Specific contributions of decaying alfalfa roots to nitrate leaching in a Kalamazoo loam soil

Following glyphosate termination of the alfalfa stands, nitrate-N released from mineralized alfalfa roots plus shoots totaled 75 kg ha−1. Alfalfa roots generated 36 kg ha−1 and alfalfa shoots generated 39 kg ha−1 which accumulated in the Ap horizons from April to July in 1997. The presence of decaying alfalfa roots in the profile quadrupled Ksat values as compared to bare fallow soils. Nitrates released from decomposing alfalfa roots combined with root-enhanced hydraulic conductivities dramatically increased NO3-N leaching following the termination of alfalfa stands. NO3-N leaching to deeper horizons approached 83 kg ha−1 in root treatments and 144 kg ha−1 in the root plus shoot treatments during the period from April to December, following alfalfa termination. Our data suggest that under temperate climate such as that of Michigan, groundwater contamination by nitrates can be reduced by terminating alfalfa stands immediately before spring-planting of the subsequent row crop, which can absorb the large quantities of N leaking from decomposing shoots and roots of the legume.