Restoring degraded agricultural peatlands: how rewetting, biochar, and iron sulphate synergistically modify microbial hotspots and carbon storage.

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The draining and conversion of peatlands for agriculture has led to their degradation globally, diminishing their carbon (C) storage capacity and functioning. However, rewetting, alongside the addition of organic/inorganic amendments, has the potential to accelerate peat formation and C accrual. The aim of this experiment was therefore to examine the combined benefits of altering water table depth and adding organic (e.g., biochar, paper waste, biosolids, cereal straw; 20 t C ha) and inorganic (e.g., FeSO; 0.5 t ha) materials on net C storage and peatland functioning (i.e., microbial communities, greenhouse gas emissions and biogeochemical cycling). The experiment consisted of outdoor agricultural peat mesocosms monitored over 1 year. The relative effectiveness of the amendments in preserving peat-C (t C ha) followed the series: biochar (18.9 t C ha) >  residues (17.3 t C ha) > biosolids (17.2 t C ha) > cereal straw (14.5 t C ha) > paper waste (13.3 t C ha) based on C additional rate (20 t C ha). Overall, a high-water table combined with biochar and FeSO addition was the most effective at suppressing enzyme activity (e.g., β-glucosidase, phenol oxidase, cellobiase), methanogen activity (e.g., ) and peat mineralization rate. We ascribe this in part to changes in the fungal and bacterial community structure (e.g., reductions in Actinobacteria by - 22% and Ascomycota by - 61%). FeSO also increased the Fe-bound C content in the non-rewetted treatment, supporting the 'iron gate' mechanism for C preservation. The mechanisms behind our results appear to be both abiotic (affecting SOC solubility through changes in redox conditions and Fe-C interactions) and biotic (via shifts in microbial community and enzyme activities), creating conditions that enhance C preservation. These findings provide evidence for implementing biochar and FeSO amendments alongside water table management as practical, scalable strategies for restoring C storage capacity in agricultural peatlands.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42773-025-00501-y.