Soil surface N balances and soil N content in a clay-loam soil under Irish dairy production systems

This study evaluated the effect of a dairy system involving grazing over the winter on a soil surface N balance (SSB) and soluble N content in a clay loam soil in comparison with early spring calving systems. The SSBs were calculated for each paddock within three dairy systems for 2 years. Inputs included N entering the soil in fertilizer, slurry, excreta, atmospheric deposition and biological N fixation. Outputs consisted of N leaving the soil in harvested and grazed herbage. Nitrogen surplus was calculated as a difference between N inputs and outputs. Soluble N was assessed in soil extracts at three depths to 0.9 m. The management of the systems resulted in N surplus from 113 to 161 kg N ha(-1) year(-1) and N use efficiency of the soil component from 63 to 72 % without significant variation between the systems. The dairy system had no effect on soil N content as its variation was likely buffered by inherent soil properties (heavy texture, high C, pH) and the presence of shallow groundwater. The biochemical anaerobic reduction processes (i.e. denitrification) likely ensured soil oxidised N consistently low (< 20 kg ha(-1)). Consequently, the system involving grazing over the winter on these soils did not create an additional environmental pressure via N losses to groundwater and N2O emissions compared with early spring calving systems. The size of soil inorganic N pools was mainly controlled by the hydrological factors and soil temperature, which are the most important factors controlling microbial activity, biochemical processes and leaching.