Objectives Plants usually adjust root traits as an adaptation to nitrogen-deficient environments. The study explored changes in the root traits of different rice genotypes under varying soil N environments, which will help the breeding of rice cultivars.

Methods Six high N-sensitive genotypes (high-NS) and six low N-sensitive genotypes (low-NS) were planted under N 0 and 180 kg/hm² (N0, N180) in a field experiment conducted in the Yellow River Irrigation Area of Ningxia. The responses of the plant genotypes (high-NS and low-NS) to N0 and N180 were observed by measuring the above-ground traits (aboveground biomass and aboveground N accumulation), root growth traits (root biomass, root-shoot ratio, root length, and root volume) and root morphological traits (root diameter, specific root length, and root tissue density).

Results The aboveground N accumulation, aboveground biomass, root biomass, root length, and root volume of the high-NS under N180 were 3.8, 2.5, 2.4, 2.4, and 2.5 times of those under N0, respectively. The traits of low-NS showed no significant difference (P>0.05) between N180 and N0, except the aboveground N accumulation under N180, which was 1.6 times of that in N0. The low-NS had higher root growth traits (root biomass, root length, and root volume) and aboveground N accumulation and biomass than high-NS under N0. Aboveground N accumulation and biomass were lower in low-NS than high-NS under N180. The plasticity of the aboveground and root growth traits of high-NS to N supply was higher than low-NS. However, both genotypes’ root morphological traits were low, and there was no significant difference (P>0.05) under N0 and N180 conditions. The results of the principal component analysis showed that the root growth traits were more aggregative and independent of root morphological traits (root diameter and specific root length) under N180. In contrast, the root traits were more dispersed under N0. The correlation among the root traits of high-NS under N180 was higher than N0. For low-NS, high correlations were recorded among the root traits under N0 and N180 treatments.

Conclusions Rice adaptation to the nitrogen environment is primarily driven by root growth traits regulation and less by morphological traits. The low nitrogen-sensitive genotypes show strong trait covariation, regardless of the nitrogen supply. The high nitrogen-sensitive genotypes show strong trait covariation only under normal N supply (N180) and had higher plasticity in plant growth traits than the low nitrogen-sensitive genotype. Therefore, the low nitrogen-sensitive genotype mainly integrates the covariation of plant traits as an adaptation strategy to the environment. On the other hand, the high nitrogen-sensitive genotype mainly improves the root’s plasticity to adapt to the environment.