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-accumulation phosphate algal 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 with the control (CK), microalgal fertilizer treatments (AF-WT, AF-HP, AF-LP) all increased soil alkaline phosphatase (ALP) activity. The AF-HP treatment exhibited the highest enzyme activity 1.02 nmol/(g·h) at 90 days of incubation, while the activities of chemical fertilizer treatments CF and APP were 28.80% and 25.58% lower than that of CK, respectively. At 90 days of incubation, AF-HP and AF-WT treatments significantly increased the proportion of aggregates with particle size of 0.053−0.25 mm and remarkably decreased the proportion of aggregates with particle size <0.053 mm. In contrast, CF and APP treatments increased the proportion of aggregates with particle size of 0.25−2 mm in the short term, but the long-term effect diminished. Microalgal fertilizer enhanced the mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates, the AF-HP treatment exhibited a relatively higher MWD value of 0.72, which was 25.10% higher than that of CK at 30 days of incubation. At 90 days of incubation, the available phosphorus contents in aggregates with particle size >2 mm under AF-HP and AF-WT treatments were 2.90 mg/kg and 2.79 mg/kg, respectively, which were 2.15 and 2.07 times higher than that of CK. Chemical fertilizer treatments showed large fluctuations in available phosphorus content, and their long-term effects were weaker than those of microalgal fertilizers. The AF-HP treatment significantly enhanced the contribution rate of available phosphorus in macroaggregates, reaching 53.58% at 90 days of incubation, which was 32.27 percentage points higher than that of CK. The increases in CF and APP treatments were relatively small, with contribution rates of 27.22% and 37.28%, respectively. Principal component analysis (PCA) revealed that AF-HP was positively correlated with available phosphorus and ALP at 90 days of incubation. AF-LP promoted enzyme activity in the short term (3 days), but the correlation weakened in the long term.

Conclusions 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, microalgal fertilizer enhances the available phosphorus content in soil aggregates of various sizes, significantly improving soil structural stability and phosphorus availability.