Corn uptake and microbial immobilization of 15N-labeled urea-N in soil as affected by composted pig manure

A pot experiment was conducted to study the effect of combined application of composted pig manure and urea on the availability of urea for corn ( Zea mays L.). Corn was cultivated for 30 and 60 days. N-15-Labeled urea (6.17 N-15 atom %) was added to soil at 0, 37.5, 75 and 150 kg N ha(-1), and unlabeled compost (0.37 N-15 atom %) was added at 0 and 150 kg N ha(-1). After 30-days growth, the uptake efficiencies of applied N by corn were 51.6 and 55.8%, for the treatments of 75 kg urea and 150 kg compost-N ha(-1), respectively. However, the efficiencies decreased to 32.5% for urea-N and 31.6% for compost-N under the mixed treatment of both N inputs at the rate of 75 kg urea and 150 kg compost-N ha(-1), due to the competition of N for corn uptake. After 60-days growth, the urea-N efficiencies were 38.7, 46.8 and 49.6% for the treatments receiving urea at 37.5, 75 and 150 kg N ha(-1), they then decreased to 32.9, 39.3 and 39.7%, respectively, by the combined application of 150 kg compost-N ha(-1). However, the efficiency of compost-N was about 60% irrespective of urea-N addition. The urea-N uptake efficiency, measured using non-isotopic regression technique, was slightly higher by about 10% than the isotopic technique, an indication of 'pool substitution'. However, a large increase in corn uptake of soil-N or compost-N was not observed. After a 60-day growth period, the percentages of applied urea-N which was immobilized in the soil in 2 M KCl non-extractable form were 13.6 similar to 21.7% for treatments without compost and 32.8 similar to 41.2% with compost. These results suggested that the high immobilization of urea-N in soils through the combined application of compost compared to treatments without compost was responsible for the lower uptake efficiency of urea-N by corn.