Effects of urea containing amino acid on rice seedling growthand rhizosphere microbial community

Objectives  The effects of urea containing amino acid (AU) on rice growth, rhizosphere nutrients, and microbial community were studied under flooding and normal soil conditions to provide base for its application in early rice cultivation.

Methods  An early rice cultivar ‘Liangyouhua 6’ was cultivated in a pot experiment. The treatments were two different urea types (common urea, U and urea containing amino acid, AU) with the same N input levels separately applied as the basal nitrogen fertilizers under two different soil conditions (dry and flooding), while non-application of either urea (no N) under both soil conditions was regarded as controls. The soil and rice plants samples were collected in all the treatments after the rice seedlings grew for 30 days. The growth and root morphological characteristics of rice, soil nutrient contents, and soil microbial community composition were analyzed.

Results  AU promoted root growth, increased root fresh weight, root length, root diameter, root surface area, and number of root tips by 9.65%, 45.06%, 25.93%, 6.17%, and 90.52%, respectively, compared with common urea (U) under dry soil condition. AU also increased these aforementioned root characteristics more than U and control under flooding, but less than the increase recorded under dry condition. The root length, root diameter, root surface area, and number of root tips of rice seedlings under the two urea treatments in dry soil were significantly lower than those under flooding condition. Compared with U, AU increased organic carbon, total N, and microbial biomass N in dry soil by 17.04%, 18.18%, and 30.00%, respectively. In flooded soil, AU increased ammonium N and microbial biomass N by 39.70% and 38.01%, respectively, but decreased nitrate N by 33.70%. AU increased the total number of bacteria in both dry and flooded soils (1.88 × 10⁷ cfu/g and 1.93 × 10⁷ cfu/g) than U (1.59 × 10⁷ cfu/g and 1.57 × 10⁷ cfu/g). AU increased the number of ammoniating bacteria and nitrifying bacteria in both flooded and dry soils than U and control. However, the number of ammoniating bacteria under AU treatment was higher in flooded soil than dry soil, whereas number of nitrifying bacteria under AU treatment was lower in flooded soil compared with dry soil. The dominant bacteria in dry soil was Sphingomonas (with relative abundance of 18.78%–22.59%), whereas Aeromonas (43.09%–52.72%) and Flavobacterium (14.86%–18.87%) were dominant in flooding condition. Compared with U treatment, the abundances of Aeromonas and Sphingomonas in AU treatment decreased significantly while uncultured_bacterium_c_Subgroup_6 increased significantly. The total N, ammonium N, and organic carbon contents were significantly correlated with microbial community structure.

Conclusions  Urea containing amino acid shows higher significance in promoting the growth, formation, and development of rice root through increase in organic carbon and microbial biomass nitrogen in the rhizosphere, reduction in nitrate nitrogen, and increase ammonium nitrogen content under flooding condition. Increase in bacteria population improves the structure of rice rhizosphere colony which is beneficial for nitrogen absorption by rice and maintenance of soil nitrogen.