Effect of pH and added slag on the extractability of Si in two Si-deficient sugarcane soils

Pressurized electro-osmotic dewatering treatment of sludge: focusing on the influences on nutrients for agricultural application

Sewage sludge requires effective dewatering and high nutrients retention before disposal for agricultural application. Pressurized electro-osmotic dewatering (PEOD) process with low energy consumption can effectively remove water from sludge, but the influences of PEOD process on nutrients for agricultural application still lacks in-depth research. In this study, the influences of PEOD process on nutrients for agricultural application were investigated, including organic matter, nitrogen, phosphorus, potassium and silicon contents. Layered experiments were conducted to investigate the layered variation of nutrients in sludge and to understand the potential change mechanisms. The experimental results showed that PEOD process caused small losses (<10%) of organic matter and total phosphorus (TP) in sludge, but caused 11.2-18.4% loss of total nitrogen (TN). PEOD process also caused 18.6-27.0% loss of total potassium (TK) and over 80% loss of available potassium in sludge, and could weaken the potential salt damage during the agricultural application of sludge. Furthermore, the available phosphorus content of sludge in the anode area increased significantly after the PEOD process, indicating that PEOD process could enhance the phosphorus bioavailability of sludge in the anode area. Besides, PEOD process caused a slight loss of silicon components in sludge, but improved the long-term silicon dissolution and release ability of sludge. This work could expand the knowledge about the influences of PEOD process on sludge nutrients and provide scientific guidance for the agricultural application of PEOD sludge.

Quantitative relationship between soil pH and electrical conductivity values and cadmium phytoavailability for Chinese cabbage under simulated conditions

Pot experiments were conducted to investigate the impacts of continuous addition of different concentrations of calcium chloride (CaCl2) and/or low-molecular-weight organic acids (LMWOAs) on soil pH, electrical conductivity (EC), and cadmium (Cd) transformation. These factors subsequently affected Cd phytoavailability in a system consisting of Cd-contaminated soil and Chinese cabbage (Brassica chinensis L.). The results indicate that CaCl2 addition had a greater impact on reducing soil pH value, increasing soil EC value, and enhancing Cd phytoaccumulation in Chinese cabbage compared to LMWOAs. When soil pH dropped by 0.3 unit and the soil EC increased by 500 µS cm-1, the Cd concentration in the Chinese cabbage shoots was 3 times higher than that in the control group. Throughout two planting terms of Chinese cabbage, the addition of CaCl2 (1.6-3.2 g kg-1) and LMWOAs (≤ 1.0 g kg-1) led to phytoextracted Cd concentration exceeding exchangeable Cd concentration in soil samples before the pot experiment. Regarding phytoextracted Cd, desorption from carbonate-bound Cd contributes more than desorption from bound to organic matter Cd and adsorption to Fe/Mn oxide Cd. This study underscores the influence of soil pH and EC value variations and Cd transformation on Cd phytoavailability. Special attention should be given to leafy vegetables grown in Cd-contaminated soil, as the phytoavailable Cd concentration reaches approximately 2.0 µg kg-1, which may lead to Cd levels surpassing acceptable limits for Chinese cabbage.

Identification of cadmium phytoavailability in response to cadmium transformation and changes in soil pH and electrical conductivity

Owing to complex changes in the soil environment, determining cadmium (Cd) phytoavailability is challenging. We devised a soil-wheat system to monitor alterations in soil pH, electrical conductivity (EC), and Cd transformation under various rates of calcium chloride and/or low-molecular-weight organic acids (LMWOAs) addition. The findings indicate that decreasing soil pH value, increasing soil EC value, and Cd transformation affect the phytoextraction of Cd. The exchangeable Cd and transformation of Cd under shifts in soil pH and EC contribute differentially to the phytoextracted Cd. The level of potentially phytoavailable Cd was identified through complete wheat cultivation in which the soil pH decreased by 0.47 unit and soil EC increased by 600-1000 μS cm-1, resembling the concentration of 0.01 M LMWOAs extractable Cd, when transitioning from paddy to dryland soil. Based on considering the phytoextracted Cd as the phytoavailable Cd throughout a complete wheat growth term, the threshold for phytoavailable Cd in soil, ensuring the safety of wheat grain (limit: 0.1 mg kg-1), is determined to be 2.90 μg kg-1. Maintaining control over Cd phytoavailability in soil emerges as the key factor in ensuring the safety of wheat grain cultivation.

