Slow-release nitrogen behavior of oxamide granules in two different types of paddy soils

Oxamide is a potential slow-release nitrogen (N) fertilizer, especially under waterlogged conditions, due to its low solubility in water and the slow-release of ammonium by soil amidases. To investigate the effects of oxamide granules (2.00–2.38 mm in diameter) after a single basal fertilization on rice growth, soil properties, and N use efficiency in terms of N recovery efficiency (NRE), we conducted field experiments in two different types of paddy soils over two rice-growing seasons. Results showed that the fertilization effects of oxamide granules varied in the two types of paddy soil. In the red clayey paddy soil, the grain yields for both rice-growing seasons were high with a significantly higher NRE in the oxamide treatment than in the urea treatment. However, in the alluvial sandy paddy soil, the grain yield in the oxamide treatment was slightly lower than in the urea treatment. Furthermore, oxamide produced little improvement in NRE in the alluvial sandy paddy soil. Soil incubation experiments over 98 d were also carried out to evaluate the factors affecting the N release behavior of oxamide granules in the two types of paddy soil. We found that the amidase activities were higher and, therefore, the oxamide hydrolysis rates were faster in the alluvial sandy paddy soil, which had a higher soil pH value and organic matter content. The faster N release and the longer growth period resulted in a mismatch between N supply by oxamide and rice demand, which, in turn, led to little improvement in NRE and a decreased grain yield in the alluvial sandy paddy soil, especially in the reduced oxamide treatment. These results could help select the appropriate size of oxamide granules for use as a slow-release N fertilizer depending on the soil properties and growth period of rice.