Kavdir, Y., D. P. Rasse, and A. J. Smucker. 2005. Specific contributions of decaying alfalfa roots to nitrate leaching in a Kalamazoo loam soil. Agriculture, Ecosystems, and Environment 109:97-106.
Alfalfa (Medicago sativa L.) contributes 430 million kg N year(-1) to the US Corn-Belt soils, according to a 1991 survey. Minimizing leaching losses from these very large N inputs requires a better understanding of the specific root dynamics that relate to the shoot-borne nitrates which have been reported to develop throughout many soil profiles. The objective of the present study was to determine the impact of decaying alfalfa roots on nitrate inputs to soils and on soil hydraulic conductivity properties which affect nitrate leaching. An experiment was initiated in 1994 and data for this report were taken from research on a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long-term ecological research) site in southwestern Michigan, during the period from 1996 through 1997. Soil extractable nitrate (NO3-N) and ammonium (NH4-N) were monitored to soil depths of 150 cm and soil soluble NO3-N and NH4-N were monitored by suction lysimeters to the depth of 65 cm. Saturated hydraulic conductivity (K-sat) of soil was measured by the double-ring infiltrometer method. 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.
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