Coupling effect of alternate wetting and drying irrigation and nitrogen rate on organic acid in rice root secretion at heading stage

Soil water and nitrogen conditions are the principal factors that affect crop growth and formation. Clarifying effects of soil water and nitrogen conditions and their coupling on grain yield of rice (Oryza sativa L.) have significant implications for high production efficiency and yield. There is extensive domestic and foreign research on the interactions of soil water and fertilizer. Specific focus has been put on ground parameters such as crop growth development, dry shoot formation, physiological function, hormone change, nutrient absorption and utilization, water use efficiency, etc. However, there is less work on the characteristics of root secretion, including changes in organic acids in roots. Research on the relationship between organic acid content and nitrogen utilization and the related interactions is also inconsistent and therefore inconclusive. The purpose of this study was to investigate the performance of root secretion under different nitrogen and water regimes and analyze the effect of the interaction of alternate wetting and drying irrigation and nitrogen levels. To that end, a pot-soil experiment was conducted using a mid-season japonica rice cultivar 'Lianjing 7' under different nitrogen rates0N, MN (240 kg·hm^-2) and HN (360 kg·hm^-2) and three irrigation regimeskeeping water (0 kPa), alternate wetting and moderate drying (-20 kPa) and alternate wetting and severe drying (-40 kPa) in 2015 and 2016. The results showed that there was remarkable interaction between nitrogen rate and irrigation regime with the results similar for the two years. Under the same nitrogen rate, tartaric acid, citric acid, oxalic acid, malic acid, succinic acid, total organic acid and amino acid secretion of rice root at heading stage were higher (respectively by 13.2%, 8.7%, 27.3%, 40.0%, 6.7%, 6.3% and 6.4%) in treatment of alternate wetting and moderate drying than in treatment of keeping water flood irrigation. The nitrogen agronomic utilization and partial productivity of rice were increased by 4.1% and 1.7% respectively. The secretion amounts of tartaric acid, citric acid, oxalic acid, malic acid and succinic acid at heading stage decreased (respectively by 16.4%, 4.5%, 12.8%, 41.7% and 5.6%) under the treatment of alternate wetting and severe drying than under the treatment of keeping water. This also reduced total organic acid and amino acid in roots and overall nitrogen use efficiency. Under the same irrigation condition, nitrogen application accelerated the root secretion of tartaric acid, acetic acid, malic acid and succinic acid, but decreased the secretion of oxalic acid and citric acid at heading stage of rice. The differences in tartaric acid, acetic acid, malic acid and succinic acid secretion by roots were small between MN and HN treatments. Nitrogen application has positive effects on total organic acid, amino acid, malic acid, succinic acid secretion. Also its interaction effect with alternate wetting and moderate drying was positive, while the interaction effect with alternate wetting and sever was negative. Correlation analysis indicated that citric and oxalic acids secretion by rice roots was significantly positively correlated with nitrogen utilization. However, acetic acid was negatively correlated with nitrogen use efficiency. The results of the study suggest that increasing root secretion capacity through appropriate regulation of irrigation coupled with nitrogen rate increased nitrogen use efficiency and thereby promoted high yield of rice.