This study aimed to use organic fertilizers, e.g., compost and manures, and a halophytic plant [wavy-leaved saltbush (Atriplex undulata)] to remediate an agricultural soil polluted with toxic elements. Compost or manure (1% w/w) was added to a polluted soil in a pot trial. The application of the organic fertilizer, whether compost or manure, led to a significant improvement in the growth of the tested plant. From the physiological point of view, the application of organic fertilizers to polluted soil significantly increased the content of chlorophyll, carotenoid, and proline and, furthermore, led to a clear decrease in malondialdehyde (MDA) in the plant leaves. The highest significant values of organic carbon in the polluted soil (SOC) and cation exchange capacity (CEC) were found for the soil amended by compost and planted with wavy-leaved saltbush. Manure significantly reduced the soil pH to 7.52. Compost significantly decreased Zn, Cu, Cd, and Pb availability by 19, 8, 12, and 13%, respectively, compared to the control. On the other hand, manure increased Zn, Cu, Cd, and Pb availability by 8, 15, 18, and 14%, respectively. Compost and manure reduced the bioconcentration factor (BCF) and translocation factor (TF) of Cd and Pb. Compost was more effective in increasing the phytostabilization of toxic metals by wavy-leaved saltbush plants compared to manure. The results of the current study confirm that the application of non-decomposed organic fertilizers to polluted soils increases the risk of pollution of the ecosystem with toxic elements. The cultivation of contaminated soils with halophytic plants with the addition of aged organic materials, e. g., compost, is an effective strategy to reduce the spreading of toxic metals in the ecosystem, thus mitigating their introduction into the food chain.

In recent years, with the frequent global occurrence of harmful algal blooms, the use of plant allelopathy to control algal blooms has attracted special and wide attention. This study validates the possibility of turning water dropwort into a biological resource to inhibit the growth of harmful Microcystis aeruginosa blooms via allelopathy. The results revealed that there were 33 types of allelopathic compounds in the water dropwort culture water, of which 15 were phenolic acids. Regarding water dropwort itself, 18 phenolic acids were discovered in all the organs of water dropwort via a targeted metabolomics analysis; they were found to be mainly synthesized in the leaves and then transported to the roots and then ultimately released into culture water where they inhibited M. aeruginosa growth. Next, three types of phenolic acids synthesized in water dropwort, i.e., benzoic, salicylic, and ferulic acids, were selected to clarify their inhibitory effects on the growth of M. aeruginosa and their mechanism(s) of action. It was found that the inhibitory effect of phenolic acids on the growth of M. aeruginosa increased with the increase of the exposure concentration, although the algae cells were more sensitive to benzoic acid than to salicylic and ferulic acids. Further study indicated that the inhibitory effects of the three phenolic acids on the growth of M. aeruginosa were largely due to the simultaneous action of reducing the number of cells, damaging the integrity of the cell membrane, inhibiting chlorophyll a (Chl-a) synthesis, decreasing the values of F₀ and F_v/F_m, and increasing the activity of the antioxidant enzymes (SOD, POD, and CAT) of M. aeruginosa. Thus, the results of this study indicate that both culture water including the rich allelochemicals in water dropwort and biological algae inhibitors made from water dropwort could be used to control the growth of noxious algae in the future.

An RND (resistance–nodulation–division)-type transporter having the capacity to export free fatty acids (FFAs) was identified in the cyanobacterium Synechococcus elongatus strain PCC 7942 during characterization of a mutant strain engineered to produce FFAs. The basic strategy for construction of the FFA-producing mutant was a commonly used one, involving inactivation of the endogenous acyl-acyl carrier protein synthetase gene (aas) and introduction of a foreign thioesterase gene (‘tesA), but a nitrate transport mutant NA3 was used as the parental strain to achieve slow, nitrate-limited growth in batch cultures. Also, a nitrogen-regulated promoter P_nirA was used to drive ‘tesA to maximize thioesterase expression during the nitrate-limited growth. The resulting mutant (dAS2T) was, however, incapable of growth under the conditions of nitrate limitation, presumably due to toxicity associated with FFA overproduction. Incubation of the mutant culture under the non-permissive conditions allowed for isolation of a pseudorevertant (dAS2T-pr1) capable of growth on nitrate. Genome sequence and gene expression analyses of this strain suggested that expression of an RND-type efflux system had rescued growth on nitrate. Targeted inactivation of the RND-type transporter genes in the wild-type strain resulted in loss of tolerance to exogenously added FFAs including capric, lauric, myristic, oleic and linolenic acids. Overexpression of the genes in dAS2T, on the other hand, enhanced FFA excretion and cell growth in nitrate-containing medium, verifying that the genes encode an efflux pump for FFAs. These results demonstrate the importance of the efflux system in efficient FFA production using genetically engineered cyanobacteria.

