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 accumu-lated 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. Ni-trogen 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 immobi-lisation under sufficient N fertilisation and was more pronounced after CCs than fallow. Effective N minerali-sation 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.

Automated summary

This study compared the effects of four different functional groups of cover crops on nitrogen immobilization, mineralization dynamics, nitrogen uptake, and biomass accumulation of subsequent silage maize. The results showed that cover crops significantly reduced soil mineral nitrogen and increased nitrogen uptake in maize. Among them, radish cover crop had the greatest net benefits for silage maize cropping systems.