Abstract: This research explored the effects of balanced fertilization on photosynthetic characteristics and yield of alfalfa, in order to clarify the contribution of photosynthetic performance to herbage accumulation in alfalfa, in a field experiment. The alfalfa variety ‘Gannong No.3’ alfalfa was used as plant material and the ‘3414’ experimental design, an incomplete factorial of N, P and K fertilizer applications widely used in China over the last 15 years, was adopted in this study. It was found that: 1) Balanced fertilization enhanced the photosynthetic characteristics of alfalfa by increasing chlorophyll content, RuBP carboxylase activity, photosynthetic rate, carbohydrate content and population leaf area index. The combined application of N at 103.5 kg·ha^-1, P₂O₅ at 105 kg·ha^-1 and K₂O at 90 kg·ha^-1 was the combination that optimized the photosynthetic characteristics of alfalfa. This nutrient combination can therefore be described as balanced fertilization. 2) Under balanced fertilization, the hay yield of alfalfa was significantly increased, and the total annual yield reached 25636.26 kg·ha^-1. Compared with other fertilizer combinations tested, balanced fertilization ensured efficient alfalfa production. 3) Through the principal component analysis of normalized data, it was found that the population leaf area index, RuBP carboxylase activity and chlorophyll were the photosynthetic factors that contributed the most to the yield development of alfalfa. The influence on yield of these factors was, respectively, 22.8%, 21.3% and 15.9%. To summarize, balanced fertilization is an important to obtaining a high yield when growing alfalfa, and the production performance of alfalfa can be effectively improved by regulating the main photosynthetic factors such as leaf area index, RuBP carboxylase activity and chlorophyll content.

Key words: balanced fertilization, alfalfa, photosynthetic characteristics, yield effect, contribution rate