Elucidating the stoichiometry and resorption patterns of multiple nutrients is of essential importance to holistically understanding plant nutrition and biogeochemical cycling. Although many studies on ecological stoichiometry have been carried out, surprisingly few of them were simultaneously done on the investigation of both nutrient resorption efficiency and stoichiometry for different-aged plantations of a perennial tree. Here, both green and senesced leaf samples were collected from four Larix kaempfer plantations aged of 8, 15, 22, and 32 years in the Qinling Mountains to examine nutrients resorption efficiency and stoichiometry characteristics. The results suggested that the nutrient concentrations of N, P, K, Ca, Mg, Al, and Fe in both green and senesced leaves of L. kaempferi (a deciduous conifer tree) did not show a regular change trends along the plantation ages in the Qinling Mountains. The concentrations of the most nutrients examined, except for Fe, in the green leaves were relatively lower than or close to the required physiological concentrations, suggesting a relative limitation of multiple nutrients exists in L. kaempferi for its above-ground biomass growth. The rank order of resorption efficiencies of four key nutrients (N, P, K, and Mg) was K (80.89%) > N (67.42%) > P (65.34%) > Mg (41.16%), whereas the nutrient Ca and Fe tended to accumulate in senesced leaves. Overall, the nutrient resorption efficiency of all examined elements did not exhibit a regular trend corresponding to the change of the plantation ages in L. kaempferi, but it was positively related to the nutrient concentrations in green leaves. The mean C:N and C:P ratios in the green and senesced leaves were significantly higher than those reported globally (on average). By contrast, the N:P ratio, at <14, was not only much lower than that reported for both China’s flora and globally (on average), but it did suggest that the N nutrient limits growth of L. kaempferi in these plantations. Taken together, the results of this study are of substantial interest and value to forest managers and for the sustainable development of the Qinling forest ecosystems.

The utilization of livestock waste has attracted increasing attention in recent years. The presence of high levels of heavy metals is a major obstacle to the utilization of biogas as a fertilizer resource. In this study, the heavy metal contents in biogas residue, slurry, and discharged sewage from three representative farms of gooseries, henneries, and dairy farms in the Yangtze River Basin were investigated and assessed. The results demonstrated that heavy metals, including Cd, Mn, As, Cu, Pb, Cr, Zn, etc., could be detected in all biogas residues, with significantly different contents between farm types (p < 0.005). Specifically, biogas residues from the goosery and the dairy farms met “China’s Organic Fertilizer Standards” (COF Standards); however, Cd concentrations in biogas residues from hennery farms exceeded the limits by five times. The concentrations of Cd and Pb in biogas slurries from all of the farms exceeded the limits of the “China Farmland Irrigation Water Quality Standard” (CFIWQ Standard). In particular, the Pb concentrations in biogas slurry from the dairy farms exceeded the limits by 29 times, and the discharged sewage from all three farm types complied with the comprehensive sewage discharge standards in China; however, only that from the goosery farms was suitable for irrigation. Thus, it is recommended to increase the feed selection, biogas engineering, and biological-purification-supporting technology, and to carry out regular sampling inspections of the biogas residue, slurry, and discharged sewage for heavy metals, so that environmental and crop pollution risks can be reduced when they are used as sources of nutrients for eco-friendly agriculture.

Farmlands fertilized with livestock manure-derived amendments have become a hot topic in the dissemination of antibiotic resistance genes (ARGs). Field ponding water connects rice paddies with surrounding water bodies, such as reservoirs, rivers, and lakes. However, there is a knowledge gap in understanding whether and how manure-borne ARGs can be transferred from paddy soil into field ponding water. Our studies suggest that the manure-derived ARGs aadA1, bla1, catA1, cmlA1-01, cmx(A), ermB, mepA and tetPB-01 can easily be transferred into field ponding water from paddy soil. The bacterial phyla Crenarchaeota, Verrucomicrobia, Cyanobacteria, Choloroflexi, Acidobacteria, Firmicutes, Bacteroidetes, and Actinobacteria are potential hosts of ARGs. Opportunistic pathogens detected in both paddy soil and field ponding water showed robust correlations with ARGs. Network co-occurrence analysis showed that mobile genetic elements (MGEs) were strongly correlated with ARGs. Our findings highlight that manure-borne ARGs and antibiotic-resistant bacteria in paddy fields can conveniently disseminate to the surrounding waterbodies through field ponding water, posing a threat to public health. This study provides a new perspective for comprehensively assessing the risk posed by ARGs in paddy ecosystems.