High level of copper affects both nitrogen transport in rice plant and nitrogen transformation in rhizosphere soil

Interactions between / among nutrients have been widely concerned and studied in recent years. Because of the global industrialization and intensive cultivation of crops in the past few decades, the Cu concentration in soil is gradually increased. Meanwhile, N fertilizers have been widely supplied in arable land to obtain high yield. Therefore, it is necessary and important to investigate the possible interaction between Cu and N in both rice plant and rhizosphere soil, and further to reveal its underlying mechanism. A short-term pot experiment was conducted during rice vegetative stage. Plants were grown in soils with different combinations of Cu and N, the plant growth, Cu and N status, enzyme activities, and expression levels of genes involved in N and Cu transport in rice plant, as well as the Cu and N transformation, bacteria and fungi communities, enzyme activities, and expression levels of genes involved in N cycling in rhizosphere soil were analyzed. Results showed that HCu significantly decreased the net photosynthetic rate and shoot dry weight of rice, while NN could effectively alleviate this repression. HCu significantly inhibited N uptake, transport, and assimilation by down-regulating OsNRTs expression and reducing NR and GOGAT activities in rice plant, while N supply decreased Cu uptake by down-regulating OsCOPTs in root, but promoted the root-to-shoot translocation and distribution of Cu in leaves by up-regulating OsHMA5 and OsYSL16 in shoot. In rice rhizosphere soil, HCu facilitated N turnover and increased N availability, but decreased soil enzyme activities and the copy numbers of almost all N cycling genes, whereas N supply did not have any impact on Cu turnover. The effect of HCu on bacterial and fungal communities was more pronounced than N treatment. HCu significantly increased the relative abundances of a metal resistant bacteria phyla, Firmicutes, but also increased the relative abundances of two pathogenic fungi, Ascomycota and Zygomycota. Taken together, although HCu facilitated N turnover and increased N availability in rice rhizosphere soil, an antagonistic effect between HCu and N was observed in rice plant that HCu significantly inhibited N uptake, transport, and assimilation. These results are still needed to be verified by field trials in the future.