Water-fertilizer regulation drives microorganisms to promote iron, nitrogen and manganese cycling: A solution for arsenic and cadmium pollution in paddy soils.

The co-contamination of arsenic (As) and cadmium (Cd) in rice fields presents a global imperative for resolution. However, understanding the complex microbially driven geochemical processes and network connectivity crucial for As and Cd bioavailability under the frequent redox transitions in rice fields remains limited. Here, we conducted a series of microcosm experiments, using flooding and drainage, alongside fertilization treatments to emulate different redox environment in paddy soils. Soil As significantly reduced in drained conditions following applications of biochar or calcium-magnesium-phosphate (CMP) fertilizers by 26.3 % and 31.2 %, respectively, with concurrent decreases in Cd levels. Utilizing geochemical models, we identified the primary redox cycles dynamically altering during flooding (Fe and S cycles) and drainage (Fe, Mn, and N cycles). PLS-SEM elucidated 76 % and 61 % of the variation in Cd and As through Mn and N cycles. Functional genes implicated in multi-element cycles were analyzed, revealing a significantly higher abundance of assimilatory N reduction genes (nasA, nirA/B, narB) in drained soil, whereas an increase in ammonia-oxidizing genes (amoA/B) and a decrease in nitrate reduction to ammonium genes were observed after CMP fertilizer application. Biochar application led to significant enrichment of the substrate-binding protein of the Mn transport gene (mntC). Moreover, Fe transport genes were enriched after biochar or CMP application compared to drained soils. Among 40 high-quality metagenome-assembled genomes (MAGs), microbial predictors associated with low Cd and As contents across different treatments were examined. Bradyrhizobacea harbored abundant Mn and FeIII transport genes, while Nitrososphaeraceae carried nitrification-related genes. Two MAGs affiliated with Caulobacteraceae, carrying diverse Fe transport genes, were enriched in biochar-applied soils. Therefore, applying CMP fertilizer or biochar in aerobic rice fields can synergistically reduce the bioavailability of Cd and As by specifically enhancing the circulation of essential elements.

Polyphosphate fertilizer impacts the enzymatic and non-enzymatic antioxidant capacity of wheat plants grown under salinity.

By 2050, the predicted global population is set to reach 9.6 billion highlighting the urgent need to increase crop productivity to meet the growing demand for food. This is becoming increasingly challenging when soils are saline and/or deficient in phosphorus (P). The synergic effect of P deficiency and salinity causes a series of secondary stresses including oxidative stress. Reactive Oxygen Species (ROS) production and oxidative damage in plants caused either by P limitation or by salt stress may restrict the overall plant performances leading to a decline in crop yield. However, the P application in adequate forms and doses could positively impact the growth of plants and enhances their tolerance to salinity. In our investigation, we evaluated the effect of different P fertilizers forms (Ortho-A, Ortho-B and Poly-B) and increasing P rates (0, 30 and 45 ppm) on the plant's antioxidant system and P uptake of durum wheat (Karim cultivar) grown under salinity (EC = 3.003 dS/m). Our results demonstrated that salinity caused a series of variations in the antioxidant capacity of wheat plants, at both, enzymatic and non-enzymatic levels. Remarkably, a strong correlation was observed between P uptake, biomass, various antioxidant system parameters and P rates and sources. Soluble P fertilizers considerably enhanced the total plant performances under salt stress compared with control plants grown under salinity and P deficiency (C+). Indeed, salt-stressed and fertilized plants exhibited a robust antioxidant system revealed by the increase in enzymatic activities of Catalase (CAT) and Ascorbate peroxidase (APX) and a significant accumulation of Proline, total polyphenols content (TPC) and soluble sugars (SS) as well as increased biomass, Chlorophyll content (CCI), leaf protein content and P uptake compared to unfertilized plants. Compared to OrthoP fertilizers at 45 ppm P, Poly-B fertilizer showed significant positive responses at 30 ppm P where the increase reached + 18.2% in protein content, + 156.8% in shoot biomass, + 93% in CCI, + 84% in shoot P content, + 51% in CAT activity, + 79% in APX activity, + 93% in TPC and + 40% in SS compared to C+. This implies that PolyP fertilizers might be an alternative for the suitable management of phosphorus fertilization under salinity.

Photosynthetic performance and nutrient uptake under salt stress: Differential responses of wheat plants to contrasting phosphorus forms and rates.

Salt stress impacts phosphorus (P) bioavailability, mobility, and its uptake by plants. Since P is involved in many key processes in plants, salinity and P deficiency could significantly cause serious damage to photosynthesis, the most essential physiological process for the growth and development of all green plants. Different approaches have been proposed and adopted to minimize the harmful effects of their combined effect. Optimising phosphorus nutrition seems to bring positive results to improve photosynthetic efficiency and nutrient uptake. The present work posed the question if soluble fertilizers allow wheat plants to counter the adverse effect of salt stress. A pot experiment was performed using a Moroccan cultivar of durum wheat: Karim. This study focused on different growth and physiological responses of wheat plants grown under the combined effect of salinity and P-availability. Two Orthophosphates (Ortho-A & Ortho-B) and one polyphosphate (Poly-B) were applied at different P levels (0, 30 and 45 ppm). Plant growth was analysed on some physiological parameters (stomatal conductance (SC), chlorophyll content index (CCI), chlorophyll a fluorescence, shoot and root biomass, and mineral uptake). Fertilized wheat plants showed a significant increase in photosynthetic performance and nutrient uptake. Compared to salt-stressed and unfertilized plants (C+), CCI increased by 93%, 81% and 71% at 30 ppm of P in plants fertilized by Poly-B, Ortho-B and Ortho-A, respectively. The highest significant SC was obtained at 45 ppm using Ortho-B fertilizer with an increase of 232% followed by 217% and 157% for both Poly-B and Ortho-A, respectively. The Photosynthetic performance index (PItot) was also increased by 128.5%, 90.2% and 38.8% for Ortho-B, Ortho-A and Poly B, respectively. In addition, Poly-B showed a significant enhancement in roots and shoots biomass (49.4% and 156.8%, respectively) compared to C+. Fertilized and salt-stressed plants absorbed more phosphorus. The P content significantly increased mainly at 45 ppm of P. Positive correlations were found between phosphorus uptake, biomass, and photosynthetic yield. The increased photochemical activity could be due to a significant enhancement in light energy absorbed by the enhanced Chl antenna. The positive effect of adequate P fertilization under salt stress was therefore evident in durum wheat plants.

