Fractions of nitrogen (including 15N) and also carbon in the soil as affected by different crop residues

. Returning crop residue can increase soil organic matter content, and residue quality has an influence over the rate of their turnover. However, there is a lack of information con - cerning the biochemical transformations of organic compounds of N and C present in the crop residues during subsequent crop growth. In this study, the contents of organic N and C fractions in soils obtained using acid and alkaline hydrolysis under two crop rotations (faba bean vs . wheat rotation) were investigated. Black fallow served as a control. The mean total N increased in the order: black fallow, wheat rotation, faba bean rotation, total C and SOM were higher in the cropped soils than in black fal - low. Hydrolysable-N (1-step acid hydrolysis) reached 83.7% total N, amino acid-N and threonine+serine-N were the highest in faba bean rotation and the lowest in black fallow, ammonia-N and aminosugar-N were lower in black fallow than in cropped soils. Hydrolysable-N (2-step sequential fractionation) reached 85.3% total N and significant differ ences were noted between the cropped soils and black fallow, with respect to both the N and C contents. 15 N was mainly accumulated in the N soluble and eas - ily hydrolysable N compounds, and these fractions were greater in cropped soils than in black fallow. N in the humic com pounds increased from black fallow to faba bean rotation. A PCA analysis showed that crop rotation and soil sampling terms had a substan - tial influence over cluster formation. An ANOSIM test revealed significant diff erences between the crop rotation and term treat - ments. The results indicated that soil with faba bean rotation is richer in N compounds than soil with wheat as a forecrop and this may result in a reduction in N fertilizers for the succeeding crop.