Soil organic matter origin and composition along a 3200 m elevation gradient on Mount Kilimanjaro

Abstract

Soil organic matter (SOM) chemistry plays a vital role in carbon (C) cycling and is influenced by a combination of biotic and abiotic factors. Along elevation gradients, SOM chemistry is shaped by litter inputs, temperature, water availability, and microbial communities. However, the specific controls over SOM composition and decomposition remain poorly understood across diverse ecosystems. To address this, we investigated the origin and decomposition state of SOM along the unique elevation gradient of Mt. Kilimanjaro (950–4200 m a.s.l.). Lignin and neutral sugars in topsoils (0–10 cm) were assessed across seven ecosystems with increasing elevation: savanna, grassland, lower-montane forest, Ocotea dominated and Podocarpus dominated cloud forests, sub-alpine Erica forest and alpine Helichrysum.

Here we show that climatic factors, especially precipitation, have a stronger influence on total lignin and sugar contents than on their respective component ratios. Lignin and sugar contents peaked at mid-elevation (2000–3000 m), correlating with optimal climatic conditions and ecosystem productivity. The ratio of plant-derived to microbial sugars decreased at both the lowest and highest elevations, reflecting microbial growth limitations due to seasonal drought (savanna) or low temperatures (alpine Helichrysum). Notably, lignin degradation indices (acid-to-aldehyde ratio, Ac/Al) were less influenced by elevation and instead depended on local ecosystem factors such as microbial community composition and stoichiometry.

These findings highlight that while large-scale climatic gradients dictate the overall distribution of sugars and lignin in Afromontane soils, local ecosystem dynamics govern their degradation and input processes. A combined biomarker approach is essential for understanding biogeochemical functioning and SOM dynamics in these ecosystems and their responses to global change.