Nitrogen, Phosphorus, and Potassium Resorption Responses of Alfalfa to Increasing Soil Water and P Availability in a Semi-Arid Environment
Authors: Kong Meng , Kang Jing , Han Cheng-Long , Gu Yan-Jie , Siddique Kadambot H.M , Li Feng-Min 
Published: February 2020

In semi-arid areas, alfalfa (Medicago sativa L.) is widely grown, but its growth is often restricted due to limited rainfall and soil nutrients, particularly phosphorus (P). Nutrient resorption is an effective strategy for dealing with nutrient shortages. Alleviation of these limited resources using film mulch and P fertilization—which are common practices in semi-arid areas—can affect the internal recycling of such nutrients. Little is known about such effects in alfalfa and the relationship between resorption efficiency and forage yield. We conducted a two-year field experiment in the semi-arid Loess Plateau of China using film mulch and P fertilization to investigate the response to long-term increasing soil water and P availability on leaf nitrogen (N), P, and potassium (K) concentrations and nutrient resorption characteristics in alfalfa. In green leaves, mulching significantly increased P concentration by an average of 5.5% but it had no significant effect on N concentration over two years, and it decreased K concentration by 16.1% in 2017. P fertilization significantly increased N concentrations to a greater degree in 2018 (8.1%) than 2017 (1.6%). P fertilization also significantly increased P concentrations by an average of 34.1% over two years. In contrast, P fertilization significantly decreased K concentration in the mulched treatment by an average of 17.3% in 2017 and 21.8% in 2018, but it had no effect in the no-mulch treatment. In senescent leaves, mulching significantly increased N concentration by an average of 3.9% and P concentration by an average of 16.7%, but it had no significant effect on K concentration over two years, while P fertilization significantly decreased N and K concentrations over two years by an average of 7.5%, and 32.8%, respectively. P fertilization significantly increased senesced P concentration by an average of 11.9% in 2017 and 17.5% in 2018; and year × mulching × P fertilization had a significant interaction on senesced leaf P concentration. For resorption efficiency, mulching decreased P resorption efficiency by an average of 3.0%, but it had no impact on N or K resorption efficiency, while P fertilization increased the N, P, and K resorption efficiencies in alfalfa by an average of 6.8%, 6.2%, and 76.4% over two years, respectively. Interactive effects of mulching and P fertilization were found on P and K resorption efficiencies over time. In addition, N and K resorption efficiencies were significantly higher in 2018 than in 2017. The application of P fertilizer without mulching resulted in positive correlations between forage yield and N, P, and K resorption efficiencies, but no correlations were observed under film mulch. That is, mulching changed the relationship between forage yield and N, P, and K resorption efficiencies in alfalfa, suggesting that N, P, and K resorption efficiencies may not be related to high yield. Our results provide new insights into the role of nutrient resorption in alfalfa in response to increasing soil water and P availability and the relationship between resorption efficiency and forage yield, which will help us to improve alfalfa yield in semi-arid regions.