Organic amendments improved soil quality and reduced ecological risks of heavy metals in a long-term tea plantation field trial on an Alfisol

Tea plantation can cause strong soil degradation, e.g. acidification, basic nutrient decrease and microbial diversity loss, naturally by its root activity and secondary by practically tremendous synthetic N input. Organic amendments application is considered a practical way to mitigate the above adverse consequence. However, the trade-off between agronomic and environmental effects on the application of the organic amendments is still under debate. Herein, we conducted a long-term field experiment with four treatments, including control (without and fertiliser) (CK), chemical fertiliser treatment (CF), chicken manure treatment (CM) and chicken manure combined with biochar treatment (CMB) to investigate the effects of organic amendments application on soil quality, heavy metal contamination and tea production in a tea plantation. Totally 16 plots were arranged randomly with a completely randomised design. The results showed that CM and CMB treatments improved soil nutrient, mitigated soil acidification and ameliorated soil porosity compared to CF treatment. CMB treatment displayed a relatively high tea yield and quality in three consecutive years of monitoring. However, CM and CMB treatments elevated the heavy metal (HM) potential ecological risk (RI) and Nemerow's composite index (Ps). CM treatment significantly increased available As, Pb, Cu and Zn concentrations compared to CF treatment, while CMB treatment significantly decreased available Cr and Cu concentrations and slightly decreased available Cd, Pb and Ni concentrations compared to CM treatment. But the increase of available As and Zn in CMB treatment compared to CM treatment also indicated adverse effects of biochar addition. The PLS-PM model showed HM risk had direct negative effects on tea quality. Moreover, soil fungal community revealed positive effects on tea yield and negative effects on tea quality. Overall, our study proved that CMB treatment could improve soil quality, reduce available Cr and Ni concentrations, maintain tea yield and increase tea quality.