Leaf quality and macrofauna are more important than the presence of trees and shrubs in riparian vegetation for leaf litter breakdown in subtropical highland grassland soil systems

Leaf litter breakdown is a critical process in ecosystems, influencing food webs and biogeochemical cycles, particularly in inorganic soils such as those found in highland grassland systems. Our experiment examined the leaf litter breakdown of two species, Eucalyptus grandis (exotic) and Inga uruguensis (native), in the soil of riparian zones at subtropical highland grasslands. During the spring season, fine and coarse mesh litterbags with exotic and native leaf litter were incubated in riparian zones with (n = 3) and without (n = 3) tree and shrub layers, resulting in a total of 72 litter bags. In each sampling site, the dry weight of the natural litter was measured, and soil samples were collected for basic physical–chemical analysis. After 42 days of incubation, the remaining material in litter bags was weighed. Our study provided partial support for our hypothesis that the presence of trees and shrubs in riparian vegetation and edaphic macrofauna would enhance litter breakdown, particularly for high-quality litter. Surprisingly, the chemical and physical properties of the soil and riparian vegetation presence did not have a significant impact on the decomposition process, indicating efficient nutrient cycling in soil systems. However, the presence of edaphic fauna, especially soil macroinvertebrates, played a critical role in facilitating leaf litter breakdown. Additionally, we observed rapid breakdown rates for high-quality litter, aligning with our expectations, despite its exotic origin. Our findings highlight the importance of considering leaf quality and macro-detritivores in studying and managing leaf litter breakdown in riparian ecosystems.