Long‐term Cropping Effects on Partitioning of Water Flow and Nitrate Loss between Surface Runoff and Tile Drainage

Surface runoff and tile drainage are the main pathways for water movement and entry of agricultural nitrate into water resources. The objective of this 5‐yr study was to characterize the partitioning of water flow and nitrate loss between these pathways for a humid‐temperate Brookston clay loam soil under 54 to 59 yr of consistent cropping and fertilization. Cropping treatments included monoculture corn (Zea mays L., MC), continuous bluegrass (Poa pratensis L.) sod (CS), and a corn–oat–alfalfa (Medicago sativa L.)–alfalfa rotation (RC–RO–RA1–RA2). Fertilization treatments included annual fertilizer addition (F) and no fertilizer addition (NF). Tile drainage and surface runoff occurred primarily during the nongrowing season (November–April), and they were highly correlated with the mean saturated hydraulic conductivity of the near‐surface soil profile. Tile drainage accounted for 69 to 90% of cumulative water flow and 79 to 96% of cumulative nitrate loss from fertilized rotation and CS, whereas surface runoff accounted for the majority of the nitrate losses in MC (i.e., 75–93% of water flow and 65–96% of nitrate loss). Cumulative nitrate losses were highest in the RC‐F (152 kg N ha⁻¹), RC‐NF (101 kg N ha⁻¹), RA2‐F (121 kg N ha⁻¹), and RA2‐NF (75 kg N ha⁻¹) plots, and these high losses are attributed to N mineralization from the plowed alfalfa and fertilization (if applicable). Fertilization increased cumulative nitrate loss in tile drainage from all treatments, whereas no fertilization increased cumulative nitrate loss in surface runoff from the rotation. Cropping system and fertilization on clay loam soil changed how water flow and nitrate loss were partitioned between tile drainage and surface runoff. CORE IDEAS: Water partitioning is highly correlated with mean saturated hydraulic conductivity. Tile drainage was the main pathway of water flow and nitrate loss in rotation plots. Surface runoff was the main pathway of water flow and nitrate loss in monoculture corn. Fertilization increased nitrate loss and water flow through tile drainage. Average nitrate loss was 36% greater in the rotation plots versus monoculture corn.