The Effects of Microplastics on Aggregates and Carbon–Nitrogen Contents in Soils with Different Iron Contents

Abstract

Exploring the interactive influences of microplastics (MPs) and iron oxides on soil aggregates and carbon–nitrogen (C-N) pools helps clarify pollution risks in subtropical paddy soils, and this study systematically identifies unresolved knowledge gaps regarding MP-Fe coupled effects to solidify its research objectives. Taking paddy soil in Guangdong as the research object, four treatments were set up: control (CK), microplastics (1 g/kg), iron (10 g/kg), and iron (10 g/kg) + microplastics (1 g/kg). The distribution characteristics of soil aggregates and the variation characteristics of C-N contents in soils with different iron contents under microplastic pollution were explored. The results showed that without iron addition, microplastics significantly increased the content of 0.250 ~ 2.000 mm aggregates by 20.37% (P < 0.05), the mean weight diameter (MWD) of soil aggregates by 26.49%, and the geometric mean diameter (GMD) by 16.22% (P < 0.05). With iron addition, microplastics significantly increased the content of < 0.053 mm aggregates by 18.38% (P < 0.05), but had no significant effect on the stability indices of soil aggregates. In terms of soil C-N contents and their component contents, without iron addition, microplastics significantly increased soil dissolved organic carbon (DOC) by 1.38 times (P < 0.05), but had no significant effect on other C-N contents. With iron addition, microplastics significantly increased soil DOC content by 1.25 times (P < 0.05), and also significantly increased the C-N contents of < 0.053 mm aggregates (P < 0.05). Correlation analysis showed that soil aggregate stability indices were significantly correlated with soil total nitrogen (TN) content (P < 0.05), indicating that TN is the main factor affecting soil aggregate stability. Principal component analysis (PCA) found that microplastics had effects on soil aggregate stability and C-N contents under different addition methods, among which the iron + microplastics treatment had the highest comprehensive score. This work further interprets surface chemical interactions between MPs and iron oxides, and supplements discussion on potential microbial driving processes to rationalize all observed differential responses. MPs reshape aggregate particle distribution and internal C-N storage via iron-dependent pathways, with distinct regulatory patterns under high vs low soil iron levels.