Human biomonitoring survey (Pb, Cd, As, Cu, Zn, Mo) for urban gardeners exposed to metal contaminated soils.

Eighty eight adult gardeners and their relatives volunteered to provide urine and blood samples for a human biomonitoring survey among users of one of the biggest allotment garden from Wallonia, showing high trace metal(oid) concentrations in soils. The purpose was to determine if environmental levels of lead (Pb), cadmium (Cd) and arsenic (As) led to concentrations of potential health concern in the study population. Blood and urine biomarkers were compared to reference and intervention cut-off values selected from the literature. The study population exhibited (i) moderately high blood lead levels with median value of 23.1 µg/L, (ii) high urinary concentrations of speciated As (inorganic arsenic and its metabolites) with a median value of 7.17 µg/g.cr., i.e. twice the median values usually observed in general populations, and (iii) very high Cd levels in urine with a median value of 1.23 µg/L, in the range of 95th-97.5th percentiles measured in general adult populations. Biomarker levels in the study population were also mostly above those measured in adults from local populations living on contaminated soils, as reported in the current literature. All biomarkers of Pb, Cd and As showed weak to strong statistically significant correlations, pointing towards a joint environmental source to these three contaminants as being at least partially responsible for the high exposure levels observed. Urine and blood biomarkers show statistically significant associations with variables related to individual characteristics (age, smoking status, …) and Pb domestic sources (Pb pipes, cosmetics, …) but involves also behavioral and consuming habits related to gardening activities on the contaminated allotment garden. At such levels, owing to co-exposure and additive effects of Cd, As and Pb regarding renal toxicity known from literature, the study strongly suggests that this population of gardeners is at risk with respect to chronic kidney diseases.

Influence of edaphic conditions and nitrogen fertilizers on cadmium and zinc phytoextraction efficiency of Noccaea caerulescens.

The success of cadmium phytoextraction operations with Noccaea caerulescens varies by a factor of 70 between sites of trials. However, soil factors driving the efficiency of cadmium (Cd) and zinc (Zn) phytoextraction are still poorly understood, as are the effects of nitrogen fertilizers. We studied biomass production and Cd and Zn uptake by two contrasting populations of N. caerulescens, Ganges (metallicolous) and Wilwerwiltz (non-metallicolous) grown in pots on a range of 24 field contaminated soils for 20 weeks. The addition of KNO3 and NH4NO3 fertilizers was also tested. Using model averaging of multiple regression models, we show that the major drivers of N. caerulescens growth are physical soil factors such as organic matter and soil bulk density while trace metal accumulation mainly relies on soil Cd and Zn exchangeable concentrations. We confirm the negative effect of soil copper (Cu) on growth, even at exchangeable concentrations below 30 mg kg-1, and therefore on uptake efficiency, while increasing soil lead (Pb) content was related to increased biomass probably due to a protective effect against soil pathogens. Finally, there is a small positive effect of nitrogen fertilization on biomass production only in soils with low initial nitrogen content (under 25 µg g-1 NO3-), while above this value, the positive impact of initial nitrogen content is offset by lower shoot Cd and Zn concentrations. Our data bring substantial information regarding the physico-chemical properties to ensure N. caerulescens growth: a soil bulk density under 1.05 kg/dm3, organic matter above 7% and pH under 7.5. We show that phytoextraction efficiency is maximal for moderate soil contamination in Cd (2-10 mg kg-1) and Zn (300-1000 mg kg-1).

Determination of zinc, cadmium and lead bioavailability in contaminated soils at the single-cell level by a combination of whole-cell biosensors and flow cytometry.

Zinc, lead and cadmium are metallic trace elements (MTEs) that are widespread in the environment and tend to accumulate in soils because of their low mobility and non-degradability. The purpose of this work is to evaluate the applicability of biosensors as tools able to provide data about the bioavailability of such MTEs in contaminated soils. Here, we tested the genetically-engineered strain Escherichia coli pP(ZntA)gfp as a biosensor applicable to the detection of zinc, lead and cadmium by the biosynthesis of green fluorescent protein (GFP) accumulating inside the cells. Flow cytometry was used to investigate the fluorescence induced by the MTEs. A curvilinear response to zinc between 0 and 25 mg/L and another curvilinear response to cadmium between 0 and 1.5 mg/L were highlighted in liquid media, while lead did not produce exploitable results. The response relating to a Zn2+/Cd2+ ratio of 10 was further investigated. In these conditions, E. coli pP(ZntA)gfp responded to cadmium only. Several contaminated soils with a Zn2+/Cd2+ ratio of 10 were analyzed with the biosensor, and the metallic concentrations were also measured by atomic absorption spectroscopy. Our results showed that E. coli pP(ZntA)gfp could be used as a monitoring tool for contaminated soils being processed.