R. Abu Bakar

In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A 15 N-recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single 15 N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58 % (average of 21%) of crop N uptake during the first growing season was derived from fertilizer. On average, 79 % of crop N was derived from the soil. When 15 N-labeled residues were applied, in the first growing season 4 % of crop N was derived from the residues. Average recoveries of 15 N-labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. Correspondin