Quality of soil organic matter and C storage as influenced by cropping systems in northwestern Alberta, Canada

Crop rotations and reduction in tillage are commonly recommended for sustained crop production and enhancing soil quality. Our objective was to evaluate the long-term effects of cropping systems (1968-1992) on soil structure, carbon storage and the quality of soil organic matter. The study was conducted on a silt clay loam soil (Typic Cryoboralf) near Beaverlodge, Alberta, The cropping systems were: (a) continuous barley (Hordeum vulgare L.) (CB); (b) continuous bromegrass (Bromus inermiss Leyess.) (CG); (c) continuous forage legume (Medicago sativa L. until 1977, and Trifolium pratense L. since 1978) (CL); and (d) 3 years of bromegrass-legume forage alternating with 3 years of barley (RF). Our data showed that the CG and CL treatments had more stable aggregates with greater mean weight diameter (MWD) than soil under continuous barley. Organic C, total N and the light fraction in soil under CG and CL were higher than those of the other two treatments. Soil under CG had the highest and CB the lowest amounts of acid-hydrolyzable monosacchrides (comprising glucose, arabinose, xylose, mannose and galactose). Higher galactose + mannose concentration in soil under CG indicated a higher soil microbiological activity. Microbial biomass C and N followed the trend among treatments in whole and light fraction organic matter, and total extracted sugars. Soil organic matter C-13-NMR spectroscopy showed that: (i) soil under CB contained the highest amounts of aromatic and the lowest content of aliphatic-C, (ii) soil under CL had the lowest phenolic-C and the least aromaticity, and (iii) soil under CG and RF had the highest amounts of aliphatic-C which includes labile substances such as amino acids and carbohydrates, indicating an improvement in the quality of organic matter. It is concluded that perennial forage crops can improve soil structure and soil organic matter quality and quantity as compared with cereal monoculture.