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. 15N-Labeled urea (6.17 15N atom %) was added to soil at 0, 37.5, 75 and 150 kg N ha−1, and unlabeled compost (0.37 15N 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∼21.7% for treatments without compost and 32.8∼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.