Nitrogen loading to offsite waters from liquid swine manure application under different drainage and tillage practices

Too much nitrogen (N) in surface water is harmful to aquatic life. Subsurface drains, some along with surface inlets, are commonly used for drainage of agricultural land in humid regions and contribute unknown N amounts to surface water. Quantification of relative amounts moving through different pathways and characterization of the sources are needed to mitigate movement. The major pathways to surface water, namely, runoff and tile drains, were monitored over 2 yr, from fields having sub-catchments with no-(Ni), hickenbottom-(Hb) or blind-(Bi) surface inlets, or natural drainage (ND) to determine the effect of drainage system configuration and associated management practices on N loading. The fields were managed under either common practice (CP) of conventional tillage with broadcast incorporated liquid swine manure (LSM) or minimum tillage (MT) with injected LSM. Tile drainage increased NO3-N load to surface water by 36 ± 17 kg ha−1 [sum of overland + subsurface over two non-growing seasons (NGSs) and one growing season] as compared with ND. Surface inlet or its type had little effect on N movement through the drainage systems. Application-induced preferential flow of LSM to drainage tile occurred with injection at one of two fields in the drain installation year, but not in the following year, contributing 80% of the annual NH4-N load from drainage tile, but only 5% of the total N (Nt) overland + subsurface movement, which consisted of 82% NO3-N (average of three tile drained catchments). Loading of N to surface water both overland and through tile drains was dominated by a few events, particularly snowmelt. Runoff waters had elevated NO3-N concentrations on some NGS events, particularly from CP management. Surface runoff accounted for up to 27% of the total (overland + subsurface) NO3-N load to surface water in the NGS under CP management, though amounts varied from different catchments. Results indicated that year-round monitoring is important when measuring systems impacts on N loading; and that to achieve watershed scale impact, along with controlling N movement to drainage tile, strategies effective on frozen ground and during snowmelts, when much of the movement occurs, are needed.