Meta-Analysis of the Impacts of Applying Livestock and Poultry Manure on Cadmium Accumulation in Soil and Crops

The return of livestock and poultry manure (LPM) to fields is necessary for sustainable agricultural development, but it is also a primary source of heavy metal contamination in agricultural land, which potentially threatens soil and crops. Conflicting results have been reported in previous studies on the impacts of returning LPM to fields on cadmium (Cd) accumulation in the soil and crops. Herein, we investigated the impacts of applying such manure on Cd accumulation in the soil and crops through meta-analysis. We also explored the relationships of Cd content in the soil and crops with the soil properties, experimental conditions, and manure properties. Moreover, we evaluated the primary reasons for the differences in the results of different studies and the factors influencing them. Upon applying LPM, the contents of soil total Cd, soil available Cd, and crop grain Cd increased by 30.96%, 86.91%, and 20.43%, respectively, and the crop root Cd content decreased by 16.91%. Random forest analysis and decision tree analysis further quantified the importance of each influencing factor and identified the primary factors influencing soil available Cd and grain Cd contents. From the perspective of safe production, some suggestions were made for returning LPM to fields: (1) applying higher pH LPM or increasing the dosage in soils with low Cd content and pH of <5.7; (2) applying LPM to soils with Cd content of <1.8 mg/kg, pH of >5.7, and soil organic matter content of >19 g/kg, which was associated with a low risk of Cd accumulation in crop grains; and (3) applying LPM while planting low-Cd-accumulating crop varieties. This study provides scientific guidance for the safe use of such manure resources and helps to reduce the risk of cadmium accumulation.