Effects of Land-Use Change and Hydrology on Soil Carbon Composition and Thermal Stability in a Tropical Freshwater Wetland: Insights from Yala, Kenya

Abstract

Wetlands are vital for mitigating climate change, but widespread conversion to agricultural land has disrupted their functioning in terms of soil carbon (C) and nitrogen (N) dynamics. This study examined the impact of land-use/cover change on soil N, C, their thermal stability, and C composition in Yala Wetland. Using a stratified random approach, soil samples were collected from permanently flooded, seasonally flooded, sugarcane, maize, and vegetable farms, across depths of 0-50 cm. Multi-Element Scanning Thermal Analysis (MESTA) was used to quantify C and N thermal stability, while solid-state 13C NMR spectroscopy characterized C composition. Results showed significant differences (P < 0.05) in SOC, nitrogen, and C:N ratios across land uses. Vegetable farms had highest SOC (117.83 ± 16.54 g kg-1) and N (7.34 ± 1.07 g kg-1), while sugarcane fields had the lowest (SOC: 13.58 ± 0.97 g kg-1; N: 1.07 ± 0.04 g kg-1). Seasonally flooded wetlands stored more SOC (98.51 ± 20.55 g kg-1) and N (5.31 ± 1.12 g kg-1) than permanently flooded wetlands, suggesting that alternate wet-dry cycles enhance humification and organic matter (OM) stabilization. Data showed dominance of thermally labile C (C < 400 °C) over thermally stable C (C> 400 °C). This was highlighted by high R400 in all land uses, (0.73-0.82). Carbon composition results indicated dominance of O-alkyl C in all land-use types. This was consistent with dominance of low-thermally stable C and a High R400 index. Overall, findings show that both wetland conversion and hydrological conditions strongly influenced OM quality and stability in the Yala wetland.