Alfalfa in rotation with annual crops reduced nitrate leaching potential

Rotation of perennial alfalfa (Medicago sativa L.) with annual crops has the potential to reduce nitrate-nitrogen (NO₃ -N) in the vadose zone and increase soil organic carbon (SOC) sequestration. The objective of this study was to determine the long-term effects on SOC, NO₃ -N, ammonium-N (NH₄ -N), and soil water in the 7.2 m depth with an alfalfa rotation compared with continuous corn (Zea mays L.). Soils from six pairs of alfalfa rotation versus continuous corn observation points were sampled to 7.2 m depth in 0.3 m increments. The uppermost 0.3 m was divided into 0-0.15 and 0.15-0.30 m. For the 0-7.2 m depth, the alfalfa rotation compared with continuous corn had 26% less soil water (0.29 vs. 0.39 g cm^-3 ) and 55% less NO₃ -N (368 vs. 824 kg ha^-1 ). The cropping system and NO₃ -N concentration did not affect NH₄ -N in the vadose zone. The alfalfa rotation compared with continuous corn had 47% higher SOC (105.96 Mg ha^-1  vs. 72.12 Mg ha^-1 ) and 23% higher total soil nitrogen (TSN) (11.99 Mg ha^-1  vs. 9.73 Mg ha^-1 ) in the 0-1.2 m depth. The greater depletion of soil water and NO₃ -N with alfalfa rotation was primarily below the rooting zone of corn, suggesting no negative implications for corn following alfalfa but greatly reduced potential of NO₃ -N leaching to the aquifer with the alfalfa rotation. Alfalfa rotation compared with continuous corn is a means to greatly reduce the leaching of NO₃ -N to the aquifer and improve the surface soil with the potential to increase SOC sequestration.