Adopting mixtures between legumes and non legumes can be an efficient tool to merge the advantages of the single species in the fall-sown cover crop practice. Nevertheless there is a lack of information on how the species proportion may affect N accumulation and C/N of the cover crops and how this can influence the N uptake and N status of different subsequent summer cash crops.

In this study the N effect of barley (Hordeum vulgare L.) and hairy vetch (Vicia villosa Roth.) grown in pure stands or in mixtures with different sowing proportion was tested on maize (Zea Mays L.) and processing tomato (Lycopersicon esculentum Mill.). Cover crop N accumulation and C/N ratio were monitored during the whole growing cycle, and CO2 flux from the soil was measured after their incorporation into the soil. N status of the following cash crops was evaluated by comparing the observed data with the appropriate critical N dilution curves.

The results highlight the effectiveness of mixtures for the management of the winter cover crop practice. In the two considered years, the species proportion influences the aboveground biomass (ranging from 2.90 to 5.94 Mg ha-1) and N accumulation (ranging from 73.8 to 183.2 kg ha-1) of the mixtures. The legume component, even at low proportion, increased the N accumulation of the cover crop of 148% (in 2006) and 134% (in 2007) compared to pure stand barley. Also the biomass quality of the cover crops was greatly affected by species proportion (e.g. C/N ranging from 12.0 to 18.9) and this aspect showed a clear effect on the N availability for the subsequent crop. N effect (Neff) of the different cover crop mixtures (especially those with high barley proportions) brought tomato much closer to the critical N value than they did with maize. The basis of the relationship between cover crop C/N and Neff was confirmed, so mixtures can be used to adjust the extent and timing of mineralisation of the incorporated biomass to the subsequent cash crop requirements. Prediction of the cash crops N status on the cover crop C/N appears to be a useful approach, but, it may be important to take the characteristics of the following cash crop into account.