Effects of water level regulation and litter addition on soil microorganisms in a typical degraded wetland

Wetland restoration is often hindered by the slow recovery of soil nutrients and microbial functions. Although water-level regulation and litter addition are commonly used restoration measures, their combined effects on soil properties, bacterial communities, community assembly, and functional stability remain poorly understood. A short-term incubation experiment was conducted under three hydrological regimes-stable low water level (W1), fluctuating water level (W2), and continuous flooding (W3)-crossed with two litter treatments. The results showed that: (1) litter addition significantly increased soil organic matter (SOM), total nitrogen (TN), and total phosphorus (TP), with W2 combined with litter addition (W2AL) producing the strongest effects, whereas soil pH decreased during the later incubation stages; (2) bacterial communities remained relatively stable and were dominated by Chloroflexota, Acidobacteriota, Pseudomonadota, Actinomycetota, and Bacillota. Water level was the main driver of community differentiation, and W2AL exhibited the highest α-diversity at the final sampling time (T4); (3) community assembly was mainly governed by deterministic processes, particularly homogeneous selection (HoS; 53.4%-59.1%), followed by drift (DR; 26.8%-38.7%). Deterministically assembled groups were predicted to be enriched in carbon metabolism, transport, and environmental information processing, whereas stochastically assembled groups were associated with genetic information processing and growth; and (4) network analysis showed that W2 had a larger, more connected, and more robust network, whereas W3 exhibited the lowest complexity and stability. Overall, under short-term controlled conditions, fluctuating water level combined with litter addition enhanced nutrient accumulation, microbial diversity, and network stability, suggesting its potential to improve soil microbial conditions in degraded wetlands. However, its long-term field effectiveness requires further validation.