Effect of cereal/grass and legume cover crop monocultures and mixtures on the performance of fall-planted cover crops, soil mineral nitrogen and short-term nitrogen availability

Winter cover crops may help to conserve N within the agricultural ecosystem and promote long term sustainability of the agricultural soils in the Fraser River Delta, British Columbia. Fall seeded cereal/grass and legume cover crops as monocultures and mixtures were investigated. Cover crop treatments included winter wheat (Triticum aestivum L.), fall rye (Secale cereale L.), annual ryegrass (Lolium multiflorum L.), spring wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.), as monoculture or in mixtures with crimson clover (Trifolium incarnatum L.), crimson clover, winter wheat + hairy vetch (Vicia villosa) and bare control. Cover crops were established in August (early planted) and of September (late planted) in 1993, 1994 and 1995. In the spring of 1995 and 1996, incubation studies using the cover crop residues were set up for a period of 16 weeks under field conditions using the in-situ resin core method.; August planted cover crop dry matter yield measured in April/May often exceeded 5000 kg ha−1 compared to an average of approximately 4500 kg ha−1 for most cover crops planted in September. Winter wheat + hairy vetch mixture accumulated consistently higher levels of dry matter and N content than cereal/grass and crimson clover mixtures. Late planted mixtures yielded proportionally more legume than early planted mixtures. The C:N ratio of early and late planted winter wheat + hairy vetch was 21–28 and 40–43% respectively lower than the winter wheat monoculture. August planted cover crops were more effective in lowering the amount of fall residual N prior to winter leaching than September planted cover crops. Cereal/grass and legume mixtures were equally effective as cereal grass monocultures at lowering the amount of fall residual soil N.; Winter wheat + hairy vetch in both years and crimson clover in 1995 released a significant amount of N by the end of the 16-week incubation period. Crimson clover and winter wheat + hairy vetch released N rapidly within the first two weeks. Cereal and grass monocultures released large amounts of N late in the growing season, while N release by the mixtures was intermediate. The presence of legumes in mixtures prevented immobilization of N. Critical N concentration above which N mineralization occurred was determined as 14.1 g kg−1 and this corresponded to a critical C:N ratio of 31.7.; Results from this study shows that winter wheat + hairy vetch mixture seems to be more reliable than cereal/grass + crimson clover mixtures in terms of performance and N contribution upon decomposition and that mixtures may reduce the potential for short-term N immobilization. (Abstract shortened by UMI.)...