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This study conducted a large root box experiment with Schefflera arboricola monoculture, Neolamarckia cadamba monoculture, and co-planting of Schefflera arboricola and Neolamarckia cadamba. The dynamic changes in Cu, Zn, Cd, and Hg contents in fresh and litter leaves of Schefflera arboricola and Neolamarckia cadamba and their litter leaves yield were analyzed for three months (September, October, and November 2020) after the surface application of 2% (W/W) sewage sludge (SS). The relationship between the heavy metal contents of fresh and litter leaves and the changes in the heavy metal return amount in litter leaves were further analyzed. The results were as follows: (1) N. cadamba had significantly higher Cu contents in fresh and litter leaves than those of S. arboricola, while had significantly lower Zn and Cd contents than those of S. arboricola. (2) The fresh leaves of S. arboricola had the lowest Zn content and the highest Hg content in November. (3) The fresh leaves of monoculture and co-planting Neolamarckia cadamba had the highest Zn, Cd, and Hg contents in November. (4) The Hg content in the litter leaves of co-planting of N. cadamba increased significantly with the time of SS application, while those of Cu, Zn, and Cd contents showed no significance. (5) The Cd content in fresh leaves was significantly and positively correlated with the Hg and Cd contents of litter leaves in Schefflera arboricola in both September and November. (6) The highest yield of litter leaves and the highest return amount of Cu, Zn, Cd, and Hg in S. arboricola occurred one month after SS application (September), while those in Neolamarckia cadamba occurred two months after SS application (October). In summary, the application time of SS showed a greater effect on the heavy metal contents in fresh leaves of N. cadamba and Schefflera arboricola than those in litter leaves; there was a positive correlation between the Cd content in the fresh leaves and the Cd and Hg contents in the litter leaves of S. arboricola; the heavy metal pollution risk of the litter leaves of S. arboricola and Neolamarckia cadamba was easy to occur in one month (September) and two months (October) after SS application, respectively. This study provides a reference for safe SS utilization and reasonable litter disposal in the landscape.

Abstract Improving soil fertility is a critical component of abandoned rare-earth mine land (ARL) revegetation. To study the effects of sewage sludge (SS), earthworms, and Jatropha curcas in ARL revegetation, SS (40% in mass ratio) and earthworms (0, 40, 60, and 80 individual adult Eisenia fetida kg−1) were applied to abandoned rare-earth mine land soil (ARLS) and then J. curcas was grown in a potting experiment. The organic carbon, nutrients (N, P, K) and heavy metals (HMs; Cd, Cu, Zn) contents in ARLS and the biomass and nutrients uptake in J. curcas were significantly increased by SS amendment. Application of 80 individual E. fetida kg−1 significantly increased availability of P and K in SS-amended ARLS relative to other treatments. Earthworms increased the height, ground diameter and biomass of J. curcas , but the promotion of biomass became weaker as earthworm density increased. After J. curcas harvest, the contents of Cd, Cu and Zn in SS-amended ARLS were reduced by 15%, 23%, and 19%, respectively. With the joint application of J. curcas and earthworms, a much larger decrease in contents of Cd (34–40%), Cu (31–44%) and Zn (24–29%) in SS-amended ARLS were observed, and the HMs ecological risks were reduced from "moderate potential" to "low potential". Moreover, J. curcas and earthworms together exerted more reduction in the exchangeable fraction HMs in SS-amended ARLS than J. curcas alone. Our results suggest that the integrated application of SS, earthworms and J. curcas is an effective approach for ARL revegetation. Highlights • Sewage sludge and earthworms joint application increased mine land soil fertility. • Earthworms stimulated Jatropha curcas growth in sludge-amended soil. • The combination of earthworms and J. curcas promoted heavy metal removal from soil. • Sludge, earthworm and J. curcas could be used together in mine land revegetation. [ABSTRACT FROM AUTHOR]

