Organic‐matter sources and stability in kettle‐hole sediments: A combined thermogravimetric and isotopic approach

Abstract

Kettle‐holes are small but biogeochemically active landscape elements that integrate terrestrial inputs and influence carbon and nitrogen cycling. However, the stability and sources of organic matter (OM) in kettle‐hole sediments remain poorly understood. We combined thermogravimetric analysis with stable isotope measurements (δ ^{1 3} C and δ ^{1 5} N) to assess the influence of arable, forest, and grassland land uses on the composition and stability of OM in kettle‐hole sediments of northeastern Germany. Thermal fraction analysis showed broadly similar OM composition among all land‐use types, although some differences were observed. Forest sediments showed highest proportion of Fraction 2 (45.3%), while grassland sediments contained the lowest (38.7%). Grassland sediments also showed a greater proportion of thermally stable fractions (Fractions 4–5: 11:0%) compared with arable (6.9%) and forest sediments (6.0%). δ ^{1 5} N values ranged from −0.3 ± 1.7‰ (Forest) to +3.7 ± 0.3‰ (arable), suggesting contrasting nitrogen sources and/or processing, while δ ^{1 3} C values (−28.7‰ to −28.3‰) showed minimal variations across land uses. Statistical analysis revealed a marginal land‐use effect on OM fractions ( p  = 0.048), with no significant pairwise differences among land‐use types. The results indicate that differences in OM stability across land uses are subtle and should be interpreted cautiously. The integration of thermal and isotopic approaches provides complementary insights into OM dynamics, although isotopic differentiation in this study was primarily driven by δ ^{1 5} N. Overall, this study highlights the importance of considering both OM stability and source indicators when evaluating sediment carbon dynamics in small landscape features.