Nitrogen mineralization of vegetable root residues and green manures as related to their (bio)chemical composition

Little is known concerning the N mineralization or N immobilization of (vegetable) root residues in soil. The (bio)chemical composition of a range of vegetable root residues (large and fine roots of red cabbage, white cabbage, Brussels sprouts, savoy cabbage, and leek), and plant parts of two green manures (ryegrass and white mustard) was determined by standard chemical analysis (total C, total N, and C:N ratio) and by a modified Stevenson fractionation method (water-soluble, hemicellulose, cellulose, and lignin fraction). Fresh chopped crop residues, homogenously mixed with a sandy loam soil, were incubated during 4 months at constant temperature (21 degreesC) and constant moisture content (14% w/w). The net N mineralization of each residue was determined by destructive sampling. All fine roots, except these of Brussels sprouts, showed a net N release throughout the incubation. All large roots showed a slight N immobilization at the start of the incubation, but at the end of the incubation a small net N release was observed, except for Brussels sprouts. The roots of Brussels sprouts immobilized N throughout the entire incubation. The leaves of the green manures released more N than both stems (only for white mustard) and roots. It was possible to fit a first-order kinetics model to 8 of the A(N) mineralization patterns: A(N) (t) = A(N) (1 - exp(-kt)). For the six other residues where the first-order model could not be fitted, the amount of N mineralized at the end of the incubation was taken as an estimate of the A(N) parameter. Both the mineralization parameters A(N) and k were correlated to a large number of (bio)chemical parameters. The amount of mineralized N, A(N), was best correlated with the C:N ratio (R = -0.86), and the rate constant k was best correlated with the lignin:N ratio (R = -0.94). The predictive relationship between the N mineralization of vegetable root residues and green manures and their (bio)chemical composition has the advantage that is independent of the length of the incubation time. The critical C:N ratio, i.e. the break point between net N mineralization and net N immobilization (AN = 0) was found to be 36.6. (C) 2003 Elsevier B.V. All rights reserved.