Effects of Phosphorus Addition on Inorganic Phosphorus Fractions and Phosphorus Accumulation in Alfalfa in Alkaline Soils

Distribution and availability of soil inorganic phosphorus fractions significantly influence plant phosphorus uptake and crop yield, particularly in alkaline soils, where phosphorus availability is often constrained by soil chemical properties. This study investigated the contribution of different phosphorus fractions to phosphorus uptake and yield of alfalfa by applying four phosphorus addition levels: 0 kg/hm2, 50 kg/hm2, 100 kg/hm2 and 150 kg/hm2, designated as P0, P50, P100, and P150, respectively, over two consecutive years. Correlation analysis and multiple linear regression analysis were employed to analyze the data. The results revealed that in alkaline soils, inorganic phosphorus fractions were dominated by aluminum-bound phosphate (Al-Pi) and decacalcium phosphate (Ca10-Pi), with storage contribution rates of 33.92% and 37.11%, respectively. In contrast, the cumulative storage contribution rates of dicalcium phosphate (Ca2-Pi), octocalcium phosphate (Ca8-Pi), iron-bound phosphorus (Fe-Pi) and occluded phosphorus (O-P) accounted for 28.97%. Although the storage contribution rate of Ca10-Pi was relatively low, its output contribution rate was high, rendering it easily absorbed and depleted by plants, thereby serving as an important source of soil phosphorus availability. Among these fractions, O-Pi was identified as the primary source of phosphorus for alfalfa, playing a critical role in P nutrition. Furthermore, Ca8-Pi exhibited a significant positive correlation with phosphorus uptake in alfalfa (R2 = 0.98, p < 0.05) and was identified as a key factor influencing alfalfa yield, making it a reliable predictor for yield estimation.