Interactive effects of irrigation volume and phosphorous application rate on yield and agronomic efficiency of applied phosphorous on alfalfa

Irrigation and phosphorous application rates are two key factors affecting the yield of alfalfa (Medicago sativa) pastures. In this study, a greenhouse pot experiment was conducted to identify an optimal combination between irrigation and phosphorous application rate. The effects of different combinations of irrigation volume and phosphorous application rates on the plant height, branch number, leaf number, stem diameter, dry yield per unit area (YPUA), dry yield per plant (YPP) and agronomic efficiency of applied phosphorous (AE_P) were investigated. Specifically, the irrigation volumes tested consisted of 40%～45%, 60%～65% and 80%～85% of saturated soil moisture and phosphorous amounts of 0, 64, 127 and 191 mg·kg⁻¹ were applied. We noticed that for the first alfalfa crop harvest, the plant height, branch and leaf numbers increased with an increase in irrigation volume, while the stem diameter, YPUA and YPP first increased and then decreased. However, the latter parameters first increased and then remained relatively stable for the second harvest. Further, the AE_P first remained relatively stable and then decreased. Furthermore, plant height, leaf number, YPUA and YPP increased with an increase in phosphorous application rate. The branch number and stem diameter first increased and then decreased, while the AE_P first increased and then remained relatively stable. The interaction between irrigation volume and phosphorous application rate displayed significant effects on the plant height, branch number, stem diameter, YPUA, YPP and AE_P (P < 0.05). The relationships between YPUA and AE_P and irrigation volume showed the downward response in 3D graph. Specifically, irrigation volume leading to 60% saturated soil moisture content, and phosphorous addition of 127 mg·kg⁻¹ led to the largest alfalfa YPUA and the highest AE_P. These data demonstrated that there exists an optimally theoretical combination between proper irrigation volume and phosphorous application rate under which alfalfa crop displays maximum yield.