| Soil heavy metal pollution is a serious threat to human health,and the increase of greenhouse gas emissions causes global climate change,which is a severe challenge to sustainable development.Heavy metal pollution and soil nitrous oxide（N2O）emission is closely related to the soil p H.Soil p H is low,heavy metal elements have high mobility and activity,and N2O emission in acidic soil is intensified.Therefore,acid soil heavy metal pollution and greenhouse gas emissions are particularly serious.Camellia oleifera is an important woody oil species in China,which has high economic and ecological value.Due to the intensive fertilization,physiological characteristics of organic acid secretion from the root system,and climate characteristics of the growing region,the soil acidification of intensive Camellia oleifera forest is serious,exceeding the acidity range of suitable soil for the growth of Camellia oleifera（p H=5.5-6.5）.Soil acidification will not only reduce production of Camellia oleifera,but also increase the risk of soil heavy metal pollution,leading to intensified greenhouse gas emissions.In order to alleviate heavy metal pollution and global climate change and improve soil acidification,many researches have been carried out.Hydroxyapatite is considered as an ideal adsorbent for the treatment of heavy metals because of its high absorbability and stability.Biochar from agricultural and forestry organic wastes has been widely used in soil improvement,heavy metal control and soil acidification mitigation.Modified biochar improves the adsorption capacity of heavy metal ions by modifying biochar.Compared with hydroxyapatite,the sources of modified biochar are more extensive.The effects of the above-mentioned amendments on heavy metal control and N2O emission reduction in acid soil of Camellia oleifera forest are still unclear.In this study,the effects of application of biochar,modified biochar and hydroxyapatite on heavy metal cadmium（Cd）pollution and N2O emission in acid soil of Camellia oleifera forest were comparatively analyzed,and the potential effects and mechanisms of biochar,modified biochar and hydroxyapatite on heavy metal pollution control and N2O emission reduction in acid soil were studied.At the same time,the abundance of soil N2O microbial functional genes under different ameliorations was discussed,and the microbial mechanism was further analyzed.The results show that:（1）Hydroxyapatite had the best treatment effect on soil Cd pollution,which was reduced by 73.4%and 64.9%in soil without nitrogen fertilizer application and nitrogen fertilizer application,respectively,followed by biochar and modified biochar.（2）Cd significantly reduced soil N2O emission by 64.1%,and inhibited the abundance of soil microbial AOA gene,which may be one of the mechanisms by which Cd reduced soil N2O emission.（3）Hydroxyapatite decreased the concentrations of NH4+-N and NO3--N,AOA and nir S gene abundance in nitrogen-applied soil,while modified biochar increased nos Z gene abundance in Cd contaminated soil,and biochar increased AOB gene abundance in Cd contaminated soil.（4）Hydroxyapatite,modified biochar and biochar significantly reduced N2O emission in soil,and the inhibition effect was in the order of hydroxyapatite>modified biochar>biochar,which was closely related to the increase of p H value.Although hydroxyapatite was more effective than modified biochar and biochar in soil N2O emission inhibition,it increased soil CO2 emission by 114.9%.Biochar promoted N2O emission in Cd contaminated soil,and hydroxyapatite and modified biochar significantly inhibited N2O emission in soil.（5）In terms of soil acidification improvement of Camellia oleifera,the p H value of modified biochar and improved biochar was between 5.4 and 6.5,which was in the suitable growth range of camellia oleifera,while hydroxyapatite greatly increased the soil p H,which was beyond the suitable p H range of Camellia oleifera.Therefore,modified biochar and biochar are more worthy of promotion in soil acidification improvement of Camellia oleifera.（6）The structural equation model showed that soil amendments affected soil N2O emission mainly through indirect effects,such as p H,ammonium nitrogen and nitrate nitrogen.The direct effect of heavy metal Cd on N2O emission is to inhibit N2O emission,and the indirect effect is to inhibit N2O emission by reducing soil nitrate nitrogen content.In conclusion,biochar,modified biochar and hydroxyapatite can inhibit soil N2O emission by changing soil microbial functional genes and nitrogen transformation.However,biochar can promote N2O emission in Cd contaminated soil,which is suitable for soil without heavy metal Cd pollution and acid improvement.Hydroxyapatite has great potential in reducing and controlling Cd pollution,but it is not suitable for acid amelioration of Camellia oleifera soil.The modified biochar has more applicability in reducing Cd pollution and N2O emission,and improving the acidity of Camellia oleifera. |
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