Thermal stability of organic matter in mountain forest and grassland soils

Abstract

Global warming is the main driver of the upward migration of treelines on mountain slopes, resulting in replacement of subalpine and alpine grasslands with forests. This affects both the quality of organic matter inputs by vegetation and the dominant mechanisms of carbon (C) stabilization in soils. We studied the stability of plant litter and soil organic matter (SOM) depending on mountain vegetation type in the North-Western Caucasus. The stability of litter and SOM was assessed by microbial decomposability, determined by CO2 efflux during a 17-week incubation, and by thermal analysis. Thermal stability of litter and SOM was analyzed by fractionating organic matter into labile, stable, and persistent pools, and by using the temperature, at which half of the organic matter was lost (TG-T50) as an integral stability marker. Microbial decomposition of litter from grassland was 1.5 times faster than that of forest litter. Decomposition of litter resulted in one-third larger CO2 efflux in grassland compared to forest. The slower decomposition of forest litter was partly attributed to its wider C:N ratio compared to grassland. The low biochemical quality of organic matter and the faster utilization of labile C increased the portion of stable organic matter in forest soil. Thermally stable and persistent SOM pools were 4.6% larger in forest soil compared to grassland, resulting in a TG-T50 that was 9.8 °C higher in the forest. Effects of mountain vegetation on thermal stability of SOM were stronger than those of plant litter. Consequently, the selective preservation becomes main mechanism of organic matter stabilization in mountain soils under treeline upward migration. Changes in the dominant stabilization mechanisms are driven by the inherent chemical complexity of forest litter and its biological and chemical transformations during decomposition. Consequently, forests are crucial for organic matter stabilization and С conservation in soils under global warming and upward treeline shift.