Mechanisms of fumigant-induced decline in soil aggregate stability under continuous cropping

Soil fumigation is widely used to control soil-borne diseases in continuous cropping systems, but its effects on soil aggregate structure remain insufficiently understood. In this study, we combined multi-site field surveys with controlled laboratory incubations to evaluate fumigation-associated changes in dry aggregate-size distribution and their relationships with microbial and binding-related indicators. The field survey showed that fumigation did not consistently alter soil aggregate distribution across all production regions; instead, the response varied markedly with soil background and fumigant type. Under controlled conditions, Qianshan loam soil from Anhui Province showed the strongest structural response, whereas Shandong soil was relatively insensitive to fumigation. In Qianshan soil, dazomet (DZ) and chloropicrin (CP) reduced dry-sieved mean weight diameter (MWD) by 12.4% and 19.1%, respectively, accompanied by decreased extracellular polysaccharide content, lower fungal abundance, reduced visible hyphal networks, and changes in selected Fe/Al oxide fractions. Although all four fumigants significantly reduced dry-sieved MWD in Qianshan soil, they produced distinct microbial and mineral-associated responses. AITC significantly enriched the EPS-producing genera Streptomyces and Microbacterium, whereas DMDS increased selected Fe and Al oxide fractions. Random forest analysis identified bacterial abundance, fungal abundance, and extracellular polysaccharide content as the variables most strongly associated with dry-aggregate MWD variation, suggesting that microbial binding-related indicators were closely linked to short-term aggregate redistribution after fumigation. Importantly, post-fumigation supplementation with Trichoderma longibrachiatum partially restored dry aggregate-size distribution after 150 d, as reflected by the alleviation of microaggregate accumulation. Overall, these findings indicate that fumigation-induced changes in mechanically stable aggregate structure are not universal but depend on soil and fumigant properties, and that beneficial fungal supplementation may promote post-fumigation structural recovery in sensitive soils. This study provides a basis for assessing the structural impacts of soil fumigation and developing integrated fumigation–restoration strategies in continuous cropping systems.