Effects Of Remedying Lead-Contaminated Soils By Different Washing Agents Coupled With Organic Fertilizer On Soil Ecological Restoration

Heavy metal contamination in soil has been a global issue due to its long-term persistence,non-biodegradability,and toxicity to organisms.Although considerable investigations have documented the metal removal rates by washing with different agents,few researches have reported the influences of washing agents on soil ecological function.In this study,citrus pericarp（CP）and pineapple peel（PP）,as the common agricultural wastes,were inoculated with lactic acid bacteria to produce fermentation liquors（FCP and FPP）for improving Pb removal efficiency.The characterizations of bio-washing eluents and changes in metal fraction were combined with optimization experiments of the process parameters to reveal the mechanisms of Pb removal by these solutions.Then,ethylenediaminetetraacetic acid（EDTA）,citric acid（CA）,and fermented pineapple peel residue（FPP）were selected as the representatives of chelates,low molecular weight organic acids,and biological materials to wash Pb-contaminated soils in a pilot scale.The different amounts of cow manure and fruit wastes composts were added to these washed-soils for ecological function revitalization.Finally,the effects of washing agents and restoration measures on soil physicochemical and biochemical properties,microbial community diversity,functional genes for nutrient transformation,and crop growth were investigated.The main results of this study were as follows:1.The Pb removal rates by FCP and FPP increased by 46.4–59.7%than those by CP and PP.The Pb removal efficiency by these agents is closely related to the process parameters such as solution concentration,p H value,contact duration,soil-liquid ratio and washing times.The higher metal removal rate was obtained at higher concentration and lower p H of solutions.Moreover,the removal rates of soil Pb by FCP and FPP increased from 0 to 120 min,and then reached a plateau.And Pb removal rates were increased under multiple continuous washings.2.Soil washing with EDTA,CA and FPP in a pilot scale could remove 50–70%of total Pb,but also changed some soil common physicochemical and biochemical properties.Washing with the three eluents obviously reduced soil aggregates in 1?2 mm and 0.25–1 mm,and increased in0.25–0.054 mm and<0.054 mm.Moreover,the carbonate-,Fe–Mn oxide-,and organic-bound fractions declined by 34.2–64.5,52.5–81.4 and 55.3–80.6%.Notably,the exchangeable Pb fraction increased by 1-fold after washing with EDTA.Soil TOC,NO₃^-–N,NH₄⁺–N,and AP were significantly decreased in CA-washed soils（P<0.05）.The AK mildly raised in FPP-washed soil.In addition,theβ-GC decreased by 3.53 and 4.73%in EDTA-and CA-washed soils,and increased by 10.81%in FPP-washed soils.3.Washing agents coupled with organic fertilizer could improve soil properties and facilitate ecological restoration of lead-contaminated soil.（1）The aggregates<0.054 mm decreased and the exchangeable Pb fraction decreased by 61.89–89.62%when cow manure and peel residue composts were added to the washed-soils.Moreover,application of organic fertilizer significantly increased the contents of soil microbial biomass carbon,nitrogen,and phosphorus.（2）Although soil bacterial and fungal Shannon index declined by 4.8–15.2 and 7.0–62.6%after washing with EDTA,CA and FPP,they improved by 2.2–9.7 and 4.1–32.4%after adding 3.0%peel residue composts in the washed-soils.Moreover,the abundances of 9,11 and 9 rare phyla reduced upon80%in EDTA-CA-and FPP-washed soils.Therefore,the rare microbiomes were more sensitive to metal removal,and they could be as suitable bio-indexes to measure the changes in soil ecological function after soil pollution,remediation and other disturbances.（3）Soil washing decreased the abundances of genes involved in the CO₂ fixation and degradation of lignin and other degradation-resistant organic matters.Especially,in the CA-washed soils,the nar G and nir S2（relate to denitrification）and ure C（encoded urease synthesis）decreased by 18.6,50.3,and 43.8%,respectively.Nevertheless,the genes involved in mineralization of soil organic carbon,nitrogen and phosphorus were enhanced in the soils after adding organic fertilizer,thus improving the bioavailability of nutrients.4.Adding organic fertilizer to the washed-soils is an effective measure to reduce the risk of heavy metals and improve the biomass of Chinese cabbage（Brassica campestris L.）.The concentrations of O₂·^-and malondialdehyde（MDA）in the Chinese cabbage increased,while the net photosynthetic and transpiration rates decreased in the Pb-contaminated soils.Soil washing with EDTA,CA and FPP decreased the Pb risk in the cabbage as well as affected their biomass,physiology,and biochemistry.The MDA of the Chinese cabbage decreased in EDTA-and FPP-washed soils,while it increased by 22.2%in CA-washed soils.Addition of organic fertilizer in the washed-soils can help to reduce the O₂·^-concentrations,improve the net photosynthetic efficiency,and promote the organic matter accumulation of Chinese cabbage.In conclusion,the washing agents coupled with organic fertilizer is a feasible practice to remedy lead-contaminated soils,restore soil function,and improve crop yield.