Nitrogen release from differently aged Raphanus sativus L. nitrate catch crops during mineralization at autumn temperatures

In temperate climates with surplus precipitation and low temperatures during autumn and winter, nitrate catch crops have become crucial in reducing nitrate leaching losses. Preferably, the N retained by the catch crop should remain in the soil and become available to the next main crop. Fodder radish (Raphanus sativus, L.) has emerged as a promising nitrate catch crop in cereal cropping, although the course of remineralization of residue N following termination of this frost-sensitive crucifer remains obscured. We incubated radish residues of different age (different planting and harvest dates) with a loamy sand soil; mineralization of residue N was determined after 1, 2, 4 and 7 months of incubation at 2 degrees C and 10 degrees C. Incubations with soil only and with residues of white mustard (Sinapis alba, L) and perennial ryegrass (Lolium perenne, L.) were included as references. Using linear regression, net N release was fitted to plant chemical characteristics (initial concentrations of N, fibre fractions, lignin and C/N ratio). Residue C/N ratio (ranging from 10 to 25) and N concentration (ranging from 17 to 40 mg N/g dry matter) showed superior fits to net N release at both temperatures (r(2), 0.64-0.94) while fibre analyses provided inferior fits (r(2), 0.12-0.64). This was true across planting date and plant age. Net N release after 7 months of incubation at 2 degrees C and 10 degrees C accounted for up to 40% and 50% of residue N, respectively. During most of the incubation period, nitrate dominated the mineral N pool at both temperatures. The N mineralization and nitrification potential at these low soil temperatures suggest that a considerable fraction of the N captured by nitrate catch crops may be remineralized, nitrified and thus available for plant uptake but also for loss by leaching and denitrification.