Greenhouse gas emissions and nitrate leaching as a result of grassland renovation practices

Grasslands are a major sink of organic carbon and for that reason are interesting as a means to achieve national and international climate goals. Grassland renovation (reseeding or conversion to arable land) is a common measure to counteract yield declines in intensively managed agricultural grasslands. The destruction of the sod that accompanies this practice induces a strong increase in mineralization of soil organic carbon and can therefore be a major source of nitrate (NO₃) leaching and emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O).

 

Most farmers prefer reseeding in autumn instead of spring because of better establishment of the sward and low weed infestation. However, limited crop nitrogen (N) demand during autumn increases the risk of NO₃ leaching and N₂O emissions. Potentially, such N losses could be mitigated by measures such as reducing tillage intensity, reducing fertilizer N application, or applying nitrification inhibitors. In 7 different experiments, we studied the effect of various grassland renovation practices on soil N cycling, greenhouse gas emissions, and nitrate leaching. We hypothesized (i) that conversion to arable land leads to greater CO₂ and N₂O losses than the reseeding of grassland; (ii) that reseeding in autumn causes enhanced risk of NO₃ leaching during winter; and (iii) that these losses can be mitigated by reducing tillage and/or N application rates.

 

Results show that NO₃ concentrations in groundwater after harvest were higher for observed after autumn reseeding combined with mitigation strategies than for reseeding in spring. This implies that confining the renewal of grassland to spring season could be a viable strategy to mitigate NO₃ leaching. Emissions of N₂O varied between experiments and could generally be linked to precipitation events and agricultural management (fertilization and renovation). Grassland renovation led to higher N₂O (and CO₂) emissions, but the effects of mitigation practices were inconsistent. The results from these experiments will be discussed in more detail. Mitigation strategies for N₂O are less straightforward than those for nitrate leaching.