Low-dose biochar added to sediment improves water quality and promotes the growth of submerged macrophytes

Submerged plant-biochar composite system exhibits effective control over residual organic pollutants in the benthic organisms of aquaculture ponds

As of now, submerged plants and biochar have demonstrated significant benefits in aquaculture pond sediment remediation. However, there is limited research on the synergistic effects of biochar and submerged plants in mitigating hydrophobic organic contaminant (HOC) accumulation in aquaculture benthic organisms and in controlling the nutrient (nitrogen and phosphorus) levels in aquaculture water. This study assesses a submerged plant-biochar system's efficacy in removing HOCs from simulated freshwater aquaculture ponds. Vallisneria natans was planted in sediment with varying levels of wheat straw biochar, while Corbicula fluminea served as the targeted benthic organism. The bioaccumulation experiment identified the optimal biochar ratio for the Vallisneria natans-biochar system in controlling HOCs in aquaculture products. Analyses included final accumulation concentrations in benthic organisms, changes in freely-dissolved concentrations in aquaculture sediment, and a mass balance calculation to explore key factors in their removal from the system. Results indicated that the Vallisneria natans-1.5% biochar composite system achieved optimal control of HOCs in sediment and aquaculture products. Biochar addition to the sediment in the composite system demonstrated a "promotion with low addition, inhibition with high addition" effect on Vallisneria natans growth. Notably, the addition of 1.5% biochar (VN1.5 group) significantly promoted the growth of Vallisneria natans leaves and roots. Comparing the final pollutant proportions in different environmental media, concentrations in water (0.20%-1.8%), clam accumulation (0.032%-0.11%), and plant absorption (0.10%-0.44%) constituted a minimal portion of the overall pollutant load in the system. The majority of pollutants (24%-65%) were degraded in the aquaculture environment, with microbial degradation likely playing a predominant role. Bacterial phyla, particularly Proteobacteria and Firmicutes, were identified as potential direct contributors to pollutant degradation in the Vallisneria natans-biochar system.

Submersed macrophytes Vallisneria natans and Vallisneria spinulosa improve water quality and affect microbial communities in sediment and water columns

Healthy aquatic ecosystems are essential for human beings. However, anthropogenic activities severely worsen water quality. In this study, using assembling mesocosms, we developed an efficient and easy-to-handle method to monitor the water quality by measuring the electrical conductivity (EC) of water. Our data demonstrate that the growth of two submersed macrophytes, Vallisnerianatans and Vallisneria spinulosa, improves water quality by decreasing EC. Furthermore, using high-throughput DNA sequencing, we analyzed the microbial community abundance and structure in sediment and water columns with or without plant growth. We generated 33,775 amplicon sequence variants from 69 samples of four sediment groups (BkM, CtM, VnR, and VsR) and three water column sample groups (CtW, VnW, and VsW). The results show that the relative abundance of bacteria was higher in the sediment than in the water column. Moreover, the diversity and composition of microbiomes were altered by Vallisneria spp. growth, and the α-diversity of the microbial communities decreased due to submersed macrophytes in both the sediment and water columns. The β-diversity of the microbial communities also varied significantly with or without Vallisneria spp. growth for both the sediment and water columns.

Aquaculture sediments amended with biochar improved soil health and plant growth in a degraded soil

Sustainable and safe management of aquaculture sediments is of great concern. Biochar (BC) and fishpond sediments (FPS) are rich in organic carbon and nutrients and thus can be used as soil amendments; however, it is not fully explored how the biochar amended fishpond sediments can affect soil properties/fertility and modulate plant physiological and biochemical changes, particularly under contamination stress. Therefore, a comprehensive investigation was carried out to explore the effects of FPS and BC-treated FPS (BFPS) on soil and on spinach (Spinacia oleracea L.) grown in chromium (Cr) contaminated soils. Addition of FPS and BFPS to soil caused an increase in nutrients content and reduced Cr levels in soil, which consequently resulted in a significant increase in plant biomass, chlorophyll pigments, and photosynthesis, over the control treatment. The most beneficial effect was observed with the BFPS applied at 35 %, which further increased the antioxidant enzymes (by 2.75-fold, at minimum), soluble sugars by 24.9 %, and upregulated the gene expression activities. However, the same treatment significantly decreased proline content by 74.9 %, Malondialdehyde by 65.6 %, H₂O₂ by 65.1 %, and Cr concentration in spinach root and shoot tissues. Moreover, the average daily intake analysis showed that BFPS (at 35 %) could effectively reduce human health risks associated with Cr consumption of leafy vegetables. In conclusion, these findings are necessary to provide guidelines for the reutilization of aquaculture sediments as an organic fertilizer and a soil amendment for polluted soils. However, more future field studies are necessary to provide guidelines and codes on aquaculture sediments reutilization as organic fertilizer and soil amendment for polluted soils, aiming for a more sustainable food system in China and globally, with extended benefits to the ecosystem and human.

