Objectives This study aimed to investigate the effects of phosphorus fertilizers, particularly microalgal fertilizers, on the composition and stability of soil aggregates and the content of available phosphorus, so as to provide a theoretical basis for selecting new, efficient, and environmentally friendly fertilizers.

Methods An indoor incubation experiment was conducted using low-phosphorus soil collected from long-term unfertilized plots at the Yucheng Experimental Base of the Dezhou Saline-Alkali Soil Improvement Experimental Station, Chinese Academy of Agricultural Sciences. The experiment treatments composed of a no fertilizer control (CK), two chemical fertilizer treatments (compound fertilizer (CF) and ammonium polyphosphate fertilizer (APP)), and three microalgal fertilizer treatments: wild-type algal fertilizer (AF-WT), high-accumulation Polyphosphate algal fertilizer (AF-HP), and low-phosphate algae fertilizer (AF-LP). Soil alkaline phosphatase (ALP) activity, aggregate composition (>2 mm, 0.25−2 mm, 0.053−0.25 mm, and <0.053 mm), aggregate stability indicators (mean weight diameter, MWD; geometric mean diameter, GMD; percentage of water-stable aggregates >0.25 mm, W_R>0.25), and available P content (AP) in aggregates were measured. Principal component analysis (PCA) was employed to analyze the dynamic effects of different treatments on soil properties.

Results Compared to the control (CK), microalgae fertilizers (AF-WT, AF-HP, AF-LP) significantly increased soil alkaline phosphatase (ALP) activity, with AF-HP showing the highest activity (1.02 nmol/(g·h)) at 90 days of incubation−a 23.47% increase. In contrast, chemical fertilizers (CF, APP) reduced ALP activity by 28.80% and 25.58%, respectively. AF-HP and AF-WT notably elevated the proportion of 0.053−0.25 mm aggregates while reducing the <0.053 mm fraction. CF and APP temporarily raised the share of 0.25−2 mm aggregates in the short term, but their effects declined over time. Microalgae fertilizers enhanced the mean weight diameter (MWD) and geometric mean diameter (GMD) of aggregates, with AF-HP achieving the highest MWD (0.84 mm, a 17.52% increase over CK) at 90 days. The available P content in >2 mm aggregates under AF-HP and AF-WT reached 2.90 and 2.79 mg/kg, respectively, which were 2.15 and 2.07 times that of CK. Chemical treatments exhibited greater fluctuation and weaker long-term efficacy. AF-HP also significantly improved the contribution rate of available P in macro-aggregates to 53.58% at 90 days, a 32.27% increase over CK, outperforming CF (27.22%) and slightly trailing APP (37.28%). Principal component analysis revealed a positive correlation between AF-HP, available P, and ALP activity at 90 days. Although AF-LP transiently stimulated enzyme activity at 3 days, its long-term correlation weakened.

Conclusion AF-HP demonstrates significant advantages in long-term phosphorus supply and enzyme activity activation, while AF-LP excels in short-term enhancement of enzyme activity. APP improves aggregate structure in the short term, whereas CF shows limited comprehensive effects. By increasing alkaline phosphatase activity and promoting the formation of 0.25−2 mm and 0.053−0.25 mm aggregates, microalgae fertilizer enhances the available phosphorus content in soil aggregates of various sizes, significantly improving soil structural stability and phosphorus availability.