The handling of sewage sludge (SS) and urban plant litter (UPL) has become an important concern. Immobilizing heavy metals (HMs) is regarded as a necessary process for recycling SS in agriculture and forestry. Here, HM removal and HM phytotoxicity in SS during vermicomposting with different additive UPLs was investigated. The results show that vermicomposting with additive UPL significantly reduced the content of HMs, and increased organic carbon content and the proportion of macroaggregates in SS. This process also significantly immobilized HMs by mainly transforming extractable and reducible HMs into residual products. The litters of Dracontomelon duperreanum and Bauhinia purpurea increased oxidizable HMs in SS and the accumulation capacity of HMs of earthworms during vermicomposting. The Cd content in vermicomposts with the B . purpurea litter addition was decreased by 31% relative to the initial SS. Maize in vermicomposts with UPL additions, especially with B . purpurea litter, exhibited significan5tly higher seed germination rates, seedling biomass, root activity, and a lower accumulation of HMs than in SS compost without UPL additions. These results suggest that vermicomposting with additive UPL can alleviate the phytotoxicity of HMs in SS and provides a new method for simultaneously recycling SS and UPL. [ABSTRACT FROM AUTHOR]

Recycling sewage sludge (SS) to soil potentially causes soil heavy metal (HM) pollution and plant phytotoxicity. Biochar plays an important role in alleviating HM phytotoxicity, and responses vary with the feedstocks and usage of biochar. However, the effect of plant adaptability on biochar-mediated alleviation is poorly understood. Here, SS-derived biochar (SB) and rice straw-derived biochar (RB) applied at rates of 1.5% and 3% (W/W, SB1.5, SB3, RB1.5, and RB3) were used to improve the properties of soil amended with SS at 50% (W/W). Alleviation of phytotoxicity by biochar was further analyzed with SS-sensitive plant Monstera deliciosa and SS-resistant plant Ruellia simplex. Results revealed that both SB and RB significantly decreased the soil's bulk density and increased water retention. They also changed soil organic matter content and HMs fractionation. The addition of SB or RB alleviated the SS phytotoxicity, and they significantly promoted the growth and the root morphology and physiological index of M. deliciosa. But for R. simplex , these significant changes only synchronously occurred in SB3 treatment. The alleviation in M. deliciosa was more prominent and more closely connected with soil property changes than in R. simplex. Also, more soil property predictors were observed to play an important role in M. deliciosa growth than in R. simplex growth. These results indicated that biochar alleviating HMs phytotoxicity in SS-amended soil is associated with the changes of soil property. Moreover, the alleviation varies more prominently with plant adaptability than with biochar feedstocks and usage. • Biochar alleviated the sludge phytotoxicity by promoted plant growth. • Biochar alleviating phytotoxicity was more prominently in sludge-sensitive plant. • Biochar detoxification mechanism was more complex in sludge-sensitive plant. [ABSTRACT FROM AUTHOR]

Application of sewage sludge to forested land has been regarded as a promising method for sewage sludge disposal, because it can improve soil fertility and productivity. It is also likely to alter the soil nutrient cycle, which is mainly mediated by soil microorganisms. However, how the soil bacterial community responds to the application of sewage sludge in subtropical planted forests is poorly understood. Unlike farmland soil, forestland soil is covered with leaf litter, which might influence the effects of sewage sludge on the soil bacterial community. In this study, we investigated the effects of sewage sludge application (30 t/ha) on the soil bacterial community at 0–10 cm depth in three forest plantations (Eucalyptus urophylla , Pinus elliottii , and Schima superba) in southern China. The treatments included sewage sludge addition (treatment S), sewage sludge addition and leaf litter removal (treatment SLR), and a control (CK). The composition of the soil bacterial community differed in the three forest plantations and was affected by treatment S, but the effects of treatment S were not greatly altered by litter removal in the three forest plantations. At the community level, the composition of the soil bacterial community was significantly related to soil pH, available phosphorus (P) and potassium (K), and cadmium (Cd). At the phylum level, the relative abundance of Actinobacteria , Bacteroidetes , and Chloroflexi was increased by treatments S and SLR, while that of Acidobacteria , Proteobacteria , and Verrucomicrobia was decreased. The changes in the abundance of these bacteria were significantly related to the increase in the contents of available nutrients and heavy metals. Sewage sludge application resulted in an increased number of positive interactions among bacterial taxa, lower modularity, and higher connectivity, which indicated an unstable bacterial community. Changes in the composition, co-occurrence pattern, and keystone taxa of the bacterial community might reflect adaptation to sewage sludge application. • The bacterial community composition shifted with sewage sludge (SS) in all plantation types. • No interaction effects were observed between leaf litters and sewage sludge on soil bacterial community. • Members of Bacteroidetes , Actinobacteria , and Chloroflexi significantly increased with SS amended. • The soil bacterial community became unstable under SS amended. • Members of α-/γ- Proteobacteria and Actinobacteria acted as keystone taxa with SS amended. [ABSTRACT FROM AUTHOR]