Responses of aquatic vegetables to biochar amended soil and water environments: a critical review

Aquatic vegetables, including lotus root, water spinach, cress, watercress and so on, have been cultivated as commercial crops for a long time. Though aquatic vegetables have great edible and medicinal values, the increasing demands for aquatic vegetables with high quality have led to higher requirements of their soil and water environments. Unfortunately, the soil and water environment often face many problems such as nutrient imbalance, excessive fertilization, and pollution. Therefore, a new cost-effective and eco-friendly solution for addressing the above issues is urgently required. Biochars, one type of pyrolysis product obtained from agricultural and forestry waste, show great potential in reducing fertilizer application, upgrading soil quality and remediating pollution. Application of biochars in aquatic vegetable cultivation would not only improve the yield and quality, but also reduce its edible risk. Biochars can improve the soil micro-environment, soil microorganism and soil enzyme activities. Furthermore, biochars can remediate the heavy metal pollution, organic pollution and nitrogen and phosphorus non-point source pollution in the water and soil environments of aquatic vegetables, which promotes the state of cultivation conditions and thereby improves the yield and quality of aquatic vegetables. However, the harmful substances such as heavy metals, PAHs, etc. derived from biochars can cause environmental risks, which should be seriously considered. In this review, the application of biochars in aquatic vegetable cultivation is briefly summarized. The changes of soil physicochemical and biological properties, the effects of biochars in remediating water and soil environmental pollution and the impacts of biochars on the yield and quality of aquatic vegetables are also discussed. This review will provide a comprehensive overview of the research progress on the effects of biochars on soil and water environments for aquatic vegetable cultivation.

Herbal plants- and rice straw-derived biochars reduced metal mobilization in fishpond sediments and improved their potential as fertilizers

Fishpond sediments are rich in organic carbon and nutrients; thus, they can be used as potential fertilizers and soil conditioners. However, sediments can be contaminated with toxic elements (TEs), which have to be immobilized to allow sediment reutilization. Addition of biochars (BCs) to contaminated sediments may enhance their nutrient content and stabilize TEs, which valorize its reutilization. Consequently, this study evaluated the performance of BCs derived from Taraxacum mongolicum Hand-Mazz (TMBC), Tribulus terrestris (TTBC), and rice straw (RSBC) for Cu, Cr, and Zn stabilization and for the enhancement of nutrient content in the fishpond sediments from San Jiang (SJ) and Tan Niu (TN), China. All BCs, particularly TMBC, reduced significantly the average concentrations of Cr, Cu, and Zn in the overlying water (up to 51% for Cr, 71% for Cu, and 68% for Zn) and in the sediments pore water (up to 77% for Cr, 76% for Cu, and 50% for Zn), and also reduced metal leachability (up to 47% for Cr, 60% for Cu, and 62% for Zn), as compared to the control. The acid soluble fraction accounted for the highest portion of the total content of Cr (43-44%), Cu (38-43%), and Zn (42-45%), followed by the reducible, oxidizable, and the residual fraction; this indicates the high potential risk. As compared with the control, TMBC was more effective in reducing the average concentrations of the acid soluble Cr (15-22%), Cu (35-53%), and Zn (21-39%). Added BCs altered the metals acid soluble fraction by shifting it to the oxidizable and residual fractions. Moreover, TMBC improved the macronutrient status in both sediments. This work provides a pathway for TEs remediation of sediments and gives novel insights into the utilization of BC-treated fishpond sediments as fertilizers for crop production.

Effects of biochar on the growth of Vallisneria natans in surface flow constructed wetland

To improve the nitrogen and phosphorus removal efficiency of surface flow constructed wetlands (SFCWs), biochar was added to an SFCW matrix. The effects of adding different amounts of biochar on water purification, the growth of Vallisneria natans (V. natans), and microbial mechanisms were explored through SFCW simulation experiments. The results showed that through the joint action of biochar and V. natans, the concentrations of total nitrogen, total phosphorus, and ammonia nitrogen in the effluent significantly decreased. The total biomass, relative growth rate, and chlorophyll content of V. natans were significantly reduced by adding biochar (≥20%, v/v), as the root activity and the root to leaf biomass ratio slightly increased at first and then decreased. The carbon and nitrogen contents of V. natans slightly increased with the addition of biochar (≥10%, v/v), but the phosphorus content slightly decreased. Moreover, the nitrogen content of the matrices decreased significantly over time (P<0.05), and the phosphorus content in the matrix showed an increasing trend in the same period. In addition, the microbial 16S rDNA sequencing results indicated that the diversity and abundance of the microbial community in the matrix of the biochar-added SFCW tended to decrease. Nevertheless, the abundance of functional bacteria related to nitrogen and phosphorus removal (i.e., Pseudomonas and Dechloromonas) slightly increased, which would benefit denitrification and dephosphorization in the SFCW. Hence, the addition of biochar to the SFCW matrix facilitated the improvement of effluent water quality, while excessive biochar addition (≥10%, v/v) restrained the growth of V. natans but did not cause death. This conclusion provides valid data support regarding the ability of biochar-added SFCW to purify lightly contaminated water.

Effects of bentonite on the growth process of submerged macrophytes and sediment microenvironment

The effects of clay mineral bentonite on the growth process of submerged macrophyte V. spiralis and sediment microenvironment were investigated in the study for the first time, aiming to determine whether it is suitable for application in the field of ecological restoration. The growth index, and physiological and biochemical index of V. spiralis in the experiments were measured once a month, and the changes of rhizosphere microorganisms and physicochemical properties of sediments were also studied at the same time. The results demonstrated that bentonite can effectively promote the growth of V. spiralis. The treatment groups of RB1/1 and MB1/5 (the mass ratios of bentonite to sediment were 1/1 and 1/5, respectively.) showed the best V. spiralis growth promotion rates which were 18.78%, and 11.79%, respectively. The highest microbial diversity and abundance existed in group of RB10 (the mass ratio of sediment to bentonite was 10/1), in which the OTUs, Shannon, Chao and Ace were 1521.0, 5.20, 1712.26, and 1686.31, respectively. Bentonite was conducive to the propagation of rhizosphere microorganisms, and further changed the physical and chemical properties of the sediment microenvironment. The nutrient elements dissolved from bentonite may be one of the main reasons that promoted the growth of V. spiralis. The purpose of this result is to prove that bentonite can be further applied as sediment improver and growing media in ecological restoration projects in eutrophic shallow lakes.