-analysis study show that the application of animal manure enhanced N 2 O emissions by 17.7%, whereas, biochar amendment significantly mitigated N 2 O emissions by 19.7%. Moreover, coarse textured soils increased [ l n R R ‾ = 182.6%, 95% confidence interval (CI) = 151.4%, 217.7%] N 2 O emission after animal manure, in contrast, N 2 O emission mitigated by 7.0% from coarse textured soils after biochar amendment. In addition, this study found that 121–320 kg N ha−1 and ⩽ 30 T ha−1 application rates of animal manure and biochar mitigated N 2 O emissions by 72.3% and 22.5%, respectively. Soil pH also played a vital role in regulating the N 2 O emissions after organic amendments. Furthermore, > 10 soil C: N ratios increased N 2 O emissions by 121.4% and 27.6% after animal and biochar amendments, respectively. Overall, animal manure C: N ratios significantly enhanced N 2 O emissions, while, biochar C: N ratio had not shown any effect on N 2 O emissions. Overall, average N 2 O emission factors (EFs) for animal manure and biochar amendments were 0.46% and −0.08%, respectively. Thus, the results of this meta-analysis study provide scientific evidence about how organic amendments such as animal manure and biochar regulating the N 2 O emission from agricultural soils. [Display omitted] • Organic amendments (animal manure and biochar) may increase SOC stocks but also influence N 2 O emissions. • Animal manure significantly increased N 2 O emission by 17.7%. • Biochar amendment significantly mitigated N 2 O emissions by 19.7%. • Maximum reduction was observed with medium doses of animal manure (121–320 kg N ha-1) and biochar (⩽ 30 T ha−1). [ABSTRACT FROM AUTHOR]