# Divergent Assembly of Bacteria and Fungi During Saline–Alkali Wetland Degradation

## TL;DR

This study shows how bacteria and fungi in wetland soils respond differently as the wetlands degrade due to drying and farming.

## Contribution

The study reveals divergent assembly patterns of bacteria and fungi during saline-alkali wetland degradation, highlighting distinct responses to environmental stress.

## Key findings

- Bacterial communities were strongly influenced by soil conditions like pH and moisture, showing deterministic assembly under stress.
- Fungal communities remained more stochastic and less affected by bulk soil properties despite degradation.
- Restoring water connectivity early after drying can prevent severe wetland degradation.

## Abstract

Saline–alkali wetlands in Northeast China are shrinking because water levels are declining and some wetlands are drained and converted to cropland, which can reduce soil fertility and weaken ecosystem functions. We compared soil microorganisms across four habitats: intact wetland, a partially dried meadow wetland, a severely salt-affected grassland with salt-tolerant plants, and cropland converted directly from wetland. We measured soil moisture, alkalinity, nutrients, and soil enzyme activities (chemicals made by soil life that help recycle nutrients), and used DNA sequencing to describe bacteria and fungi. Drying caused a sharp drop in soil moisture, and the most salt-affected habitat had the highest alkalinity and the lowest nutrient levels and enzyme activities. Bacteria tracked these soil changes closely and were increasingly shaped by harsh environmental filtering, meaning only tolerant types persisted under extreme stress and farming disturbance. Fungi were less tightly linked to bulk soil conditions, were more influenced by chance and small habitat differences, and kept relatively stable links under stress. These results highlight an early restoration window: restoring water connectivity soon after drying begins may prevent severe degradation. Where soils are already highly alkaline, lowering alkalinity, rebuilding nutrients, and supporting vegetation recovery can aid ecosystem recovery.

To clarify microbial assembly during saline–alkali wetland degradation, we analyzed bacterial (16S rRNA) and fungal (ITS) communities across four habitats: pristine wetland (PW), transitional meadow wetland (TMW), halophytic herbaceous community (HHC), and converted farmland (CF). Soil water content collapsed from PW (42.22%) to ≤18.40% elsewhere, and soils were alkaline with pH highest in HHC (10.08). Nutrient pools and enzyme activities were highest in PW (SOC 35.03 g kg−1; URE 142.58 mg g−1; SUC 527.83 mg g−1) but declined sharply under natural degradation, reaching minima in HHC (SOC 8.02 g kg−1). ACP and CAT were also lowest in HHC. Bacterial communities were dominated by Actinomycetota and Pseudomonadota, with Acidobacteriota and Bacillota enriched in CF. Bacterial diversity peaked in CF, whereas fungal richness was highest in CF and Shannon diversity peaked in TMW. Ordination and redundancy analyses indicated stronger edaphic control on bacteria than fungi, with pH, SOC, and moisture as key drivers. Null-model analyses showed bacterial assembly shifted toward deterministic selection under saline–alkali stress and agricultural conversion, whereas fungal assembly remained predominantly stochastic. Co-occurrence networks further suggested higher bacterial vulnerability under extreme degradation but comparatively higher fungal robustness. Overall, bacteria and fungi follow divergent assembly rules during saline–alkali wetland degradation.