Spatial and seasonal variation in soil organic matter responses to landfill disturbance in the Hyrcanian Forests

Forest ecosystems are increasingly exposed to anthropogenic pressures that threaten soil functioning and long-term ecosystem stability. Among these pressures, landfill-based waste disposal has emerged as a growing concern in developing regions, particularly where forest soils play a central role in organic matter turnover and nutrient retention. However, the responses of these systems to waste-derived inputs under contrasting climatic settings remain insufficiently constrained. In this study, we examine how municipal solid waste disposal alters soil organic matter characteristics, with emphasis on carbon (C)- and nitrogen (N)-related properties, across climatically distinct forest regions of northern Iran. In this regard, soil samples from landfill-impacted sites and nearby forest control sites across four seasons and three soil depths were obtained from four forested areas (i.e., Chalous, Noor, Shirgah, and Behshahr) arranged along a west–east climatic gradient with progressively lower precipitation and higher temperatures. The results revealed that control forest stands, particularly in Chalous and Noor, exhibited significantly higher macro- and micro-aggregate-associated C and N, as well as greater stocks of labile organic matter fractions in surface soils, compared with landfill-affected sites. These differences were most pronounced in the upper soil layers and during summer and fall. In contrast, landfill sites, especially in the warmer and drier regions of Shirgah and Behshahr, showed reduced C contents in free and occluded light fractions and in cold- and hot-water-soluble organic C and N, indicating changes in physical stabilization and nutrient retention. Particulate and dissolved organic C and N were generally higher in surface soils (0–10 cm) than in deeper soil layers, with relatively higher values observed in the Chalous and Noor forest stands. Overall, the findings indicate that unengineered landfill activities were associated with significant changes in soil C and N stabilization pathways in forest ecosystems, with greater differences observed under warmer and drier climatic conditions. These results highlight the role of climate in regulating soil vulnerability to waste-related disturbances and underscore the need for improved solid waste management strategies to protect soil functions and ecosystem sustainability in forested landscapes.