| Heavy metal contamination in farmland soil has been one of the great concerns in China. It is urgent and necessary to remediate such heavy metal polluted agricultural soils to improve food security. Studies have shown that converting biomass (e.g. straw and manure) into biochar and applying biochar to soil can stabilize soil heavy metals effectively and reduce plant uptake. However, the efficiencies of biochar in reducing heavy metal accumulation in plants varied based on published researches due to the differences of soil properties, biochar types, application rate and plant species. In order to improve biochar's efficiency to immobilize soil heavy metals and reduce heavy metal accumulation in plant, a meta-analysis was conducted by collecting published papers to estimate the overall effects of biochar on soil heavy metal (Cd, Pb, Cu, Zn and As) availabilities, their uptake by plants and on soil properties (pH, SOC, CEC et al.). Besides, the effects of soil properties (such as pH, SOC and texture), biochar properties (including feedstocks, pyrolysis temperature, pH,application rates et al.) and plant species on biochar's effect on reducing plant metal uptake were investigated. Based on the meta-analysis, two field studies were conducted to study the effects of wheat straw biochar on Cd uptake, translocation and accumulation in different rice cultivars; and to study the effects of wheat straw biochar on Cd and Pb accumulation in wheat and maize grains.Main results were listed as follows:1. The results of meta-analysis showed that, biochar significantly increased soil pH,SOC, CEC and EC by an average of 9, 69, 123 and 93%, respectively, compared to control.Available Cd, Pb, Cu and Zn content in soils were reduced by 52, 46, 29 and 36%,respectively, while soil available As content was increased by 64%. Biochar significantly reduced Cd, Pb, Cu and Zn concentration in plant by 38, 39, 25 and 17%, respectively compared to control. Although biochar significantly increased available As in soil, As concentration in plants were significantly reduced by 12%.2. Soil conditions, biochar properties and application rate and plant species can affect the decrease extent of heavy metals in plant caused by biochar application. Generally,application of biochar in soils with lower pH, there will be greater decreasing extent of Cd and Pb concentration in plant, while for Cu and As, there was a reverse effect. The reduction extent of Cd, Pb, and As in plant were greater in coarse soils than in fine soils after biochar application. There were greater reduction extents of Cd, Pb, Cu, Zn and As when biochar was applied to soils with higher organic carbon content.3. Manure derived biochar had much better efficiency to reduce Cd and Pb accumulation (reduced by 73 and 65%, respectively) in plant than other biochars. Sewage sludge biochar did not significantly reduce Cd accumulation in plant. Besides, other biochars derived from agricultural wastes, wood and green waste can effectively reduce Cd,Pb and Cu in plants. There was an inconsistent effect of biochar derived from different feedstocks on changes of plant Zn and As concentration. Agricultural waste biochar and green waste biochar can effectively reduce Zn concentration in plant, while green waste biochar and sewage sludge biochar can significantly reduce As accumulation in plants.Biochar with higher pH had greater efficiency to reduce Cd, Cu and Zn concentration,while there was a reverse effect for As. Higher application rate of biochar had greater reduction of heavy metals in plants.4. Biochar effectively reduced heavy metal concentration in different part of plant(including above-ground and below-ground, editable-part and uneditable-part). There was a much greater reduction extent of Cd in above-ground than in below-ground in plant after biochar application. As concentration in editable significantly reduced while there was no significant change of As concentration in uneditable part. Generally, Cd, Pb and Cu concentration were significantly reduced in different plant species after biochar application.There were inconsistent changes of Zn or As in plant after biochar application. The reduction extents of heavy metal concentrations in plants were significanty different among plant species. Besides, there were greater efficiencies when soils had higher heavy metal concentration or biochars had lower heavy metal concentration.5. According to a two-year field experiment in rice paddy, the results showed that soil pH and soil organic carbon (SOC) were sustainably increased while soil Cd availability was effectively decreased in acidic paddy soils following wheat straw biochar amended at 20 and 40 t ha-1. Biochar at 40 t ha-1 reduced CaCl2 extractable Cd by 70, 85, 54 and 43%during four rice seasons, which had much better effects than 20 t ha-1. Biochar significantly reduced Cd translocation from rice root to shoot, the corresponding translocation factor were reduced by 20-80%. Both biochar and low Cd cultivar treatments effectively reduced grain Cd concentration. Except for the first season, Cd concentration in different rice cultivars were significantly reduced in the next three seasons after biochar was applied,corresponding grain Cd concentration were reduced by 61, 86, and 57%, respectively.Grain Cd concentration in indica conventional cultivars was significantly lower than in indica hybrid cultivars. Greater reduction of Cd was observed in low accumulation cultivars than in higher accumulation cultivar after biochar was applied. For late rice,biochar in combination of low Cd affinity cultivars had a maximum decrease of grain Cd by 69-80%, which was greater than biochar or low accumulation cultivars alone.6. Biochar significantly reduced available Zn content by 35-91%, while it had no effect on Zn concentration in rice grains. Indica hybrid rice cultivars had significantly higher Cd but lower Zn in grains than in indica conventional cultivars, which makes indica conventional rice cultivars have lower Cd/Zn ratio, which means higher consumption safety and higher potential to resist Zn deficiency risk caused by biochar application.Biochar in combination of low Cd affinity cultivars reduced grain Cd/Zn ratio in greater extent (by 72-80%), reducing consuming risk more efficiently.7. According to a two crop-season field experiment in upland, the results showed that soil bulk densities were significantly reduced (18-29%) while soil organic carbon content were significantly increased by 45-218% in both wheat and maize season in as a result of biochar application. Soil pH and EC were significantly increased by biochar only in wheat season. Grain yield of wheat and maize were not affected by biochar. Soil CaCl2 extractable Cd and Pb were significantly reduced in both wheat and maize seasons. Cd and Pb concentration in wheat grains were significantly reduced by biochar at a maximum of 21%and 43%, respectively; while their concentration in maize grains were not affected by biochar. Cd and Pb concentration in maize grains were significantly lower than in wheat grains and stayed at a safety level according to the limit of maximum contaminant levels in food of China.In conclusion, biochar can effectively improve soil properties, increase crop yield,reduce heavy metal availability (except for As) in soils and reduce plant uptake, thus significantly improving the safety of agricultural food; and such effects are proved to be consistent in field condition. However, attentions should be paid that the efficiency of biochar's effects on stabilizing soil heavy metal and reducing plant uptake can be affected by plant cultivars, plant species, soil properties, biochar properties and heavy metals. All these factors should be considered when using biochar to remediate heavy metal polluted soils so that biochar can be used more effectively and agricultural food can be produced more safely. |
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