Long-term effects of drainage and rewetting on the degradation and preservation of peat organic matter in sub-tropical climate

Peatlands cover about 3 % of the earth's land surface, while storing about 20 % of the total global soil organic carbon. These carbon stocks are largely at risk, as many peatlands have deteriorated since the Industrial Revolution due to conversion to agricultural land by drainage. Globally, peatland drainage is responsible for over 3.5 % of anthropogenic greenhouse gas emissions. Approximately 75 % of these emissions originate from warm climate regions. Mitigation of these emissions can be achieved by rewetting degraded peatlands. This study focuses on a warm Mediterranean sub-tropical climate peatland that has been cultivated for the past ∼70 years (Hula Valley, Israel). The historic marsh was drained in 1957 for agricultural use and underwent a hydrological restoration project for elevating and stabilizing groundwater table since 1994. This land management history resulted in a sedimentary peat column that can be divided into three distinct sub-sections: drained, rewetted and pristine peat. This setting enables studying the drainage and rewetting effects on soil organic matter (SOM) degradation and preservation under warm climates. For this purpose, five sediment cores, 4 m long each, were excavated from cropland located over the historic marsh area. Locations were chosen to match previous studies on this site. Each soil profile was characterized using Rock-Eval^® thermal analysis of the organic matter, and short-term soil aerobic respiration experiments. Integration of these results with historic SOM content data and with SOM modelling was used to explore the long-term process and rate of degradation. We found that the mean SOM content in the top one meter of the soil profile declined from 68±4 % to 21±2 % by weight over the past 66 years, excluding the compaction effect. In comparison to the drained section, the rewetted and pristine sub-sections have a mean SOM of 33±2 % and 64±2 %, respectively. A peak in pyrite concentration beneath the recent water table-level, was observed in most profiles, indicating anaerobic conditions and sulfur recycling. Rock-Eval^® thermal analysis demonstrated that during decomposition, the residual SOM became more oxidized and contained a lower proportion of thermally labile SOM, with a significant difference found between drained and rewetted peat. These results imply that the raising of the water table (∼30 years ago) effectively helped preserving organic matter compared to the drained section. Long-term SOM field data were integrated and studied using an SOM decomposition model and by incorporating respiration fluxes. The resulting trends highlighted that the first few decades of exposure are highly significant for the fate of the carbon stock, leading to substantial CO₂ emissions. These emissions were lower by 60 %–85 % after 70 years. Furthermore, our results suggest that currently, approximately 13 %–21 % of SOM persists as resistant organic matter in the degraded peat.