Nitrogen leaching from soil lysimeters irrigated with dairy shed effluent and having managed drainage

The leaching of nitrogen (N) from agricultural soils is undesirable for env ironmental and health reasons. We investigated the effects of adding dairy shed effluent (DSE), irrigated on a weekly basis during the milking season, on the amounts and forms of N leached from large undisturbed soil monolith lysimeters of a Gley Soil over a period of 2 years. Drainage was managed u sing a weir that maintained the water table at 3 depths: 25 (high), 50 (med ium), or 75 (low) cm below the soil surface. The low water table treatment represented the usual situation for the soil when drained. If undrained, it would be usual during wet periods in the field for a perched water table t o form on the slowly permeable horizon at 75 cm depth. The total amount of N irrigated onto the lysimeters in the first milking se ason was equivalent to a total of 511 kg N/ha.year, and up to 33.3 kg N/ha. year leached from the soil. The losses from lysimeters receiving effluent w ere nearly double those from lysimeters receiving an equivalent amount of w ater only, when the high and medium water tables were imposed. Adding efflu ent caused only a small increase (7 kg N/ha) in total N leached in the low drainage treatment. In the second milking season, the effluent-N loading wa s increased to 1518 kg N/ha.year and the pasture was managed to simulate a 'cut and carry' land treatment system. Under these conditions, up to 131.4 kg N/ha.year leached from the soil, which was nearly 100 kg N/ha more than lysimeters receiving only water. The total N leaching losses represented a similar proportion of added N (7% and 9%) for years 1 and 2, respectively. Most of the leached N (80-90%) was in organic N form. The managed drainage treatment in which the water table was nearest the soi l surface resulted in less N being leached in the nitrate-N (NO3-N) form (< 2.5 kg N/ha.year) than the other drainage treatments (6-12 kg N/ha.year); however, it did result in the greatest amount of organic and total N leache d (33 and 131 kg N/ha for Year 1 and 2, respectively). The smaller amount o f NO3-N leached from the high water table treatment is attributed to enhanc ed denitrification, and the greater amount of organic N is attributed to pr eferential flow. Although NO3-N concentrations in leachate generally remain ed below World Health Organisation (WHO) standards in all treatments, the l arge amount of N leached in organic form would suggest that inorganic N sho uld not be the only form of N considered when measuring N leaching losses.