Effects of water management and grassland renewal on the greenhouse gas emissions from intensively used grassland on bog peat

Abstract

Artificial drainage is prerequisite for conventional agricultural use of peatlands, but causes high emissions of greenhouse gases (GHG), mainly carbon dioxide (CO2). Furthermore, grassland renewal is regularly practiced to maintain high fodder quality, but might cause high emissions of nitrous oxide (N2O). Raising water levels is necessary to reduce CO2 emissions. Water management by subsurface irrigation (SI) and ditch blocking (DB) is thus discussed as potential compromise between maintaining intensive grassland use and reducing GHG emissions. Here, we present results of a four year study on the effects of SI and DB in combination with grassland renewal on GHG emissions from an intensively used grassland on bog peat in North-Western Germany. The water management itself was successful and lead to average mean annual water levels of -0.33 m at the parcels with SI. This was 0.38 m higher than at the control parcels. Ditch blocking also raised the mean water levels to -0.33 m, but the parcel was dryer in summer and wetter in spring than those with SI. Despite clear effects on water levels, CO2 and total GHG emissions were much (38 % and 31 %) higher from SI parcels than from the control parcels. CO2 and GHG emissions of the DB parcel were similar to those of the control. Shallow ploughing increased N2O emissions for around 1.5 years, but there was no clear effect of direct sowing. Methane emission from all parcels were low. The surprising results regarding CO2 might be explained by an interaction of increased soil moisture in the topsoil and improved nutrient retention during periods of high soil temperatures facilitated by SI and, concurrently, by limitations of microbial activity due to dry conditions at the control parcels. Thus, results of this study do not support subsurface irrigation as a GHG mitigation measure at intensively used bog peatlands.