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Modifications of Soil Nitrogen Pools in Response to Alfalfa Root Systems and Shoot Mulch

Lab. for Planetary and Atmospheric Physics, Univ. of Liége, 5 Ave. de Conte, B-4000 Liége, Belgium;

Alvin J. M. Smucker^* and Oliver Schabenberger

Crop and Soil Sciences Dep., Plant and Soil Sciences Bldg., Michigan State Univ., East Lansing MI 48824-1325.

^* Corresponding author (smucker{at}pilot.msu.edu).

Decomposing alfalfa (Medicago sativa L.) shoots and roots generate large amounts of NO₃-N available to the next crop but also susceptible to deep leaching. This study was aimed at determining the specific contributions of above- and belowground alfalfa biomass to soil N pools. Dynamics of soil and plant N pools were studied in a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalfs) over a 2-yr period under hare fallow (BF), hare fallow to which alfalfa shoot mulch was applied (BFSM), living alfalfa plants with shoots removed after harvest (A), and living alfalfa with shoot mulch remaining on the soil surface after harvest (ASM). Organic N pools were monitored in alfalfa plant parts, soil-incorporated debris, and soil organic matter to depths of 150 Em. Inorganic N pools were monitored by suction lysimeters, soil extraction, and evaluation of soil denitrification rates. Living alfalfa stands kept soil inorganic N at very low levels, whether shoot mulch was applied or not. Soluble inorganic N concentrations decreased earlier in the fall in the upper horizons of bare fallow soils receiving alfalfa shoot mulch, suggesting enhanced leaching from bare soil under alfalfa mulch. Alfalfa crown and roots contained an average of 115 kg N ha^– after 2 yr of treatment. In conclusion, alfalfa shoot mulch contributed little to sustained increases in soil N pools, while crowns and roots contributed larger quantities to the soil N pool.

Received for publication April 17, 1998.

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