The responses of cadmium phytotoxicity in rice and the microbial community in contaminated paddy soils for the application of different long-term N fertilizers.

An eight-year field trial was conducted to investigate the effects of four different N fertilization treatments of urea (CO(NH2)2, the control), ammonium sulfate ((NH4)2SO4), ammonium chloride (NH4Cl), and ammonium hydrogen phosphate [(NH4)2HPO4]) on cadmium (Cd) phytotoxicity in rice and soil microbial communities in a Cd-contaminated paddy of southern China. The results demonstrate that the different N treatments exerted different effects: the application of (NH4)2HPO4 and (NH4)2SO4 significantly increased rice grain yield and decreased soil-extractable Cd content when compared with those of the control, while NH4Cl had a converse effect. Expression of genes related to Cd uptake (IRT and NRAPM genes) and transport (HMA genes) by roots may be responsible for Cd phytotoxicity in rice grown in the different N fertilization treatments. Our results further demonstrate that N fertilization had stronger effects on soil bacterial communities than fungal communities. The bacterial and fungal keystone species were identified by phylogenetic molecular ecological network (pMEN) analysis and mainly fell into the categories of Gammaproteobacteria, Acidobacteria and Actinobacteria for the bacterial species and Ascomycota for the fungal species; all of these keystone species were highly enriched in the (NH4)2HPO4 treatment. Soil pH and soil available-Cd content emerged as the major determinants of microbial network connectors. These results could provide effective fertilizing strategies for alleviating Cd phytotoxicity in rice and enhance the understanding of its underlying microbial mechanisms.