This work provides information relevant to the nitrogen (N) management strategy of multiple cropping in upland systems in the humid tropics of the transmigration areas of Sumatra, Indonesia. The experiment was conducted on a red yellow podzolic (orthoxic palehumult) soil at Kota Bumi in Lampung Province, Indonesia, where the annual rainfall of 2430 mm allows three consecutive crops per year to be grown. Two sequential systems were studied--rice-soybean-cowpea and rice-corn-cowpea. For each crop, crop residues and fertilizer applied to subplots were labelled with ~SN so that the crop N uptake for the planted crop and subsequent crops could be partitioned into that derived from N fixation (legume only), fertilizer, crop residues and soil. The experiment was conducted over two years (6 crops).The percentage of soybean and cowpea N derived from N fixation was 33% for soybean and 12-33% for cowpea. Removal of N in seed exceeded the amount derived from fixation.Efficiency of utilization of fertilizer N ranged from 9-18% of that applied for wet season upland rice (900-1300 mm rainfall) to 32-40% for dry season corn (410-840 mm rainfall). Residual fertilizer N recovery by subsequent crops was as high as 14% of that applied to corn and as low as 2% of that applied to rice.Legume residues were an effective source of N to the following crop, particularly cowpea residues applied to rice, where percent recovery was higher than from fertilizer. Cereal residues were of lower value as a source of N. Percent utilization of N in crop residues by the following crop was related to the % N in the residues and the rainfall received by the following crop (R 2 = 0.69, P = 0.01).

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Field experiments (20 m 2 plots) were conducted to compare Azolla and urea as N sources for rice (Oryza sativa L.) in both the wet and dry seasons. Parallel microplot (1 m 2) experiments were • -1 ° conducted using 15N. A total of approximately 60 kg N ha was applied as urea, Azolla, or urea plus 1 Azolla. Urea or Azolla applied with equal applications of 30 kg N ha-at transplanting (T) and at maximum tillering (MT) were equally effective for increasing rice grain yields in both seasons. Urea at 30 kg N ha-i at T and Azolla 30 kg N ha-1 at MT was also equally effective• Urea applied by the locally recommended best split (40 kg at T and 20 kg at MT) gave a higher yield in the wet season, but an equal yield in the dry season. The average yield increase was 23% in the wet season, and 95% in the dry season. The proportion of the N taken up by the rice plants which was derived from urea (%NdfU) or Azolla (%NdfAz) was essentially identical for the treatments receiving the same N split. Recovery of 15N in the grain plus straw was also very similar. Positive yield responses to residual N were observed in the succeeding rice crop following both the wet and dry seasons, but the increases were not always statistically significant. Recovery of residual lSN ranged from 5.5 to 8.9% for both crops in succeeding seasons. Residual recovery from the urea applications was significantly higher than from Azolla in the crop succeeding the dry season crop• Azolla was equally effective as urea as an N source for rice production on a per kg N basis.

The use of organic materials aimed at reducing the dependence on inputs such as chemical fertilizers can contribute to sustainability and improving the low N fertilizer efficiency of rice plants in paddy soils.  Therefore, better understanding of N transformation in flooded soils, particularly the microbial transformation of N-organic amendments to plant-available N and gaseous N forms is needed for most efficient use of soil and organic materials N, for determining the potential of denitrification and for aiding in the selection of N management practices for sustainable agriculture.  The experiments were conducted in the laboratory and glasshouse of Soil Department, Faculty of Agriculture, Bogor Agricultural University.  An incubation experiment was conducted in the laboratory at room temperature during 120 d to analyze the mineralization-immobilization patterns in flooded soils amended with rice straw, rice straw composts 4 mo and 8 mo and their combinations with urea.  The first time of soil flooding, mineral N (N-NH4+ and N-NO3-) concentrations in all of the amended soils sharply decreased and those gradually increase since 7 d of incubation.  It might be due to the reduction of N-NO3- to N2O and N2, and immobilization.  This data was consistent with the data of N2O emission derived from pot experiment.  The high emission of N2O was observed at the first time of soil flooding of the amended soils, and the soils added rice straw showed the largest N2O emission than the other treatments. It might be due to the change of soil condition from aerobic to anaerobic condition, and the higher decomposable C as energy source for denitrifyer contained in rice straw.  Regardless the organic materials added to soils, the longer anaerobic condition, nitrification sharply decreased, so that nitrate availability limits denitrifications.decreased, so that nitrate availability limits denitrifications.