Silicon-mediated alleviation of cadmium toxicity in soil-plant system: historical review

The contamination of crops by Cd is a worldwide problem that needs to be addressed for minimizing risk for human health. Today, numerous investigations have demonstrated that Si plays a role in reducing Cd toxicity and accumulation in cultivated plants. The evolution of scientific understanding - the Cd behavior in soil and in plant is discussed for the first time. Our analysis evidences that the research on Si-Cd interactions in the soil-plant system has quickened only in recent years, although basic interactions between silicic acid and Cd cations in aqueous systems were studied over 40-50 years ago. Today, numerous direct and indirect mechanisms of the Si impact on mobility and translocation of Cd in soil and in plants are reported. More productive studies in this area are those that considered the soil-plant system as a whole. Analysis of the development of the Cd-Si-related ideas suggests the prospects of further studies aimed at finding synergetic action of Si and other substances on Cd behavior in the soil-plant system.

Chitosan/silica: A hybrid formulation to mitigate phytopathogens

Long-term and indiscriminate use of synthetic pesticides to mitigate plant pathogens have created serious issues of water health, soil contamination, non-target organisms, resistant species, and unpredictable environmental and human health hazards. These constraints have forced scientists to develop alternative plant disease management strategies to reduce synthetic chemical' dependency. During the last 20 years, biological agents and resistance elicitors have been the most important used alternatives. Silica-based materials/chitosan with a dual mode of action have been proposed as promising alternatives to prevent plant diseases through direct and indirect mechanisms. Moreover, the combined application of nano-silica and chitosan, due to their controllable morphology, high loading capacity, low toxicity, and efficient encapsulation, act as suitable carriers for biological agents, pesticides, and essential oils, making them proper candidates for mitigation of phytopathogens. Based on this potential, this literature study reviewed the silica and chitosan properties and their function in the plant. It also assessed their role in the fighting against soil and aerial phytopathogens, directly and indirectly, as novel hybrid formulations in future managing platforms.

Regulating soil bacterial diversity, community structure and enzyme activity using residues from golden apple snails

It has been shown that the golden apple snail (GAS, Pomacea canaliculata), which is a serious agricultural pest in Southeast Asia, can provide a soil amendment for the reversal of soil acidification and degradation. However, the impact of GAS residue (i.e., crushed, whole GAS) on soil bacterial diversity and community structure remains largely unknown. Here, a greenhouse pot experiment was conducted and 16S rRNA gene sequencing was used to measure bacterial abundance and community structure in soils amended with GAS residue and lime. The results suggest that adding GAS residue resulted in a significant variation in soil pH and nutrients (all P < 0.05), and resulted in a slightly alkaline (pH = 7.28-7.75) and nutrient-enriched soil, with amendment of 2.5-100 g kg^-1 GAS residue. Soil nutrients (i.e., NO₃-N and TN) and TOC contents were increased (by 132-912%), and some soil exocellular enzyme activities were enhanced (by 2-98%) in GAS residue amended soil, with amendment of 1.0-100 g kg^-1 GAS residue. Bacterial OTU richness was 19% greater at the 2.5 g kg^-1 GAS residue treatment than the control, while it was 40% and 53% lower at 100 g kg^-1 of GAS residue and 50 g kg^-1 of lime amended soils, respectively. Firmicutes (15-35%) was the most abundant phylum while Bacterioidetes (1-6%) was the lowest abundant one in GAS residue amended soils. RDA results suggest that the contents of soil nutrients (i.e., NO₃-N and TN) and soil TOC explained much more of the variations of bacterial community than pH in GAS residue amended soil. Overuse of GAS residue would induce an anaerobic soil environment and reduce bacterial OTU richness. Soil nutrients and TOC rather than pH might be the main factors that are responsible for the changes of bacterial OTU richness and bacterial community structure in GAS residue amended soil.

