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

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−1) and inorganic (e.g., FeSO4; 0.5 t ha−1) 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−1) followed the series: Miscanthus biochar (18.9 t C ha−1) > Miscanthus residues (17.3 t C ha−1) > biosolids (17.2 t C ha−1) > cereal straw (14.5 t C ha−1) > paper waste (13.3 t C ha−1) based on C additional rate (20 t C ha−1). Overall, a high-water table combined with biochar and FeSO4 addition was the most effective at suppressing enzyme activity (e.g., β-glucosidase, phenol oxidase, cellobiase), methanogen activity (e.g., Methanosarcina) 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%). FeSO4 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 FeSO4 amendments alongside water table management as practical, scalable strategies for restoring C storage capacity in agricultural peatlands.