Organic fertilization integrated with water management to remediate As and Cd contamination in a paddy soil.

Soil heavy metal pollution is the main risk for sustainable agriculture, especially the combination of As and Cd pollution in paddy fields which may lead to the superimposed accumulation in rice. There is an urgent need for environmental-friendly and cost-effective strategies to remediate the contamination of As and Cd in soils. In this work, a pot culture experiment was conducted in a As and Cd polluted paddy soil to explore the effects of organic fertilization (OF) and two water managements (continuous flooding, CF; intermittent irrigation, II) on the fractionation of As and Cd in soil, and the uptake of As and Cd by rice. The results showed that OF integrated with intermittent irrigation performed best in reducing the contents of As and Cd in rice grains by 58.9 % and 69.3 %, respectively, under compound pollution. The significant conversion of available As and Cd to stable species (specifically adsorbed and Fe-Mn/Al oxide bound) under OF + II were supported by the changes in an array of soil attributes such as pH, Eh, soluble Fe and dissolved organic carbon (DOC). Intermittent irrigation was more conducive to the accumulation of As outside the roots, and Fe-plaque prevented As uptake by roots and the translocation to shoots. While more accumulation of Fe-plaque along with Cd on root surface induced by continuous flooding is helpful for depressed assimilation of Cd by rice. Considering the combined contamination of As and Cd polluted in paddy soils, a management approach was proposed based on intermittent irrigation and application of organic fertilizer at the rate of 0.1 % (∼ 2.3 t/ha) in two phases (two weeks before planting or drainage). Organic fertilization will hold great promise in restoring polluted soils and maintaining soil health via suppressing the lability of heavy metals and providing nutrients.

Distinct Responses of Rare and Abundant Microbial Taxa to In Situ Chemical Stabilization of Cadmium-Contaminated Soil.

Soil microorganisms, which intricately link to ecosystem functions, are pivotal for the ecological restoration of heavy metal-contaminated soil. Despite the importance of rare and abundant microbial taxa in maintaining soil ecological function, the taxonomic and functional changes in rare and abundant communities during in situ chemical stabilization of cadmium (Cd)-contaminated soil and their contributions to the restoration of ecosystem functions remain elusive. Here, a 3-year field experiment was conducted to assess the effects of five soil amendments (CaCO3 as well as biochar and rice straw, individually or in combination with CaCO3) on rare and abundant microbial communities. The rare bacterial community exhibited a narrower niche breadth to soil pH and Cd speciation than the abundant community and was more sensitive to environmental changes altered by different soil amendments. However, soil amendments had comparable impacts on rare and abundant fungal communities. The assemblies of rare and abundant bacterial communities were dominated by variable selection and stochastic processes (dispersal limitation and undominated processes), respectively, while assemblies of both rare and abundant fungal communities were governed by dispersal limitation. Changes in soil pH, Cd speciation, and soil organic matter (SOM) by soil amendments may play essential roles in community assembly of rare bacterial taxa. Furthermore, the restored ecosystem multifunctionality by different amendments was closely related to the recovery of specific keystone species, especially rare bacterial taxa (Gemmatimonadaceae and Haliangiaceae) and rare fungal taxa (Ascomycota). Together, our results highlight the distinct responses of rare and abundant microbial taxa to soil amendments and their linkage with ecosystem multifunctionality. IMPORTANCE Understanding the ecological roles of rare and abundant species in the restoration of soil ecosystem functions is crucial to remediation of heavy metal-polluted soil. Our study assessed the efficiencies of five commonly used soil amendments on recovery of ecosystem multifunctionality and emphasized the relative contributions of rare and abundant microbial communities to ecosystem multifunctionality. We found great discrepancies in community composition, assembly, niche breadth, and environmental responses between rare and abundant communities during in situ chemical stabilization of Cd-contaminated soil. Application of different soil amendments triggered recovery of specific key microbial species, which were highly related to ecosystem multifunctionality. Together, our results highlighted the importance of rare bacterial as well as rare and abundant fungal communities underpinning restoration of soil ecosystem multifunctionality during the Cd stabilization process.

Bio-organic stabilizing agent shows promising prospect for the stabilization of cadmium in contaminated farmland soil.

In situ immobilization of cadmium (Cd) has been considered as a cost-effective and non-disruptive remediation technique for Cd-contaminated soils. In this study, several immobilization approaches were compared in a Cd-contaminated agricultural farmland. The soil was treated with different combinations of the immobilizing agents such as biochar (C), rice straw (RS), lime (L), and engineered bacteria P. putida X4/pIME (B). The plant yield and Cd uptake of lettuce as well as soil Cd fractionations were measured. The Cd content in lettuce leaves and roots decreased by 46.8~67.2% and 36.8~60.2%, respectively. Among the five treatments, combined rice straw, lime, and engineered bacteria treatment showed the lowest Cd concentration in lettuce leaves (0.14 mg/kg) and the highest plant yield (21.5 t/ha). The alleviating effects are assigned to the significant transformation of water soluble and exchangeable Cd to humic substance bound, strong organic bound and residual Cd in the soils. This study suggests that this bio-organic stabilizing agent is more cost-effective than some other immobilization agents reported previously, and shows a great application prospect in improving agriculture production of heavy metal-polluted agricultural soils.