Using golden apple snail to mitigate its invasion and improve soil quality: a biocontrol approach

The invasive and widespread golden apple snail (GAS, Pomacea canaliculata) is a harmful crop pest in many parts of Asia. The heavy use of molluscicides to control GAS could result in soil and water pollution as well as in loss of biodiversity. A sustainable and pollution-free control method is urgently needed to counteract this invasion. In this study, we proposed using dried and powdered GAS residue to neutralize and fertilize soils. We compared the effects of adding GAS residue (i.e., ground GAS shell and meat residue) to the effects of adding lime upon soil properties and microbes in a greenhouse pot experiment. Each pot was incubated for 120 days, and soil pH, nutrients, microbial species, and enzyme activity were assessed. Results showed that addition of GAS residue significantly improved soil pH, contents of total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), and available nitrogen but decreased soil available phosphorus (AP) content due to phosphorus sorption induced by soil organic matter (OM) and high pH. The GAS residue added to soil released nutrients and alleviated soil acidity, as well as provided more resources to soil microbes to increase their bioactivity, although lime addition was better at mitigating soil acidity. We found that with added GAS residue of 25 g kg^-1, the soil nitrate nitrogen (NO₃-N) content increased by 10 times; microbial biomass increased by 43%; and enzyme activity of β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, and β-D-cellobiosidase also were enhanced, compared to the control. Our findings suggest that GAS residue functions well as a fertilizer and soil amendment to aid the remediation of barren and acidic soils, making it a valuable and useful option in the control of the invasive GAS.

Silicon and Plant Natural Defenses against Insect Pests: Impact on Plant Volatile Organic Compounds and Cascade Effects on Multitrophic Interactions

Environmental factors controlling silicon (Si) accumulation in terrestrial plant are key drivers to alleviate plant biotic stresses, including insect herbivory. While there is a general agreement on the ability of Si-enriched plant to better resist insect feeding, recent studies suggest that Si also primes biochemical defense pathways in various plant families. In this review, we first summarize how soil parameters and climate variables influence Si assimilation in plants. Then, we describe recent evidences on the ability of Si to modulate plant volatile emissions, with potential cascade effects on phytophagous insects and higher trophic levels. Even though the mechanisms still need to be elucidated, Si accumulation in plants leads to contrasting effects on the levels of the three major phytohormones, namely jasmonic acid, salicylic acid and ethylene, resulting in modified emissions of plant volatile organic compounds. Herbivore-induced plant volatiles would be particularly impacted by Si concentration in plant tissues, resulting in a cascade effect on the attraction of natural enemies of pests, known to locate their prey or hosts based on plant volatile cues. Since seven of the top 10 most important crops in the world are Si-accumulating Poaceae species, it is important to discuss the potential of Si mobility in soil-plant systems as a novel component of an integrated pest management.

Silicon ameliorates iron deficiency of cucumber in a pH-dependent manner

Strategy I plants may respond to iron (Fe) deficiency by rhizosphere acidification. Here, the role of medium pH-values in silicon (Si)-induced mitigation Fe deficiency in Strategy I plants (Cucumis sativus) was investigated, particularly the metabolites regulated by a lack of Fe, using a target metabolomics approach. Plants were grown hydroponically, either with (+Fe) or in Fe-free (-Fe) nutrient solution, with (+Si) or without (-Si) a Si supply. The nutrient solution was adjusted to pH 5.0 or 6.0 and checked daily. Leaf metabolites potentially involved in Fe transport were determined. The typical Fe responses of cucumber (e.g., decrease in leaf chlorophyll, Fe imbalance) were more pronounced when plants were grown at pH 6.0 than 5.0, during long-term Fe deficiency (15 days). Major metabolites up-regulated by Fe deficiency and found in young leaf were succinic, citric and glutamic acids, respectively; their maximal concentrations occurred in Fe-starved plants grown at pH 6.0 without Si supply. Silicon (Si)-induced effects accompanied with alleviation chlorosis symptoms, were most distinct in plants grown at pH 6.0 for an extended period without Fe. Changes in abundance of metabolites specifically up-regulated by a lack of Fe may be manifested before any Si-induced changes in plant Fe content were apparent, suggesting that metabolite responses are highly sensitive to a Fe-dependent signal altered by Si treatments under Fe deficiency. The results indicate that Si supply was more evident when plants were more stressed by an increase in nutrient solution pH under Fe-limited conditions.