Catch crops store more nitrogen below-ground when considering Rhizodeposits

One means of reducing nitrate leaching in temperate farming is to include catch crops in crop rotations, which immobilize residual nitrogen (N) in their biomass. For an accurate quantification of the N stored in catch crops and subsequently released from residues, their total biomass, including roots and rhizodeposits has to be assessed. In a pot experiment under controlled conditions, oil and forage radish (Raphanus sativus L. var. oleiformis Pers.) and winter turnip rape (Brassica rapa L. var. silvestris [Lam.] Briggs) plants were leaf-labelled every five to seven days with N-15-urea (99at%) five times during the vegetation. At harvest, plants were separated into shoot, coarse, medium and fine roots by hand picking and wet sieving, respectively. The amount of N derived from rhizodeposition (NdfR) was calculated using two different calculation approaches. In addition to the pot experiment, a field experiment with unlabelled plants was set up to extrapolate the results from the pot experiment to the field scale, at the same time evaluating the influence of additional mineral N fertilization on biomass distribution. The contribution of rhizodeposition to total N in the field was estimated by extrapolating the root-N-to-rhizodeposition-N ratio of the pot experiment to the field experiment. In the pot trial, between 4.6 and 10.3% of the total assimilated nitrogen of the catch crops was found as rhizodeposits, which is at the lower end of values from other studies on legumes and non-legumes. In the field experiment the shoot-to-root ratio was lower compared to the pot experiment. Thus, the contribution of rhizodeposition to total N under field conditions is substantially higher. Fertilization in the field trial mostly influenced the formation of above-ground plant biomass. Considering the rhizodeposition reveals, that the investigated catch crops store more N than previously assumed. As a consequence, catch crops have to be evaluated with a stronger focus on below-ground biomass to make sure the right amounts of N are considered for fertilization schemes in crop rotations.