Effect of alternative wetting and drying irrigation and nitrogen coupling on rhizosphere environment of rice

Abstract: Soil moisture and nitrogen nutrient are the principal factors affecting rice (Oryza sativa L.) production. Elucidation of their influences and coupling effects on grain yield of rice would have great significance for high yield and high efficiency. Domestic and foreign scholars have conducted extensive research on the interaction of water and fertilizer. The former focuses on the ground, such as crop growth development, physiological function, hormone change, nutrient absorption and utilization, water use efficiency and other aspects of the studies, few papers are for soil and root secretion characteristics, and their interaction and the conclusions are not consistent. The purposes of this study were to investigate the effects of water and nitrogen coupling on soil enzyme activity, microorganism quantity, root secretion and coupling effect. A mid-season japonica rice cultivar of Xindao 20 was pot-grown. Three treatments of different nitrogen levels, i.e. 0N, MN (240 kg/hm2) and HN (360 kg/hm2) and three irrigation regimes, i.e. submerged irrigation (0 kPa), alternate wetting and moderate drying (-20 kPa) and alternate wetting and severe drying (-40 kPa) were conducted in 2014 and 2015. Some indices, such as urease enzyme, sucrose enzyme, catalase enzyme, bacteria, fungi, actinomycetes quantity and total organic acid in root at different stages were investigated in the experiment. Results showed that there was a significant interaction between irrigation regimes and nitrogen levels, and no significant difference was observed between the 2 years. In the same nitrogen levels, urease enzyme, sucrose enzyme, catalase enzyme activity in soil at main growth stages were higher under the condition of alternate wetting and moderate drying compared with the submerged irrigation, and meanwhile bacteria, fungi and actinomycetes quantity in soil were also increased at main stages, and total content of organic acid was enhanced. So mild water stress and MN enhanced soil enzyme, microorganism and organic acid content, and formed the best mode in this paper, which was referred as the water-nitrogen coupling management model. The opposite result was observed under the condition of alternate wetting and severe drying. Soil enzyme, microorganism quantity and organic acid content in the root secretion at main stages were decreased significantly. In the same irrigation regime, soil enzyme and microorganism quantity at main growth stages were higher under the condition of MN treatment when compared with no nitrogen applied, and meanwhile total content of organic acid was enhanced significantly. The opposite result was observed under the condition of HN treatment, which indicated that heavy nitrogen application decreased soil enzyme and microorganism quantity, and organic acids of root secretion were also reduced significantly. Correlation analysis showed that there was significant or extremely significant positive correlation between soil enzyme, microorganism quantity and total organic acid content at main growth stages. Positive effects were observed in the effect of nitrogen fertilizer for soil enzyme activities, microbial quantity and total content of organic acid, and water stress and interaction effect were also positive under the condition of alternate wetting and moderate drying, while negative effect was observed under the condition of alternate wetting and severe drying. These results suggest increasing soil enzyme activity and microorganism quantity, and improving organic acids of root secretion through the appropriate regulation of water and nitrogen, will create a good rhizosphere environment for the growth of rice.