Effects of water-saving irrigation and reducing phosphorus fertilizer on rice yield, phosphorus utilization and soil phosphorus balance in black soil

Abstract: This study aims to explore the effects of water-saving irrigation and phosphorus fertilizer reduction on the phosphorus utilization of rice and soil phosphorus balance in the black soil region of China. A field experiment was carried out in 2020 and 2021. Compared with the conventional flooding irrigation and phosphorus fertilizer application (CK, 45 kg/hm2), five phosphorus fertilizer application levels were set under water-saving irrigation mode (C): 0 kg/hm2 (P0, no phosphorus fertilizer), 18 kg/hm2 (P1, 60% phosphorus reduction), 27 kg/hm2 (P2, 40% phosphorus reduction), 36 kg/hm2 (P3, 20% phosphorus reduction), and 45 kg/hm2 (P4, conventional phosphorus application). The balance of soil phosphorus and the phosphorus fertilizer utilization rate were calculated to determine the effects of phosphorus fertilizer reduction in different levels on the phosphorus uptake by aboveground biomass, rice yield, and available phosphorus content in the soil layer under water-saving irrigation. The results showed that the content of available phosphorus in the surface soil of water-saving irrigation and phosphorus fertilizer reduction treatment after harvest in 2020 was significantly lower than that of the CK treatment (P<0.05). The available phosphorus content in the surface soil of CP3 treatment was significantly higher than that of the rest under water-saving irrigation (P<0.05) after the rice harvest in 2021. All phosphorus reduction treatments under the water-saving irrigation in 2021 were significantly improved the content of soil available phosphorus in the 20-40 cm soil layer (P<0.05). The phosphorus accumulation in the aboveground biomass of CP3 treatment in 2020 and 2021 was significantly higher than that of CP4 and CK treatment, which were 14.64 and 15.86 kg/hm2, respectively (P<0.05). The phosphorus accumulation in each organ of the rice aboveground part was ranked in the order of the grain > stem and sheath > leaf. The phosphorus accumulation in the grain, stem, and sheath of CP3 treatment was significantly higher than that of the rest (P<0.05) in 2020 and 2021. The rice yield of the CP3 treatment was significantly higher than that of the CK treatment in 2020 and 2021 (P<0.05), the same as the CP4 treatment in 2021, whereas, there was no significant difference in 2020 (P>0.05). The rice yield of the CP3 treatment was the largest in the two-year experiment, which was significantly higher than that of the rest treatments (P<0.05). The two-year CP3 treatment had significantly improved the absorption and utilization efficiency of phosphorus fertilizer, as well as the agronomic utilization rate, indicating less surplus of soil phosphorus (P<0.05) in 2020 and 2021, compared with the conventional fertilization. Consequently, the 20% reduction of conventional phosphorus fertilizer application rate was optimal under water-saving irrigation in the black soil region. Therefore, the higher rice yield and phosphorus fertilizer utilization rate can be achieved in the two-year experiment, particularly for the less surplus of soil phosphorus.