Freeze-thaw carry-over effect promotes decomposition of recalcitrant carbon in peatlands by nitrogen limitation

Abstract

Peatlands are pivotal in global carbon sequestration initiatives. However, studies of winter ecological factors and their subsequent effects on soil carbon–nitrogen (C-N) coupling processes remain limited, particularly amidst altering snowpack conditions due to climate change. Here, an in situ field experiment focusing on snowpack manipulation (presence and absence) was conducted within a northern peatland, China. The N functional groups and availability, bacterial community’s structure, succession and metabolic function, and carbohydrate-active enzymes (CAZymes) were determined at 0–30 cm (topsoil) and 30–60 cm (subsoil) employing synchrotron radiation X-ray absorption near-edge structure (XANES) and metagenomic sequencing technologies. The findings revealed that snowpack absence augmented the number of freeze–thaw cycles by 9 times, causing the subsoil that initially did not experience freeze–thaw cycles to undergo 17 cycles. This amplification of freeze–thaw cycles significantly influenced soil N processes during the freeze–thaw period and subsequent seasons. Specifically, it resulted in a 40.2 % and 1.8 % increase in the metabolic potential of denitrification in the topsoil and subsoil, respectively. Concurrently, there was a reduction in inorganic N content by 4.1 % and 4.4 % in the topsoil and subsoil, respectively. Furthermore, the diminished N availability (ammonium and inorganic N) intensifying soil N limitation subsequently altered microbial assembly processes. This shift led to a significant increase in the abundance of CAZymes encoding the decomposition of lignin (19.2 % and 4.8 %), chitin (4.8 % and 1.4 %), and murein (9.0 % and 0.8 %) in the topsoil and subsoil. Additionally, the content of pyridine, primarily derived from the decomposition of lignin and microbial cell walls, increased by 2.2 % and 1.9 % at two studied depths under snowpack absence conditions. These results uncover a cascading relationship between snowpack conditions, N availability, and the decomposition of recalcitrant carbon in peatland soils, highlighting the need for further comprehensive studies in this domain.