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Soil organic components and aggregation as influenced by cover and ley crops

University of British Columbia

Structural degradation of silty clay loam soils in Delta, British Columbia, has resulted from intensive cultivation for vegetable crops. Field and laboratory incubation studies were conducted to determine the changes in soil total C, total N and total and labile polysaccharides produced by cover crops in relation to aggregate stability of these soils. Mean weight diameter (MWD) was used as a measure of soil aggregate stability. The field experiment included two cover crop sites, a pasture site and a Reclamation Site with severe structural degradation. The results on the cover crop site in 1992-1993 were not significantly different between bare soils and winter cover crop treatments because of problems of variability and high precipitation in April of 1993 when soils were sampled. In 1993-1994, cover crops which overwintered, annual ryegrass and fall rye, increased soil aggregate stability, and all of the cover crops significantly increased soil labile polysaccharides, but did not change total N during their growing period. Soil aggregate stability was significantly correlated with the content of labile polysaccharides and followed the order: annual ryegrass > fall rye > spring barley = bare soil. Long time pasture significantly increased the contents of total C, total and labile polysaccharides and total N , which effectively increased soil aggregate stability. At the Reclamation Site, grass (tall fescue) ley treatments significantly increased M W D values and total carbon and labile polysaccharide contents in comparison with a cash crop/clover combination. However, MWD and total and labile polysaccharide contents did not show the differences between the drained and undrained soil in the sampling in May of 1993, and the total carbon content in the undrained soil was higher than in the drained soil. In a laboratory experiment, chopped shoots and coarse roots of green cover crops, fall rye and annual ryegrass, and starch were added to a subsoil from the 1993-1994 cover crop site and incubated for 2, 4 and 8 weeks. The added cover crop and starch treatments increased soil total C and N and labile polysaccharides, which significantly increased soil MWD. The starch amendment was more effective in forming larger soil aggregates than the double-dose fall rye (the amount added into the soil was twice as much as the fall rye amendment) and fall rye amendments for 2 and 4 weeks incubations, but less effective than the double-dose fall rye and fall rye amendments for the 8 weeks incubation. A l l amendments increased the proportion of 2-6 mm aggregates relative to the control at all sampling dates except the fall rye amendment after 4-weeks incubation. M W D values correlated with soil total and labile polysaccharides, total C and total N in the incubation samples. Macroaggregates contained higher contents of total C, total polysaccharides and labile polysaccharides and total N than microaggregates.

Thesis/Dissertation

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2009-02-06

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http://hdl.handle.net/2429/4194

Soil Science

University of British Columbia

UBCV

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