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Sewage sludge (SS) application on forest plantation soils as a fertilizer/soil amendment is increasingly becoming a forestry management measure. However, the potential risk of SS-derived potentially toxic elements (PTEs) is still a cause for concern. This research was carried out to evaluate PTEs behavior in SS applied as an amendment for forestland application purposes. Speciation and transformation perspectives on residual effects during a 3-year field experiment were evaluated. Sewage sludge was applied to soils under Eucalyptus urophylla S.T. Blake, Schima superba Gardn. et Champ, and Pinus elliottii Engelm plantations at 30 Mg·ha−1 (dry weight). We investigated the total concentrations of Cu, Zn, Pb, Cd, and Ni, as well as their concentrations of five fractions: exchangeable (F1), bound to carbonates (F2), Fe–Mn oxides (F3), organic matter (F4), and residual (F5) along with the 0–10 cm of a surface A horizon (topsoil) one day, one, two, and three years after SS application, respectively. Sewage sludge caused significant increase in PTEs total concentrations and proportion of fraction F1 or F2. The mobility index and potential ecological risk confirmed that the risk was linked to the presence of these PTEs, especially for Cd. The total concentration and corresponding ecological risks of PTEs in most cases decreased, suggesting a potential loss of PTEs from the topsoil system. The percentage of all PTEs in fractions F1–F4 decreased and that in F5 increased with time. These results illustrate that the residual and mobility of PTEs in the topsoil during SS utilization was lower than expected and more attention should be paid to the risk of PTEs transferring to other environments.

The ecological effects of eucalypt plantations (EPs) have garnered increasing attention. To understand their effect on soil quality at a landscape scale, and to determine whether soil quality parameters differ due to different stand types, we evaluated soil characteristics in twenty-one groups of EPs, Pinus massoniana Lamb. plantations (PMPs) and natural broadleaved forests (NBFs) across Guangdong Province, China. Both the physical characteristics of soil hydrology and the properties of soil nutrients in A and B horizons were determined. Results showed that, compared to NBFs, EPs and PMPs produced a shallower litter layer, reduced canopy density, higher soil bulk density, significantly lower total porosity, non-capillary porosity, total water volume, and hygroscopic water in the A horizon (P

Recycling sludge as a soil amendment has become a viable option for sludge disposal. However, such application can lead to soil pollution because of its enrichment in contaminants, especially heavy metals. To identify an effective means to ensure the ecological safety of sewage sludge landscape utilization, a glasshouse experiment was conducted by mixing sewage sludge at 0%, 15%, 30%, 60%, and 100% (V/V) amendment ratios to landscape soils and planting five common landscape plants (Aphelandra ruellia, Syngonium podophyllum, Schefflera odorata, Alocasia macrorrhiza, and Dianella ensifolia). Sludge amendment significantly improved fertility and moisture retention capacity of soil, but dramatically increased the contents of Cd, Pb, Cu, and Zn. Compared with lateritic red soil (CK), sludge amended-soils increased the relative growth rate of landscape plants and enhanced their nutrient uptake and heavy metals accumulation. A rate of 30% or less of sludge application showed positive growth effects for all five landscape plants, and did not cause potential ecological risks of heavy metals to landscape soils. However, rates of 60% or higher posed very high potential ecological risk in multiple metals, particularly in Cd. Interestingly, S. odorata, A. ruellia and A. macrorrhiza could alleviate the ecological risks of Cd, Cu and Zn, respectively, and these results were confirmed by data of relative changes in heavy metal contents in post-experiment soil. Our results suggest that attention to reducing the environmental risks of heavy metals in sludge utilization as landscape soil-amendments should not only be paid to controlling the application dosage of sludge but also to planting suitable plant species.

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.

Enriched planting of native tree species in monoculture plantation forests is a commonly recommended forest practice. However, its effect on various ecological processes is generally lacking. Here, we carried out an experiment in a 16-year-old Eucalyptus plantation in South China to assess the effects of enriched planting of native tree species on surface water, soil erosion and nutrient losses. Two treatments were conducted in 2008: (1) enriched planting of native broadleaved tree species with uniform thinning of 60% of Eucalyptus trees (TEP); and (2) enriched planting of native broadleaved tree species without thinning (NEP). The original Eucalyptus plantation stands was used as the control (CK). Runoff plots (total n = 9, 3 for each treatment or CK) were established in 2009, and surface water flow, sediment, nitrogen (N) and phosphorus (P) losses were monitored from major rainfall events in 2010–2012. Results showed that enriched planting in Eucalyptus plantation significantly reduced surface water flow and soil erosion. Compared with CK, TEP and NEP reduced annual surface water flow by 29–43% and 11–16%, and reduced annual soil erosion by 38–54% and 20–33% throughout the study period, respectively. TEP and NEP had significantly lower annual mean concentrations of N and P in surface water. Compared with CK, TEP reduced annual N and P losses through surface water by 42–60% and 44–64%, respectively, while NEP reduced them by 25–28% and 24–34%, respectively. N and P losses were significantly related to surface water flow. Between the two treatments, TEP was better for retaining water and soil, and for preventing nutrient loss. These results clearly demonstrated that the enriched planting of native tree species effectively retained surface water and nutrients.

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.

Application of sewage sludge compost (SSC) as a fertilizer on landscaping provides a potential way for the effective disposal of sludge. However, the response of landscape trees to SSC application and the impacts of heavy metals from SSC on soil are poorly understood. We conducted a pot experiment to investigate the effects of SSC addition on Mangifera persiciforma growth and quantified its uptake of heavy metals from SSC by setting five treatments with mass ratios of SSC to lateritic soil as 0%:100% (CK), 15%:85% (S15), 30%:70% (S30), 60%:40% (S60), and 100%:0% (S100). As expected, the fertility and heavy metal concentrations (Cu, Zn, Pb and Cd) in substrate significantly increased with SSC addition. The best performance in terms of plant height, ground diameter, biomass and N, P, K uptake were found in S30, implying a reasonable amount of SSC could benefit the growth of M. persiciforma. The concentrations of Cu, Pb and Cd in S30 were insignificantly different from CK after harvest, indicating that M. persiciforma reduced the risk of heavy metal contamination of soil arising from SSC application. This study suggests that a reasonable rate of SSC addition can enhance M. persiciforma growth without causing the contamination of landscaping soil by heavy metals.