Phosphorus fraction characteristics and influencing factors of soil aggregates in pure and mixed forests.

Phosphorus components in the soil aggregates of different particle sizes is critical for improving phosphorus availability in north subtropical regions. We investigated two representative stand types, pure stands and the mixed stands of and in Lishui District, Nanjing. We measured soil aggregates from two soil layers (0-20 cm and 20-40 cm) to determine the stability of soil aggregates, labile phosphorus fraction content and the acid phosphatase activity of each particle size, as well as litter biomass and fine root biomass, to analyze the differences of active phosphorus fractions of soil aggregates between different stand types. The results showed that the content of macroaggregates (>2 mm) was higher than those of other particle sizes in different soil layers of both stand types. The proportion of macroaggregates was 48.4% in mixed stands and 43.7% in pure stands. In the 0-20 cm soil layer, the stability of soil aggregates was significantly greater in mixed stands than in pure stands. In the 0-20 cm soil layer, the total phosphorus (TP) content in aggregates of both stands ranged from 181.82 to 273.34 mg·kg, without significant difference between the two stands, while that in the 20-40 cm layer ranged from 172.51 to 251.49 mg·kg, with the mixed stands exhibiting significantly higher TP in macro-aggregates (>2 mm) than the pure stands. In the 0-20 cm soil layer, available phosphorus (AP) content in aggregates ranged from 18.29 to 33.10 mg·kg, without difference between the two stands, while that in the 20-40 cm layer ranged from 11.78 to 25.22 mg·kg, with the mixed stands having higher AP across all aggregate particle sizes than the pure stands. Acid phosphatase activity in mixed stands was significantly higher than in pure stands across all soil aggregate sizes and depth layers. Fine root biomass in the mixed stands was significantly greater than in the pure stands in both soil layers. In contrast, litter biomass did not differ between stands. Generalized linear mixed models revealed that soil pH, acid phosphatase activity of soil aggregates, and stand types exerted significant effects on labile phosphorus content of soil aggregates in the 0-20 cm soil layer, explaining 32.0%, 15.7%, and 9.0% of the variance, respectively. In the 20-40 cm layer, stand type, acid phosphatase activity of soil aggregates, fine root biomass, and litter biomass all showed significant influences, accounting for 22.0%, 16.5%, 16.2%, and 10.6% of the variance. Compared to pure stands, mixed stands enhanced soil labile phosphorus content by influencing multiple environmental factors.