Impact of afforestation on GHG fluxes and related microbiome in abandoned peat extraction areas

Abandoned peat extraction areas are significant hotspots of major greenhouse gas (GHG) emissions, including CO₂, CH₄, and N₂O, compared to drained or undisturbed peatlands. These areas are subject to restoration through either rewetting or afforestation. However, the long-term successional dynamics of GHG fluxes and the underlying microbial mechanisms remain poorly understood. We present GHG flux monthly dynamics and related microbial functional gene abundances across four different-aged afforested sites in Estonia sampled from 2023 to 2025: a young plantation (YP; 1-3 yrs) of Silver birch (SB), Scots pine (SP), Norway spruce (NS), Black alder (BA), and a reference area without trees, a mid-age plantation (MP; 17 yrs) of SB, SP, NS and a reference area, an older plantation (OP-SB, 30 yrs), and a natural riparian forest (NF-BA,  ~80 yrs) on a river bank.

In YP, all tree species showed excellent growth in the first three years, particularly silver birch, which demonstrated that this species is highly suitable for the afforestation of abandoned peat extraction areas. In YP and MP plantations, soil CO₂ emissions were higher in areas with trees than in the reference area without trees, which was possibly caused by additional autotrophic respiration and the addition of fresh, easily decomposable carbon from tree roots. On the temporal scale, CO₂ fluxes increased significantly across YP, OP-SB, and NF-BA during the latter part of the study period, yet remained stable in MP. Methane dynamics were strongly influenced by stand age and species; the oldest forest (NF-BA) consistently acted as a CH₄ sink (mean, -31.6 ± 2.7  µg C m^− 2 h^− 1), supported by the higher oxygen content in river water and the highest abundance of pmoA-containing methanotrophs. Due to intensive precipitation and increasing soil water content (SWC), the older birch plantation (OP-SB) transitioned from a minor to a major CH₄ source (23.8 ± 9.61  µg C m^− 2 h^− 1), while all young plantations remained persistent sources (75.6 ± 17.1 - 85 ± 8.25 µg C m^− 2 h^− 1). This was due to the elevated water table in YP throughout the entire study period. Across all sites, CH₄ fluxes negatively correlated with pmoA abundance, highlighting the critical role of aerobic methanotrophic potential in peat soils.

Nitrous oxide emissions were highest in the old alder forest (NF-BA, 13.7 ± 2  µg N m^− 2 h^− 1), followed by mid-age plantation (MP, 8.92 ± 1.14  µg N m^− 2 h^− 1), which were particularly high during freeze-thaw cycles and post-precipitation periods. Overall, N₂O fluxes showed a positive correlation with SWC. In the riparian Black alder forest, N₂O fluxes were negatively correlated with the C: NO₃^- ratio and positively linked to a high abundance of all Nitrogen-cycling functional genes and soil NO₃^- levels.  Random forest modeling identified total Carbon, SWC, and nirK gene proportions as the primary predictors of N₂O emissions.

These findings demonstrate that while afforestation of abandoned peat extraction areas can eventually establish CH₄ sinks in peatlands, the tree species and stand age significantly modulate the net radiative forcing of the restored ecosystem through altered N-cycling and microbial community structures.