Carbonate rock dissolution regulates soil microbial iron and nitrogen cycle to reduce cadmium accumulation in rice

Carbonate rock dissolution (CRD) in karst areas generates abundant ionic components. CRD is a widespread natural phenomenon with properties that affect microbial activity and function. However, little is known about how CRD influences nitrogen (N) and iron (Fe) cycling-related microbial communities in soils to decrease Cd accumulation in rice. The karst limestone soil (LS), non-karst areas of yellow soil (YS), and yellow soil with the addition of carbonate rock minerals (YSC) were prepared, and extraneous Cd was added to conduct rice cultivation experiments. Metagenomics sequencing was used to investigate microbial communities related to the Fe N cycling. The results revealed that the content of CRD ions in LS and YSC were substantially higher than those in YS. Compared to YS, CRD in LS and YSC effectively inhibited the absorption of Cd by roots, leading to the decrease of Cd content in grains by 94.08–97.52 % and 63.44–74.72 %, respectively. The reduction in contents of available Fe (AFe) and nitrate nitrogen (NO 3 − -N) in LS and YSC indicated that the Fe N cycling driven by the soil microorganism was affected by CRD. The CRD ions altered the relative abundance of differential flora between LS and YS groups. CRD inhibited the complete nitrification, comammox process driven by Planctomycetes, while it promoted assimilatory nitrate reduction to ammonium (ANRA) driven by Nocardioides and Fe(II) oxidation coupled with dissimilatory nitrate reduction to ammonium (DNRA) driven by Anaeromyxobacter . The partial least squares-structural equation modeling (PLS-SEM) model revealed that CRD primarily promoted microbial nitrate reduction to ammonium (MNRA), which indirectly decreased the contents of NO 3 − -N and available Cd (ACd) in soil. Therefore, CRD in the karst regions promotes MNRA and inhibits nitrification, thereby reducing ACd content and facilitating the mitigation of Cd accumulation in rice.