Abstract:

In order to explore the effects of nitrogen and phosphorus fertilizer supply on forage dry matter yield and regrowth of alfalfa （Medicago sativa） in seasonal cultivation systems in the Jianghuai area， field experiments were conducted in 2017 and 2018 with 4 phosphorus （P₂O₅） rates （0， 50， 100， 150 kg·ha^-1） and 4 nitrogen （N） rates （0， 60， 120， 180 kg·ha^-1）. The forage dry matter yield and yield components， the content and accumulation of nitrogen and phosphorus in the shoot， and the regrowth characteristics at 6 days and 12 days after cutting were examined. It was found that： 1） The application of nitrogen and phosphorus fertilizers significantly promoted alfalfa growth. The forage dry matter yield increased with increasing nitrogen supply when P₂O₅ supply was lower. In the treatments with high P₂O₅ supply， the optimal nitrogen application rate for alfalfa growth was 120 kg·ha^-1. The dry matter yield increased significantly with increase in phosphorus application rate， and this effect was independent of nitrogen supply. 2） There were significant positive correlations between forage dry matter yield and shoot nitrogen content， shoot nitrogen accumulation， and shoot phosphorus accumulation. 3） Nitrogen and phosphorus fertilizer application significantly promoted alfalfa regrowth. For P₂O₅ application rates of 0， 50， 100 and 150 kg·ha^-1， the optimal nitrogen application rates for alfalfa regrowth were 180， 120， 120 and 60 kg·ha^-1， respectively. At 6 days after cutting， both the shoot length and leaf area were positively correlated with alfalfa biomass. At 12 days after cutting leaf area was positively correlated with alfalfa biomass. In conclusion， the application of phosphate fertilizer was beneficial for reducing nitrogen fertilizer rates in a seasonal alfalfa cultivation system in the Jianghuai area and the optimal nitrogen fertilizer application rate decreased as phosphate fertilizer application rate increased. Based on these data the most suitable combination of annual phosphorus and nitrogen application rates for seasonal alfalfa cultivation systems in the Jianghuai area was 100 kg·ha^-1 phosphorus and 120 kg·ha^-1， nitrogen fertilizer. These results provide reference data for development of fertilizer management recommendations in seasonal alfalfa cultivation system in the Jianghuai area.

Key words: alfalfa, nitrogen fertilizer, phosphorus fertilizer, regrowth, forage dry matter yield