| dc.description.abstract | 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 mono cultures 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 mono culture 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 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" 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-I) 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
mono cultures 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 ofN, except for annual ryegrass + crimson clover in
1996. Critical N concentration above which N mineralization occurred was determined as
14.1 g kg" and this corresponded to a critical C:N ratio of 3l.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
contribution upon decomposition and that mixtures may reduce the potential for short-term
N immobilization | en |
