AUTHOR=Yang Kaixin , Li Shengyi , Sun Yanliang , Cartmill Andrew D. , López Ignacio F. , Ma Chunhui , Zhang Qianbing 

TITLE=Effects of combined nitrogen and phosphorus application on soil phosphorus fractions in alfalfa (Medicago sativa L.) production in China

JOURNAL=Frontiers in Plant Science

VOLUME=15

YEAR=2024

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2024.1380738

DOI=10.3389/fpls.2024.1380738

ISSN=1664-462X

ABSTRACT=<p>Nitrogen (N) and phosphorus (P) fertilizers change the morphological structure and effectiveness of P in the soil, which in turn affects crop growth, yield, and quality. However, the effects and mechanism of combined N and P application on the content of P fractions and the transformation of effective forms in alfalfa (<italic>Medicago sativa</italic> L.) production is unclear. This experiment was conducted with four levels of N: 0 (N<sub>0</sub>), 60 (N<sub>1</sub>), 120 (N<sub>2</sub>) and 180 kg·ha<sup>-1</sup> (N<sub>3</sub>); and two levels of P (P<sub>2</sub>O<sub>5</sub>): 0 (P<sub>0</sub>) and 100 kg·ha<sup>-1</sup> (P<sub>1</sub>). The results indicated that, under the same N level, P application significantly increased soil total N, and total P, available P, and content of various forms of inorganic P when compared to no P application, while decreasing the content of various forms of organic P and pH value. In general, under P<sub>0</sub> conditions, soil total N content tended to increase with increasing N application, while total P, available P content, pH, inorganic P content in all forms, and organic P content in all forms showed a decreasing trend. When compared to no N application, insoluble P (Fe-P, O-P, Ca<sub>10</sub>-P) of the N application treatments was reduced 2.80 - 22.72, 2.96 - 20.42, and 5.54 - 20.11%, respectively. Under P<sub>1</sub> conditions, soil total N and O-P tended to increase with increasing N application, while, pH, Ca<sub>2</sub>-P, Al-P, Fe-P, Ca<sub>10</sub>-P, and organic P content of each form tended to decrease. Total P, available P, and labile organic P (LOP) of N application reduced 0.34 - 8.58, 4.76 - 19.38, and 6.27 - 14.93%, respectively, when compared to no application. Nitrogen fertilization reduced the soil Ca<sub>2</sub>-P ratio, while P fertilization reduced soil Fe-P, moderately resistant organic P (MROP), and highly resistant P (HROP) ratios, and combined N and P elevated the Ca<sub>8</sub>-P to LOP ratio. The results of redundancy analysis showed that soil total N content, available P content, and pH were the key factors affecting the conversion of P fractions in the soil. Nitrogen and P reduced the proportion of soil insoluble P, promoted the activation of soil organic P, resulting in accumulation of slow-acting P in the soil, thereby improving the efficiency of soil P in alfalfa production.</p>