[Effects of Soil Acidification on the Distribution and Availability of Arsenic in Aggregates]

The spatiotemporal substitution method was used to collect yellow soil and purple soil with different acidification degrees； analyze the effects of acidification on the composition and properties of soil aggregates； explore the distribution characteristics, occurrence forms, and availability of arsenic （As） in soil aggregates with different acidification degrees； and reveal the key factors restricting the availability of soil As. The results showed that the distribution of total As in each aggregate was positively affected by its mass fraction, and the larger the mass fraction, the more the total As distribution. Soil acidification destroyed the aggregate structure, reduced the mass fraction of large aggregate S1 （>2 mm） by 5 to 15 percentage points, led to the increase in amorphous iron content by 2 to 7 times, and decreased the total As distribution of S1. The available As was positively regulated by non-obligate/obligate adsorptive arsenic （F1, F2） （F1+F2 correlation coefficient 0.76） and negatively affected by residual arsenic （F5） （correlation coefficient -0.89）. Soil acidification significantly reduced the ratio of F1 to F2 after exogenous As entered the soil, resulting in a decrease in ω （available As） from 6.71-15.37 mg·kg^-1 to 2.36-9.26 mg·kg^-1, and the highest exogenous available As content was found in S1 and microaggregate S4 （<0.25 mm）. Simultaneously, exogenous available As was also negatively affected by organic matter （correlation coefficient -0.72） and positively regulated by available phosphorus and CEC （coefficients 0.40 and 0.52, respectively）. There were differences in soil property factors that restricted the availability of As in aggregates of different particle sizes. Soil acidification in S2 （1-2 mm）, S3 （0.25-1 mm）, and S4 aggregates significantly increased the amorphous iron and thus reduced the availability of As, and the exogenous available As in S3 was not significantly affected by CEC. However, the effective As from all agglomerated sources was affected by organic matter, nitrogen, and potassium. The results provide a basis for the accurate control of soil As availability and pollution remediation.