Quantities of mineral N in soil and concentrations of nitrate‐N in groundwater in four grassland‐based systems of dairy production on a clay‐loam soil in a moist temperate climate

This study examined the quantity of mineral N in soil and nitrate-N losses to groundwater from grassland-based dairy production in 2001 and 2002. There were four treatments with different inputs of N, through fertilizers, concentrates and biological fixation, and four associated stocking densities. Nitrogen inputs were 205, 230, 300 and 400 kg ha−1, respectively, and annual stocking densities were 1·75, 2·10, 2·50 and 2·50 cows ha−1. There were 18 cows per treatment. Grazed herbage accounted for 0·64, grass-silage for 0·26 and concentrates 0·10 of annual DM consumed by the cows. Quantities of mineral N (nitrate-N and ammonium-N) in soil were measured, following extraction in 2 M KCl (1:2 w/v) shaken continuously for 2 h, on three occasions between late September and early February each winter. Concentrations of nitrate-N in groundwater from wells inserted vertically to a depth of 1 m were determined throughout both winters. Quantities of mineral N in the soil increased (P < 0·001) with higher N inputs and declined (P < 0·001) with later sampling date. There were no relationships between nitrate-N concentrations in groundwater and N inputs, N surpluses, deposition of excreta-N at the soil surface and soil mineral N during both winters. Low losses of nitrate-N to groundwater were primarily attributed to high rates of denitrification associated with a heavy soil texture, wet anaerobic soil conditions, relatively high organic carbon contents throughout the soil profile and mild soil temperatures throughout the year. Uptake of N by herbage made an important contribution to low N losses over the winter.