Soil nitrogen and nutrient dynamics after repeated application of treated dairy-waste

Improved understanding of the effects of dairy-waste treatment and land application on microbial processes and products is required to predict the outcome of waste applications and avoid undesirable environmental impacts. Our objective was to assess effects of treated dairy-waste on soil N pools, nitrification, plant N availability, and yield in a silage cornfield (Zea mays L.) treated with ammonium sulfate (AS), dairy-waste compost (DC), or liquid dairy-waste (LW) as N sources at two levels of application over 5 yr. Increases in soil C and N, nitrate, and available P and K were observed for the DC treated soils throughout the 5-yr period. Soil organic C increases for the high-level DC treated soil doubled the C pool resulting in an increase of 14 Mg C ha(-1). The highest nitrate accumulation was at the 60- to 90-cm depth for soils receiving high level of DC (200 kg N ha(-1)), which moved to lower depths in subsequent years. Soils receiving a high-level of DC or LW showed a three-fold increase in nitrifier activity compared with the control. There was a positive silage corn yield response with all the treatments, with DC having the highest yields. While N from AS and LW are available for plant uptake almost immediately, the organic N in compost continued to mineralize throughout the growing season, after harvest and in subsequent years. Careful management of application rates to optimize the timing of N release versus plant demand and of post-harvest nutrient pools are suggested for the prevention of excessive nitrate accumulations and movement from repeated dairy-waste applications.