Evaluation of dairy manure nitrogen-15 enrichment methods on short-term crop and soil nitrogen budgets

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 15 N for direct measurement of manure N availability to crops. The forage method involved the labeling then feeding of 15 N-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 15 N-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 15 N 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 15 N the first year and 4 to 8% the second year after application. Most 15 N recovery in soil inorganic and total N was found in the upper 30 cm of soil, indicating little downward movement of applied manure 15 N. On average, 68% of applied manure 15 N 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 15 N derived from the forage enrichment method.