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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 concerning 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 fallow. 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 differences were noted between the cropped soils and black fallow, with respect to both the N and C contents. 15N was mainly accumulated in the N soluble and easily hydrolysable N compounds, and these fractions were greater in cropped soils than in black fallow. N in the humic compounds increased from black fallow to faba bean rotation. A PCA analysis showed that crop rotation and soil sampling terms had a substantial influence over cluster formation. An ANOSIM test revealed significant dif­ferences between the crop rotation and term treatments. 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. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in years 2016–2018 in crop-rotation: faba bean-winter wheat-winter wheat. After seedling emergency (2016) mineral fertilizer (15NH4)2SO4 having 20.1 at% 15N (dose of 30 kg N ha−1) was applied. The faba bean biomass contained 194.1 kg ha−1of nitrogen (140.0 kg of N in seeds and 54.1 kg in crop residue). This amount included 58.7 kg N ha−1 in seeds and 21.2 kg of N in crop residuesthat were fixed from the air. Crop residue were ploughed into soil. Plots with winter wheat cultivated after faba bean (2017) were divided on two sub-plots 0 and 100 kg N ha−1. Overall, winter wheat cultivated for two subsequent years took up from total nitrogen of crop residue 16.1 kg ha−1 of N in control and 29.8 kg ha−1 in fertilized plots, but from biologically fixed nitrogen 6.29 and 11.6 kg ha−1, respectively. Applied fertilizer contained 5.920 kg 15N ha−1 from which 1.577 kg 15N ha−1 was cumulated in faba bean seeds, 0.662 kg 15N ha−1 in crop residue (sum 2.239 kg 15N ha−1), 0.195 kg 15N ha−1 and in winter wheat harvested (2017) and 0.084 kg 15N ha−1 in winter wheat harvested (2018). [ABSTRACT FROM AUTHOR]