Recycling of residual soil nitrogen in a lowland rice-sweet pepper cropping system

Residual mineral N after the dry season (DS) crop in an intensive rainfed l owland is lost upon flooding for rice (Oryza sativa L.) planting. The conse rvation and recycling of this N are essential for maintaining groundwater q uality and system sustainability. Experiments conducted in rice-sweet peppe r (Capsicum annuum L.) cropping systems in farmers' fields aimed (i) to qua ntify the Levels of soil mineral N after the incorporation of residues of d ifferent dry-to-wet (DTW) transition crops in combination with two formulat ions of fertilizer N, as well as their effects on rice yields and N use eff iciencies, and (ii) to estimate the soil N balance. Significant amounts of NH4-N accumulated in soil at 15 d after incorporation of residues of indigo (Indigofera tinctoria L.) alone (12 kg ha(-1)) and indigo mixed with mungb ean (Vigna radiata L.) residue (24 kg ha(-1)), and at 60 d after incorporat ion of maize (Zea mays L.) residue (8 kg ha(-1)). Soil NH4-N in treatments with maize residue was lower than that from indigo and mungbean, but it was improved, when maize residue was mixed with fertilizer N. Nitrate N peaked in the upper soil leger before flooding occurred, followed by its Leaching and disappearance later. Crop residues incorporated in the plot maintained low NO3 throughout the soil profile. The crops during DTW transition reduc ed N losses by 33 to 72%, and residue incorporation supplied N equivalent t o 87 kg ha(-1) to rice. The results suggest that a transition crop alone ca nnot completely reduce the N losses; therefore, strategies for reducing N f ertilizer rates to better match N demand of the DS crop are needed.