Short communication

Effects of the land application of sewage sludge on soil heavy metal concentrations and soil microbial communities

The overuse of chemical fertilizers and intensive cropping systems often results in soil quality degradation in the tropical and subtropical regions and affects cadmium (Cd) availability to crops. Therefore, studying the impact of agronomic managements on soil Cd adsorption and related mechanisms is critical for sustainable agriculture in tropical regions. Here, a three-year field experiment was conducted with five different agronomic managements [corn-pepper rotation with NPK fertilizers as control treatment (NPK), corn-pepper rotation with NPK fertilizers and sheep manure (NPKM), continuous corn cropping (CC), corn-pepper intercropping, and then rotation with green manure (CPGM), and corn-pepper rotation and then fallow (CPRF)]. The chemical characterization of bulk soils and soil aggregates from different treatments was performed and the isotherm adsorption experiments of Cd on different bulk soils and soil aggregates were conducted to investigate the effects of agronomic managements on characters of bulk soil and soil aggregates and subsequent effects on Cd adsorption. The results revealed that the maximum adsorption of Cd by bulk soil and various soil aggregates in the five treatments ranged from 0.235 to 0.627 and 0.222–1.360 mg g⁻¹, respectively. The change of maximum Cd adsorption with management was in the order of CPGM > CPPF > NPKM > CC > NPK, regardless of bulk soil and soil aggregates. Compared to the NPK treatments, total P and Fe contents were significantly increased in the bulk soil and various soil aggregates of the CPGM and CPRF treatments, respectively. Furthermore, SEM-EDS analysis implied that the micro-zone distribution of Cd in the bulk soil was closely related to P and Fe, especially in the CPGM treatment. Random forest model revealed that soil pH predominantly contributed to the Cd adsorption by both the bulk soil (8.7%) and 0.25–2.0 mm soil aggregates (10.8%) in the five treatments. However, the contribution of humic P, amorphous Fe oxide, labile inorganic P, labile organic P, and total P were significantly higher than soil pH for 0.053–0.25 mm and < 0.053 mm soil aggregates. Overall, corn-pepper intercropping with green manure rotation was promising for Cd immobilization by tropical soil because of the increased effective contents of P fractions and Fe oxides for Cd adsorption.

Application of Zn fertilizer can improve cereal Zn concentration, and affect soil microbial ecology by increasing soil Zn concentration. We investigated the effects of three-year continuous applications of different Zn fertilizers (i.e., ZnSO₄ and ZnEDTA) on wheat Zn biofortification, soil Zn fractions and bacterial community on a Zn deficient calcareous soil. The results showed that ZnEDTA application induced a higher grain Zn concentration and bioavailability than those in the case of ZnSO₄ application, which met the target of wheat Zn biofortification within three years. Correspondingly, ZnEDTA application resulted in a higher Zn availability in soil than ZnSO₄ application by facilitating Zn transformation into exchangeable and organic matter loosely bound fractions. Bacterial diversity and richness were not affected, but the bacterial community was altered after three years Zn applications. ZnEDTA application significantly decreased the relative abundances of Nocardioides, Arthrobacter, Blastococcus, Gemmatimonas, and Streptomyces as compared with ZnSO₄ application. Meanwhile, ZnEDTA application lowered α-glucosidase, β-glucosidase, and β-xylosidase activities in soil as compared with ZnSO₄ application. These results indicated that ZnEDTA application is an effective practice to achieve wheat Zn biofortification, but attention needs to be paid to the potential environmental risks associated with the decreased activities of microbes and enzymes in soil.

Zinc (Zn) fertilizer application can certainly improve the production and nutritional quality of cereal crops. However, Zn accumulation in the soil may lead to some deleterious environmental impacts in agroecosystems. The effects of long-term Zn application on soil microbial properties remain unclear, but it is imperative to understand such effects. In this study, we collected soil samples from a nine-year field experiment in a wheat-maize system that continuously received Zn applied at various rates (0, 2.3, 5.7, 11.4, 22.7 and 34.1 kg ha⁻¹) to evaluate the soil enzymes, microbial biomass and microbial community structure. The results showed that Zn application at the rate of 5.7 kg ha⁻¹ significantly increased the activities of urease, invertase, alkaline phosphatase and catalase in the soil, while the rate of 34.1 kg ha⁻¹ significantly decreased the evaluated enzyme activities. The microbial biomass carbon (C) and nitrogen (N) were not affected by Zn application rates, although an increase in the microbial biomass C was observed in the 11.4 kg ha⁻¹ treatment. Moreover, the alpha diversity of the bacterial and fungal communities did not vary among the nil Zn, optimal Zn (5.7 kg ha⁻¹) and excess Zn (34.1 kg ha⁻¹) treatments. However, the bacterial communities in the soil receiving the optimal and excess Zn application rates were slightly changed. Compared to the nil Zn treatment, the other Zn application rates increased the relative abundances of the Rhodospirillales, Gaiellales and Frankiales orders and decreased the abundance of the Latescibacteria phylum. The redundancy analysis further indicated that the soil bacterial community composition significantly correlated with the concentrations of soil DTPA-Zn and total Zn. These results highlight the importance of optimal Zn application in achieving high production and high grain quality while concurrently promoting soil microbial activity, improving the bacterial community and further maintaining the sustainability of the agroecological environment.

