The impact of grassland ploughing on CO2 and N2O emissions in the Netherlands

The contribution of ploughing permanent grassland and leys to emissions of N2O and CO2 is not yet well known. In this paper, the contribution of ploughing permanent grassland and leys, including grassland renovation, to CO2 and N2O emissions and mitigation options are explored. Land use changes in the Netherlands during 1970-2020 are used as a case study. Three grassland management operations are defined: (i) conversion of permanent grassland to arable land and leys; (ii) rotations of leys with arable crops or bulbs; and (iii) grassland renovation. The Introductory Carbon Balance Model (ICBM) is modified to calculate C and N accumulation and release. Model calibration is based on ICBM parameters, soil organic N data and C to N ratios. IPCC emission factors are used to estimate N2O-emissions. The model is validated with data from the Rothamsted Park Grass experiments. Conversion of permanent grassland to arable land, a ley arable rotation of 3 years ley and 3 years arable crops, and a ley bulb rotation of 6 years ley and one year bulbs, result in calculated N2O and CO2 emissions totalling 250, 150 and 30 ton CO2-equivalents ha-1, respectively. Most of this comes from CO2. Emissions are very high directly after ploughing and decrease slowly over a period of more than 50 years. N2O emissions in 3/3 ley arable rotation and 6/1 ley bulb rotation are 2.1 and 11.0 ton CO2-equivalents ha-1 year-1, respectively. From each grassland renovation, N2O emissions amount to 1.8 to 5.5 ton CO2-equivalents ha-1. The calculated total annual emissions caused by ploughing in the Netherlands range from 0.5 to 0.65 Mton CO2-equivalents year-1. Grassland renovation in spring offers realistic opportunities to lower the N2O emissions. Developing appropriate combinations of ley, arable crops and bulbs, will reduce the need for conversion of permanent pasture. It will also decrease the rotational losses, due to a decreased proportion of leys in rotations. Also spatial policies are effective in reducing emissions of CO2 and N2O. Grassland ploughing contributes significantly to N2O and CO2 emissions. The conclusion can be drawn that total N2O emissions are underestimated, because emissions from grassland ploughing are not taken into account. Specific emission factors and the development of mitigation options are required to account for the emissions and to realise a reduction of emissions due to the changes in grassland ploughing.