Interactive effects of nitrogen and potassium on: Grain yield, nitrogen uptake and nitrogen use efficiency of rice in low potassium fertility soil in China

Elucidating the mechanism underlying the interactive effects between nitrogen (N) and potassium (K) on rice yield is requisite to further study soil K deficiency in paddy ecosystems in Southeast Asia. Field studies with combined application of four N rates (0, 90, 180, and 270 kg N ha(-1)) and four K rates (0, 60, 120, and 180 kg K20 ha(-1)) in 2013, 2014 and 2016 and a hydroponic experiment in 2017 were conducted to investigate their effects on rice yield as well as N use efficiencies (NRE). Co-application of N and K significantly promoted the growth and development of root and leaf, grain yield and NRE of rice. The application of N combined with zero-K increased the grain yield by 10.6-22.0%, 24.0-37.4% and 12.7-21.9% in 2013, 2014 and 2016 respectively. Similarly, leaf area index (LAI) increased by 5.9-17.9%, 8.1-19.7% and 27.9-62.7% across the three years. An increase of grain yield by 15.5-32.5%, 26.6-46.1% and 11.5-23.0% and LAI by 22.6-31.5%, 34.2-42.8% and 26.7-79.1% across the three years was recorded due to combined application of N and K. Application of higher doses of K resulted in relatively higher N uptake and N NRE. Application of K improved the N uptake and NRE by 5.6-10.0%, 7.4-16.2%, 10.9-26.4% and 12.0-22.3%, 16.9-36.4%, 20.6-43.3 % respectively across the three years. To deduct the effect of soil N and K background value, we also did a hydroponic experiment to do some physical test. In the hydroponic experiment, the leaf and root glutamine synthetase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were significantly improved with the improvement of N and K. The leaf and root GS, GOGAT and GDH activity of the treatment with sufficient N and K (+N + K) was significantly increased by 402.3%, 332.5%, 265.2% and 200.0%, 410.4%, 239.0% respectively compared with that of the treatment with deficient N and K (-N-K). These results indicated that the combined use of N and K could further improve the grain yield as well as NRE in rice production.