Effect of water management on nitrous oxide emission and heterotrophic soil respiration from drained grassland on peat soil

Raising groundwater levels in drained peatlands used for dairy farming aims to reduce peat decomposition, land subsidence and associated carbon dioxide (CO₂) emissions. However, higher groundwater levels also affect soil biological nitrogen (N) transformations by decreasing N mineralization and influencing microbial processes such as nitrification and denitrification, which may affect nitrous oxide (N₂O) emissions. This creates a potential trade-off, where reduced peat mineralization and CO₂ release may coincide with higher N₂O emissions. We quantified the effects of ditch water level and water infiltration systems (WIS) on N₂O emissions from peat grassland in a five-year field experiment in the Netherlands. Four peat grassland fields differed in ditch water level (20 vs 55 cm below the soil surface) and were subdivided into control without infiltration, passive WIS (PWIS) and active WIS (AWIS) treatments. N₂O fluxes were measured weekly using closed chambers and gap-filled with a generalized additive model. Averaged over five years, deep ditch levels resulted in higher dry matter yield (10.3 vs 8.8 t DM ha ⁻1 yr ⁻1 ) and plant N uptake (282 vs 241 kg N ha ⁻1 yr ⁻1 ), indicating higher N turnover. Annual cumulative N₂O emissions were also higher under deep ditch conditions (3.6 vs 1.9 kg N₂O–N ha ⁻1 yr ⁻1 ), whereas WIS had no significant effect on N 2 O emissions. In contrast, heterotrophic soil respiration was reduced by 10% under PWIS and 20% under AWIS compared to the control. These findings show that raising ditch water levels can reduce N₂O emissions by up to 47%, offering a promising water management strategy to mitigate N₂O losses with only minor yield penalties.