Humipedon dynamics in lowland Amazonian forests: are there Amphi humus forms even in tropical rain forests?

Abstract

Understanding humipedon dynamics is crucial to capture forest ecological processes. However, studies disentangling humipedon dynamics in tropical forests are scarce. The humipedon structure classification is important because it shows how forest ecosystems interact and react with (a)biotic factors. Here, we used the humus morpho-functional approach to investigate how soil texture and reduced impact-logging affect forest humipedon structure and dynamics in the eastern Amazon. We used a morpho-functional classification of humus forms as the foundation to visualize, classify, and code 133 soil profiles in two contrasting soil textures and different post-logging practices, to identify litter leaching, fragmentation, and humification processes coupled with soil aggregation and root dynamics. The classification of humus forms resulted in the identification of three humus systems: Mull, Moder and Amphi. Sandy and clayey textures differ statistically (χ2 = 51.83; p < 0.001), with sandy texture predominating in Moder system, while clayey texture exhibits Amphi or Mull systems. Sandy textures presented a weak soil aggregation, but strong association with root (rhiOF) and humification (OH) horizons, leading to a clear transition between organic and mineral pools. In contrast, clayey textures show a strong carbon storage and soil aggregation. The humipedon horizons differ significantly (p = 0.009; stress = 0.1142 and R2 = 0.987) and their dynamics clearly evolve towards an opposite direction. The classification framework introduced here supports environmental management by promoting speed and viability in diagnosing forest ecosystem soils towards forest protection or use in the landscape, and specific climate mitigation goals both in local and regional planning.