Nitrogen mineralization in a vertisol from organic manures, green manures and crop residues in relation to their quality

The nitrogen (N) mineralization characteristics of organic manures, green manures and crop residues in relation to their quality were investigated in a Vertisol. The proportion of N mineralized from different organic materials ranged from -28% in wheat straw to 95% in gliricidia. The N release from good quality materials (green manures & poultry manure) having C:N ratio of 12 to 14:1 was fast. Whereas the organic materials (farmyard manure, soybean and chickpea straw) with intermediate quality having C:N ratio of 29 to 33:1 resulted in immobilization of N during the first 6 weeks of incubation and subsequent release of mineral N from 8 to 16 weeks of incubation. Addition of urea with these materials to raise the N content of the materials to 2%, and narrow the C:N ratio to 18 to 22:1, resulted in net mineralization of N from the first week onwards. Addition of poor quality wheat straw with a C:N ratio of 79:1 resulted in a marked decrease in mineral N, due to immobilization of N, throughout the incubation period. Addition of urea N to bring down the C:N ratio to 22 helped in faster mineralization of N from the wheat straw + urea mixture. Correlations between quality parameters of the organic materials and the proportion of N mineralized at 4, 8, 12 and 16 weeks after incubation showed that N mineralization was strongly influenced by N content and C:N ratio of the organic materials. In contrast, lignin and polyphenol contents had no effect on mineralization, but ratios of lignin/polyphenol, polyphenol/N, and lignin+polyphenol/N had significant negative relationships with N mineralization.