Response of cadmium adsorption to three-year different agronomic managements in tropical soil: The role of P fractions and Fe oxides

Abstract

The overuse of chemical fertilizers and intensive cropping systems often results in soil quality degradation in the tropical and subtropical regions and affects cadmium (Cd) availability to crops. Therefore, studying the impact of agronomic managements on soil Cd adsorption and related mechanisms is critical for sustainable agriculture in tropical regions. Here, a three-year field experiment was conducted with five different agronomic managements [corn-pepper rotation with NPK fertilizers as control treatment (NPK), corn-pepper rotation with NPK fertilizers and sheep manure (NPKM), continuous corn cropping (CC), corn-pepper intercropping, and then rotation with green manure (CPGM), and corn-pepper rotation and then fallow (CPRF)]. The chemical characterization of bulk soils and soil aggregates from different treatments was performed and the isotherm adsorption experiments of Cd on different bulk soils and soil aggregates were conducted to investigate the effects of agronomic managements on characters of bulk soil and soil aggregates and subsequent effects on Cd adsorption. The results revealed that the maximum adsorption of Cd by bulk soil and various soil aggregates in the five treatments ranged from 0.235 to 0.627 and 0.222–1.360 mg g−1, respectively. The change of maximum Cd adsorption with management was in the order of CPGM > CPPF > NPKM > CC > NPK, regardless of bulk soil and soil aggregates. Compared to the NPK treatments, total P and Fe contents were significantly increased in the bulk soil and various soil aggregates of the CPGM and CPRF treatments, respectively. Furthermore, SEM-EDS analysis implied that the micro-zone distribution of Cd in the bulk soil was closely related to P and Fe, especially in the CPGM treatment. Random forest model revealed that soil pH predominantly contributed to the Cd adsorption by both the bulk soil (8.7%) and 0.25–2.0 mm soil aggregates (10.8%) in the five treatments. However, the contribution of humic P, amorphous Fe oxide, labile inorganic P, labile organic P, and total P were significantly higher than soil pH for 0.053–0.25 mm and < 0.053 mm soil aggregates. Overall, corn-pepper intercropping with green manure rotation was promising for Cd immobilization by tropical soil because of the increased effective contents of P fractions and Fe oxides for Cd adsorption.