Nitrate leaching in an organic dairy/crop rotation as affected by organic manure type, livestock density and crop.

Summary

In dairy farming systems the risk of nitrate leaching is increased by mixed rotations (pasture/arable) and the use of organic manure. We investigated the effect of four organic farming systems with different livestock densities and different types of organic manure on crop yields, nitrate leaching and N balance in an organic dairy/crop rotation (barley – grass-clover – grass-clover – barley/pea – winter wheat – fodder beet) from 1994 until 1998. Nitrate concentrations in soil water extracted by ceramic suction cups ranged from below 1 mg NO3-N/l in 1st year grass-clover to 20-50 mg NO3-N/l in the winter following barley/pea and winter wheat. Peaks of high nitrate concentrations were observed in 2nd year grass-clover, probably due to urination by grazing cattle. Nitrate leaching was affected by climatic conditions (drainage volume), livestock density and time since ploughing in of grass-clover. No difference in nitrate leaching was observed between the use of slurry alone and farmyard manure from deep litter housing in combination with slurry. Increasing the total-N input to the rotation by 40 kg N/ha per year (from 0.9 to 1.4 livestock units/ha) only increased leaching by 6 kg NO3-N/ha. Nitrate leaching was highest in the second crop (winter wheat) following ploughing in of the grass-clover (61 kg NO3-N/ha). Leaching losses were lowest in 1st year grass-clover (20 kg NO3-N/ha). Averaged over the four, years nitrate concentration in drainage water was 57 mg/l. Minimising leaching losses requires improved utilisation of organic N accumulated in grazed grass-clover pastures. The N balance for the crop rotation as a whole indicated that accumulation of N in soil organic matter in the fields of these systems were small.