Interspecific interactions contribute to higher forage yield and are affected by phosphorus application in a fully-mixed perennial legume and grass intercropping system

Although intercropping is a common technique that has been widely applied, the effect of phosphorus (P) fertilization on productivity of a fully-mixed perennial intercropping system composed of four species, including two legumes and two grasses, has rarely been studied. The objective of this study was to determine the effect of interspecific interactions on forage yield and the influence of P application on interspecific competitiveness and yield in a fully-mixed intercropping system of alfalfa (Medicago sativa L.), sainfoin (Onobrychis viciifolia Scop.), orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.). A three-year (2016-2018) field experiment was conducted in central China with an annual rainfall and temperature of 456 mm and 9.5 degrees C, respectively. Five legume and grass seeding ratios (L: G) of 5:5, 4:6, 3:7, 6:4, and 7:3 were included, with four monocultures used as the control. The total forage yield was measured when 20% of the alfalfa flowers had opened (BBCH 62). Competitiveness indices, including the land equivalent ratio (LER), aggressivity (A), relative crowding coefficient (K values), competitive ratio (CR), and actual yield loss (AYL), were assessed. The total forage yield increased under fully-mixed intercropping in 2017-2018 when a transgressive overyielding advantage was attained, which was contributed by the interspecific interactions and P application. Moreover, the maximum yield and highest intercropping advantage appeared in L: G 3:7 and L: G 5:5 under 80 kg ha(-1) P application in 2017-2018. Additionally, adequate P (80 kg ha(-1)) contributed to higher A values, K values, CR, and AYL of alfalfa and orchardgrass than in sainfoin and tall fescue, which suggested greater interspecific benefits and land-use efficiency. The fully-mixed intercropping system of alfalfa, sainfoin, orchardgrass, and tall fescue realized these potential advantages by coordinating interspecific interactions and environmental resources, which were manipulated by P application.