Investigations into aspects of nitrogen and carbon dynamics in grassland used for dairy production on a clay loam soil

Increasing concentrations of nitrogen (N) in the waterbodies along with increasing concentrations of nitrous oxide (N2O) and carbon dioxide in the atmosphere have become an international environmental concern. Permanent grasslands are important sources of feed for intensively managed dairy and beef farming systems in North West Europe and represent around 90% of agricultural land in Ireland. However, there is a potential for substantial N losses following grazing during the winter when a significant effective rainfall occurs. In addition, ploughing and reseeding of grassland in order to increase its productivity may lead to a substantial decrease in soil organic matter (SOM) followed by an increase in soil inorganic N. This two year study investigated (i) the environmental impact of a dairy production system involving grazing over the winter through calculations of soil surface N balances and its effect on soil N dynamics and N loses to groundwater; (ii) the impact of permanent grassland renovation on soil N, N2O emissions, N leaching and soil organic carbon (SOC) in a poorly drained clay-loam soil at Solohead Research Farm. The correlations between grazing management and soil N dynamics in the soil profile or N concentrations in the groundwater were difficult due to high natural buffering capacity of the soils associated with heavy texture, high SOC, high soil pH, anaerobic conditions and presence of shallow groundwater. For this reason, grazing over the winter period had no effect on soil N dynamics and groundwater quality on this site. In contrast, grassland renovation decreased topsoil SOC and total N, increased oxidized N leaching to groundwater and N2O emissions from the soil surface as a result of soil disturbance, enhanced decomposition of SOM followed by nitrification and denitrification. Although, the overall losses due to net SOM mineralisation were high (3 t N and 32 t C ha-1), the proportion lost via N leaching and direct N2O emissions was low (27 kg N ha-1 y-1) suggesting that nitrate were instantaneously reduced by dissimilatory nitrate reduction to ammonium or complete denitrification.