Effects of cover crops on soil aggregate stability, total organic carbon, and polysaccharides

Structural degradation of silt clay loam soils in Delta, British Columbia, has resulted from intensive cultivation of vegetable crops. A field experiment and a laboratory. incubation study were conducted to assess the ability of nonleguminous winter cover crops, spring barley (Hordeum vulgare L.), fall rye (Secale cereale L.), and annual ryegrass (Lolium multiflorum Lam.), to affect soil organic C, total and dilute acid extractable polysaccharides, and aggregate stability, expressed as mean weight diameter (MWD). The field experiment included four treatments: three cover crops (spring barley, fall rye, and annual ryegrass) and control (bare soil) arranged in a randomized complete block design. Annual ryegrass and fall rye increased MWD, and all of the cover crops increased soil dilute acid extractable polysaccharides. In the incubation experiment, starch (2.68 g C kg(-1) soil) or chopped shoots and coarse roots of fall rye (single- [4.14 g C kg(-1) soil] and doubledose [8.28 g C kg(-1) soil]) and annual ryegrass (4.62 g C kg(-1) soil) were added to a soil from the cover-crop site and incubated for 2, 4, and 8 wk. Cover crop and starch amendments increased soil organic C, dilute acid-extractable polysaccharides, and soil MWD. After 2-wk incubation, the starch amendment had the greatest MWD in all the treatments, increasing by 25, 44, and 45%, compared with the annual ryegrass, double-dose fall rye, and fall rye amendments, respectively (P < 0.05). After 8-wk incubation, however, the MWD in the starch amendment containers decreased by 18% compared with that in the double-dose fall rye amendment treatments (P < 0.05). All the cover crop amendments increased MWD and percentages of water stable 2- to 6-mm aggregates at all incubation periods (P < 0.05). Soil aggregate stability highly correlated with dilute acid-extractable polysaccharides in the field and in incubation experiments. This study suggests that the dilute acid-extractable polysaccharide fraction represents active binding agents under short-term cover crops. It has been shown that soil aggregate stability can be increased under 8-mo nonleguminous cover crops in the intensively cultivated soils.