PRDI can maintain aboveground biomass and increase economic benefits in alfalfa through regulating N:P ratios in roots and leaves

Abstract

N:P ratios is often used to identify nitrogen and/or phosphorus limitation status of crop growth and further provides optimal fertilizer management for crop production. Although partial root-zone drying irrigation (PRDI) has been verified to influence N:P ratios in nitrophilic and nitrogen-demanding plants, the effect of PRDI on N:P ratios in N-fixing plants is not well documented. This study, conducted in 2014–2016, focused on alfalfa (Medicago sativa L.) to investigate the effect of PRDI on N:P ratios in roots, stems and leaves of N-fixing plant and on the resulting net economics. The field experiment was designed with two irrigation modes: PRDI and conventional furrow irrigation (CFI); and four irrigation volumes: 70 % ETc (evapotranspiration), 85 % ETc, 100 % ETc, 115 % ETc. PRDI maintained aboveground biomass, and it increased both economic benefits and N:P ratios in alfalfa roots, stems and leaves. PRDI transitioned alfalfa plants from N and P co-limitation to P limitation. The N:P ratios in roots were most useful for identifying N and/or P limitation status in alfalfa plants in a PRDI environment. A structural equation model (SEM) showed that irrigation mode indirectly impacted aboveground biomass by positively affecting N:P ratios in roots and leaves, and irrigation volume directly affected aboveground biomass. This study further found that the optimal combination were PRDI with 97 % ETc irrigation volumes in alfalfa production. These results suggest a mechanism for N-fixing plant adaptation to the changing environment caused by PRDI and provide useful information for water and nutrient management in alfalfa production.