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Although amounts and patterns of nitrogen mineralization from decomposing crop residues and other organic materials are known to be affected by their initial chemical composition and quality (e.g., C/N ratio, lignin and soluble polyphenol concentrations), published results differed in which properties correlated best with the N release. Six crop residues (leguminous and non-leguminous) and two organic materials (spent mushroom compost and dairy pond sludge) were incubated to determine the release of mineral N and its relationships to residue chemical composition. A previously proposed equation for calculating plant residue quality index (PRQI) was modified to PRQIM (Plant residue quality index-modified) to include C/N, lignin/N and polyphenol/N ratios of crop residues and organic materials so as to integrate these three major variables, which control the residue decomposition and nutrient release. The PRQIM was significantly and highly correlated with N release not only for the data obtained in the present study but also for three independent data sets obtained from the literature for a wide variety of organic materials ranging from arable crop residues, tropical savanna woodland litter and leguminous tree prunings. [ABSTRACT FROM AUTHOR]