Renovation and conversion of permanent grass-clover swards to pasture or crops: Effects on annual N2O emissions in the year after ploughing

Abstract

The need for improved herbage yield and forage quality, on intensively managed grassland, in dairy farming systems, often results in land being brought into tillage, either by grassland reseeding or conversion to arable cropping (particularly maize (Zea mays L.) for silage). However, both options are likely to increase the greenhouse gas emissions (GHG) after grassland cultivation, particularly when this involves ploughing, although there is limited information about the level of emissions. In order to estimate whether the time of ploughing and type of forage crop would affect the annual emissions of the important GHG nitrous oxide (N2O) in the year after soil disturbance of grassland, a field experiment was established on a Eutric Luvisol in northern Germany. The three factorial experiment comprised the effect of land use management (four treatments: ploughing and reseeding in autumn; ploughing and reseeding in spring, ploughing and conversion to maize in spring; control treatment of intact permanent grassland); the effect of nitrogen (N) fertilizer input (two treatments: 0 and 240kgNha−1year−1 applied as cattle slurry); and the experimental year (two years). Treatments were arranged with three replicates in a randomized block design. N2O emissions were measured on a weekly basis with the static chamber method and N drainage during winter was estimated using suction cups. Thus, direct and indirect N2O emissions (viaN leaching) were calculated per ha as well as per unit of forage yield. Results showed that ground-frost and freeze-thaw cycles were major drivers of enhanced N2O fluxes during the winter following ploughing and reseeding of grassland in autumn, resulting in the highest direct emissions of 21.31kgN2O-Nha−1year−1. Emissions following grassland ploughing in spring were mainly driven by high soil mineral N-concentrations but with maximum figures of 3.90 and 6.32kgN2O-Nha−1year−1 after reseeding and conversion to maize respectively. Emissions in the intact permanent grassland were lowest in both years. Forage yield related emissions were also highest for grassland ploughing and reseeding in autumn and lowest for intact permanent grassland, but with no significant differences for reseeding and maize cultivation after grassland ploughing in spring. Application of slurry increased annual N2O emissions, particularly when applied in the year of reseeding, but with a lower calculated emission factor as advised by the IPCC guidelines for N fertilizers. We conclude that sustaining highly productive permanent grass-clover swards with moderate N fertilization and without tillage is the best option for ‘climate-smart forage production’ followed, when unavoidable, by tillage operations only in early spring for direct grassland reseeding, or delayed reseeding using a high yielding forage crop like maize.