Abstract: In the black soil region of Northeast China, long-term excessive application of phosphorus fertilizers has led to soil phosphorus accumulation and reduced availability. Activating soil available phosphorus (AP) is therefore crucial for sustainable agricultural development. Previous studies have shown that low-molecular-weight organic acids are effective phosphorus activators, with oxalic and citric acids exhibiting particularly significant effects. Their mechanisms involve the synergy of acid dissolution, complexation, and competitive adsorption. However, systematic studies are still lacking regarding the comparative effects of various organic acids on the dynamic transformation of phosphorus forms in the black soils of Northeast China, which are non-calcareous. To address this gap, this study selected four low-molecular-weight organic acids-oxalic, citric, malic, and ascorbic acids and conducted an incubation experiment. The experiment aimed to investigate the activation performance and mechanisms of these acids at different concentrations (0% to 2%) on available phosphorus in both dark brown soil and typical black soil. We employed the Hedley sequential extraction procedure to fractionate soil phosphorus. The phosphorus content in each fraction was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) to analyze its dynamic changes. Redundancy analysis (RDA) was used to elucidate the relationships between phosphorus fractions and soil environmental factors. The results showed that: (1) The addition of organic acids significantly increased soil AP content, which peaked around day 20 of incubation and then gradually stabilized. The enhancement effect strengthened with increasing concentration, with the 2% treatment being the most pronounced. (2) Oxalic and citric acids exhibited the strongest activation capacity. Their 2% treatments maximally increased AP content in the two soils by 143.9% and 120.1%, respectively. This is consistent with the existing conclusions that both have significantly enhanced the activation effect of organic phosphorus in non-limestone soils. (3) Analysis of phosphorus transformation revealed distinct activation pathways: oxalic acid primarily decreased calcium-bound phosphorus (HCl-P); citric acid concurrently promoted the transformation of moderately stable organic phosphorus (NaOH-P_o) and HCl-P; malic acid mainly affected iron/aluminum-bound inorganic phosphorus (NaOH-P_i); whereas ascorbic acid activated residual phosphorus (Res-P). RDA confirmed that despite being non-calcareous, the black soils exhibited high calcium content (HCl-P accounted for 19.0%) and high organic matter. Consequently, phosphorus activation was predominantly governed by calcium complexation, with soil pH and dissolved organic carbon (DOC) identified as the key environmental factors driving phosphorus form transformation. The activation pathway primarily focused on the acidolysis and complexation of calcium-bound phosphorus (HCl-P), diverging significantly from the mechanism in typical acidic non-calcareous soils, which is dominated by the release of iron/aluminum-bound phosphorus. This highlights the differences in activation mechanisms among non-calcareous soils due to varying substrate properties. In Northeast China’s black soils, oxalic and citric acids hold an advantage in enhancing phosphorus availability through the combined effects of acidolysis, ligand exchange, and the promotion of organic phosphorus mineralization. This insight provides a basis for the rational regulation of phosphate fertilizer use in the black soil region.