Effects of winter cover crops from different functional groups on soil-plant nitrogen dynamics and silage maize yield

Cover crops (CCs) are known for their multiple benefits in agro-ecosystems. This is especially true in intensive cropping systems with high nitrogen (N) inputs and considerable post-harvest residual N. Furthermore, research has shown that CCs have the potential to reduce nitrate N leaching losses. The efficacy of ecosystem services by implementing CCs is related to site-specific management optimisation as well as species individual traits. In this study, four winter CCs from different functional groups (oil radish, saia oat, spring vetch, winter rye) were compared to a bare fallow which served as control. Furthermore, the CCs were evaluated in terms of the effects on N immobilisation, mineralisation dynamics, N uptake and biomass accumulation of silage maize used as a subsequent cash crop. Field trials were conducted at two locations, during two consecutive establishment years, with silage maize fertilised with two different N levels (no/sufficient fertiliser). Successful CC growth accumulated between 0.89 (volunteer oilseed rape (OSR)/vetch mixture) and 3.50 Mg dry matter ha⁻¹ (rye). The CCs accumulated between 39 (rye) and 79 kg N ha⁻¹ (radish) in aboveground biomass and significantly reduced soil mineral nitrogen (SMN) by 12 kg N ha⁻¹ during the winter growth period. In the subsequent maize growing season all CCs increased SMN by 45 kg N ha⁻¹ on average compared to the fallow. The C/N ratios in the aboveground biomass of CCs varied between 10.5 and 27.0 in the order: OSR/vetch < radish < rye < oat. Nitrogen uptake of the subsequent silage maize was significantly increased by 53 and 54 kg N ha⁻¹ compared to the fallow without and with added N, respectively. Net N mineralisation indicated periods with net SMN immobilisation under sufficient N fertilisation and was more pronounced after CCs than fallow. Effective N mineralisation during maize growth was significantly higher following CCs without N fertilisation (51 kg N ha⁻¹); however, the increase was lower with added N (43 kg N ha⁻¹). Cover crops increased apparent fertiliser N immobilisation rates during the maize cropping period by 28 % (maize following bare fallow) to 34 % (OSR/vetch > radish, no effect for rye). Maize dry matter yield (15.58/17.33 Mg ha⁻¹ without/with N) did not significantly differ between CCs and bare fallow treatments. Overall, there was a trade-off between N buffering during winter (CC biomass as main driver for N uptake), N supply and utilisation of the subsequent cash crop (C/N ratio of CC residues as main driver for mineralisation) where radish indicated the greatest net benefits for silage maize cropping systems without yield effects.