Conventional subsoil irrigation techniques do not lower carbon emissions from drained peat meadows

The focus of current water management in drained peatlands is to facilitate optimal drainage, which has led to soil subsidence and a strong increase in greenhouse gas (GHG) emissions. The Dutch land and water authorities proposed the application of subsoil irrigation (SSI) system on a large scale to potentially reduce GHG emissions, while maintaining high biomass production. Based on model results, the expectation was that SSI would reduce peat decomposition in summer by preventing groundwater tables (GWTs) from dropping below -60gcm. In 2017-2018, we evaluated the effects of SSI on GHG emissions (CO2, CH4, N2O) for four dairy farms on drained peat meadows in the Netherlands. Each farm had a treatment site with SSI installation and a control site drained only by ditches (ditch water level -60g/g-90gcm, 100gm distance between ditches). The SSI system consisted of perforated pipes -70gcm from surface level with spacing of 5-6gm to improve drainage during winter-spring and irrigation in summer. GHG emissions were measured using closed chambers every 2-4 weeks for CO2, CH4 and N2O. Measured ecosystem respiration (Reco) only showed a small difference between SSI and control sites when the GWT of SSI sites were substantially higher than the control site (>20cm difference). Over all years and locations, however, there was no significant difference found, despite the 6-18gcm higher GWT in summer and 1-20gcm lower GWT in wet conditions at SSI sites. Differences in mean annual GWT remained low (<g5gcm). Direct comparison of measured N2O and CH4 fluxes between SSI and control sites did not show any significant differences. CO2 fluxes varied according to temperature and management events, while differences between control and SSI sites remained small. Therefore, there was no difference between the annual gap-filled net ecosystem exchange (NEE) of the SSI and control sites. The net ecosystem carbon balance (NECB) was on average 40 and 30tCO2ha-1yr-1 in 2017 and 2018 on the SSI sites and 38 and 34tCO2ha-1yr-1 in 2017 and 2018 on the control sites. This lack of SSI effect is probably because the GWT increase remains limited to deeper soil layers (60-120cm depth), which contribute little to peat oxidation. We conclude that SSI modulates water table dynamics but fails to lower annual carbon emission. SSI seems unsuitable as a climate mitigation strategy. Future research should focus on potential effects of GWT manipulation in the uppermost organic layers (-30gcm and higher) on GHG emissions from drained peatlands.