Alfalfa in rotation with annual crops reduced nitrate leaching potential.

Abstract

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 7.2m 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 7.2m depth in 0.3m increments. The uppermost 0.3m was divided into 0-0.15 and 0.15-0.30m. For the 0-7.2m depth, the alfalfa rotation compared with continuous corn had 26% less soil water (0.29vs. 0.39g cm-3 ) and 55% less NO3 -N (368vs. 824kg 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 0-1.2m 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.