The disposal of biosolids poses a major environmental and economic problem. Agricultural use is generally regarded as the best means of disposal. However, its impact on soil ecosystems remains uncertain. Biosolids can improve soil properties by supplying nutrients and increasing organic matter content but there is also a potentially detrimental effect arising from the introduction of heavy metals into soils. To assess the balance between these competing effects on soil health, we investigated soil bacterial and fungal diversity and community structure at a site that has been dedicated to the disposal of sewage sludge for over 100 years. Terminal restriction fragment length polymorphism (T-RFLP) was used to characterize the soil microbial communities. The most important contaminants at the site were Ni, Cu, Zn, Cd, and Pb. Concentrations were highly correlated and Zn concentration was adopted as a good indicator of the overall (historical) biosolids loading. A biosolids loading, equivalent to 700–1000 mg kg⁻¹ Zn appeared to be optimal for maximum bacterial and fungal diversity. This markedly exceeds the maximum soil Zn concentration of 300 mg kg⁻¹permitted under the current UK Sludge (use in agriculture) Regulations. Redundancy analysis (RDA) suggested that the soil microbial communities had been altered in response to the accumulation of trace metals, especially Zn, Cd, and Cu. We believe this is the first time the trade-off between positive and negative effects of long term (>100 years) biosolids disposal on soil microorganisms have been observed in the field situation.

Kenaf (Hibiscus cannabinus L.) was experimentally cultivated with the use of digested, dried sewage sludge (130 t/ha) and water from a municipal Sewage Treatment Plant (STP) in order to assess their potential to replace conventional fertilization (100 kg N/ha, 75 kg P₂O₅/ha and 75 kg Κ₂O/ha) and irrigation. Tap water and treated wastewater were used for irrigation in quantities corresponding to 6500 m³/ha. Four different treatment combinations were applied as follows: (a) wastewater irrigation and conventional fertilization, (b) wastewater irrigation and sewage sludge fertilization, (c) tap water irrigation and sewage sludge fertilization, and (d) tap water irrigation and conventional fertilization. The dry plant biomass collected in the final harvest (140 days after plant emergence) from the four treatment plots was 12.3 t/ha, 12.6 t/ha, 12.4 t/ha and 12.8 t/ha respectively. These differences were not statistically significant (ANOVA, P = 0.05) and, therefore, it was concluded that the use of municipal wastes had similar effects on dry biomass production with that of conventional fertilization. An earlier harvest (125 days after plant emergence) gave 11.3% lower dry biomass on average in relation to the second harvest, and this difference was statistically significant (ANOVA, P = 0.05). Premature harvest may lead to significant biomass losses, so the plant must be collected during its technological maturity stage. There was not any statistically significant difference among the four treatments and between the two harvests in fiber dimensions and derived values (suitability indices for paper manufacture). On the other hand, cellulose and lignin content in the second harvest were significantly higher compared to the first one, whereas no significant differences were detected among the four treatments.

Enzyme activities have the potential to indicate biological functioning of soils. In this study, soil urease, dehydrogenase, acid phosphatase and invertase activities and fluorescein diacetate (FDA) hydrolysis were measured in two red soils spiked with Pb²⁺ ranging from 0 to 2 400 mg kg⁻¹ to relate the enzyme activity values to both plant growth and the levels of available and total Pb²⁺ concentrations in soils, and to examine the potential use of soil enzymes to assess the degrees of Pb contamination. Soil samples were taken for enzyme activities assaying during 3 month's incubation and then after planting of celery (Apium graveolens L.) and Chinese cabbage (Brassica chinensis L.). Enzyme activities in the red soil derived from arenaceous rock (RAR) were generally lower than those in the red soil developed on Quaternary red earths (REQ). At high Pb²⁺ loadings, in both incubation and greenhouse studies, urease activity and FDA hydrolysis were significantly inhibited. But there were no significant relationships between soil dehydrogenase, acid phosphatase or invertase activity and soil Pb²⁺ loadings in both RAR and REQ soils. The growth of celery and Chinese cabbage increased soil urease activity and FDA hydrolysis, but had minimal effect on dehydrogenase and invertase activities. There were positive correlations between celery biomass and soil urease activity and FDA hydrolysis. These results demonstrate that urease activity and FDA hydrolysis are more sensitive to Pb²⁺ than acid phosphatase, dehydrogenase and invertase activities in the RAR and REQ soils.