# Ecotone-Driven Vegetation Transitions Reshape Soil Nitrogen Cycling Functional Genes in Black Soils of Northeast China

## TL;DR

This study shows how changes in vegetation in forest-wetland ecotones affect soil microbial communities and nitrogen cycling in black soils of Northeast China.

## Contribution

The study reveals that vegetation transitions restructure microbial communities through hydrological and biogeochemical heterogeneity, not linear gradients.

## Key findings

- Forest soils show higher microbial diversity and nitrogen fixation potential under oxic conditions.
- Wetland soils have denitrification-enriched communities and stronger carbon-nitrogen coupling.
- The wetland edge acts as a functional hotspot with coexisting aerobic and anaerobic processes.

## Abstract

Understanding how vegetation transitions influence soil microbial processes is essential for predicting nutrient cycling and greenhouse gas dynamics in ecotone ecosystems. In this study, we examined soils along a forest–wetland gradient in Northeast China, where fertile black soils serve as both agricultural and ecological resources. By integrating analyses of nitrogen-cycling functional genes, microbial diversity, community assembly, ecological networks, and predicted metabolic functions, we revealed that vegetation transitions restructure microbial communities through hydrological and biogeochemical heterogeneity rather than a simple linear gradient. Forest soils exhibited greater microbial diversity, more complex network connectivity, and higher potentials for nitrogen fixation and nitrification under oxic conditions. In contrast, wetland and edge soils harbored denitrification-enriched taxa and stronger carbon–nitrogen coupling under fluctuating redox states, indicating enhanced capacity for N2O reduction and metabolic resilience. The results also demonstrate that the wetland edge acts as a functional hotspot where aerobic and anaerobic processes coexist, and that hydrological and nutrient variability jointly shape microbial assembly, interaction networks, and functional stability. Overall, this study provides mechanistic insights into how vegetation-driven transitions regulate nutrient turnover and greenhouse gas fluxes, offering a scientific basis for the sustainable management of black-soil ecotones under changing environmental conditions.

Forest–wetland ecotones are transitional ecosystems characterized by pronounced hydrological and biogeochemical heterogeneity, yet the microbial mechanisms regulating nutrient cycling in these zones remain insufficiently understood. This study investigated how vegetation transitions across a forest–wetland ecotone in the black-soil region of Northeast China shape soil microbial communities and nitrogen–cycling functions. Soils were collected from four vegetation types: mixed forest (MF), coniferous forest (CF), wetland edge (WE), and natural wetland (NW). Quantitative PCR was used to quantify key nitrogen–cycling functional genes (nifH, amoA, amoB, norB, nosZ), and PICRUSt2 was applied to predict microbial functional potentials. Forest soils (MF and CF) exhibited higher microbial diversity, stronger network connectivity, and greater abundances of nifH and amoA, indicating enhanced nitrogen fixation and nitrification under oxic conditions. In contrast, wetland soils harbored denitrification-enriched communities with higher norB and nosZ abundances but lower diversity. The WE vegetation type acted as a functional hotspot where alternating oxic–anoxic conditions facilitated the coexistence of nitrifiers and denitrifiers, thereby enhancing carbon–nitrogen coupling and functional resilience. Redundancy and Mantel analyses identified soil organic carbon, total nitrogen, water content, and enzyme activities as major environmental drivers of microbial structural and functional variation. This study reveals that vegetation transitions reorganize microbial community assembly and nitrogen-cycling functions through hydrological and biogeochemical heterogeneity, providing mechanistic insights into nutrient turnover and ecological regulation in black-soil ecotones.