Alternative dry-wet irrigation improves the rhizospheric oxygen environment and nitrogen transformation, and increases nitrogen absorption by rice plants

Objectives This study investigated the characteristics of the oxygen environment in rice rhizospheric and non-rhizospheric soils, its effects on soil C and N transformation, and rice N utilization under different irrigation and fertilization regimes. The aim was to reveal the intrinsic mechanism of suitable water and N coupling management to improve rice growth and N use efficiency.

Methods The test soils were collected from a long-term field experiment, including two irrigation regimes, i.e. conventional flood irrigation (CF) and alternate wet and dry irrigation (AWD), and three N application levels, i.e. N₀ (N 0 g/kg), N_0.8 (N 0.8 g/kg), and N_1.2 (N 1.2 g/kg). Japonica rice cultivar of nipponbare (Nip) and indica rice cultivar Yangdao 6 were grown in root boxes. The unisense microelectrode system and ¹⁵N isotope tracer techniques were used to detect soil oxygen environment, soluble organic C, and microbial biomass C and N. Enzyme activities related to C and N transformation and their relationships with rice growth and N utilization were also analyzed.

Results 1) Compared to CF, AWD (P < 0.05) increased the dissolved oxygen content and redox potential in rhizospheric and non-rhizospheric soils. However, the dissolved oxygen content in the rhizospheric soil of Yangdao 6 was higher than Nip. 2) AWD (P < 0.05) increased the soil microbial biomass C and N and the soluble organic C content in the rhizospheric soil of Nip and Yangdao 6 under N_0.8 and N_1.2 levels compared with N₀. AWD conditions increased the activities of urease, invertase, atalase and N-acetyl-β-D-glucosaminidase in Nip under N_0.8 and N_1.2 and Yangdao 6 under N_0.8. 3) AWD (P < 0.05) increased soil nitrification rate, extracted total N, NO₃^– and free amino acids under N_0.8 and N_1.2 compared with N₀, but decreased NH₄⁺ content in the rhizosphere. AWD also increased root NH₄⁺ uptake rate, rice dry matter weight and N utilization index. The increments mentioned above were higher in Yangdao 6 than Nip. 4) Rice biomass and N use index were (P < 0.05) positively correlated to soil nitrification rate, microbial biomass C and N, the availability of soil soluble organic C and N, and root NH₄⁺/NO₃^– uptake rate in the rhizosphere.

Conclusions AWD could improve the dissolved oxygen and redox potential in the rhizosphere, establish a well rhizospheric environment, thus improve the content of soil soluble organic carbon, soil microbial biomass-carbon/nitrogen and the enzyme activity related to C/N metabolism. This benefits the rhizosphere nitrification rate and root NH₄⁺/NO₃^– uptake and the improvement of the accumulation of rice dry matter and nitrogen use efficiency.