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This article in AJ

  1. Vol. 97 No. 1, p. 333-337
    Received: May 19, 2004

    * Corresponding author(s): jmpowel2@wisc.edu
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  1. J. Mark Powell *a,
  2. Keith A. Kellingb,
  3. Gabriela R. Muñozb and
  4. Paul R. Cusickb
  1. a USDA-ARS, Dairy Forage Research Center, 1925 Linden Dr. West, Madison, WI 53706
    b Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706


Indirect estimates of manure N availability to crops are highly variable. We developed two methods that label dairy manure N components with the stable isotope 15N for direct measurement of manure N availability to crops. The forage method involved the labeling then feeding of 15N-enriched forage to dairy cows (Bos taurus) to label urine N, fecal endogenous N, and fecal undigested feed N. The urea method involved the direct feeding of 15N-enriched urea to label urine N and fecal endogenous N. Manure from each enrichment method was applied to a Plano silt loam (fine-silty, mixed, mesic, Typic Argiudolls) using field plots in 1999 and 2000; corn (Zea mays L.) was grown for 2 yr after each application. No significant differences were observed in manure 15N recoveries in corn, soil inorganic N, or soil total N due to manure application year or manure enrichment method. Corn took up 14 to 16% of manure 15N the first year and 4 to 8% the second year after application. Most 15N recovery in soil inorganic and total N was found in the upper 30 cm of soil, indicating little downward movement of applied manure 15N. On average, 68% of applied manure 15N was accounted for, either in crop uptake (21%) or in the soil (47%). The less laborious and less costly urea enrichment method may be adequate for short-term (2 yr or less, the range of this study) manure–soil–crop–N cycling studies. Longer-term studies may need to include fecal undigested feed 15N derived from the forage enrichment method.

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