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UBC Theses and Dissertations

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 Odhiambo, Jude Julius Owuor


Winter cover crops have potential to be a valuable tool in efficiently managing N while providing biological N accumulation and green manure for subsequent summer crops. This 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. The objective was to determine the effect of these cover crop combinations on cover crop productivity, fall residual soil N recovery, spring soil water content, spring biomass and N management and N availability to subsequent summer crops. 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 villosd) and bare control. Cover crops were established in the third week of August (early planted) and third week 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. Averaged over the three year study, August planted cover crop dry matter yield measured in April/May (except of crimson clover) often exceeded 5000 kg ha⁻¹ compared to an average of approximately 4500 kg ha⁻¹ for most cover crops planted in September. Late planted cover crops accumulated most dry matter in the spring. Among the cereals and grass, winter wheat and annual ryegrass had consistent dry matter yields and N contents. While mixtures also performed well, 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. This reduction was 3-10, 5-9 and 10-22% for early planted and 18-23, 15-18 and 12-18% for late planted winter wheat, fall rye and annual ryegrass respectively when planted in mixture with crimson clover. 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. In 1995, the critical soil water content (0.32 kg kg⁻¹) for trafficability was attained by the third week of April. In the relatively wet spring of 1996, soil water content was still greater than the critical value by the final sampling date on 7 May. 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, regardless of the proportion, prevented immobilization of N , except for annual ryegrass + crimson clover in 1996. 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.

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