SOIL AGGREGATION AND FUNGAL AND BACTERIAL BIOMASS UNDER ANNUAL AND PERENNIAL CROPPING SYSTEMS

Understanding the mechanisms controlling crop effects on soil aggregat ion is necessary to develop sustainable soil management practices. Cha nges in soil aggregation, carbohydrates, and fungal vs. bacterial biom ass were monitored following a shift from fallow (bare soil) to variou s cropping systems. Faba bean (Vicia faba L.) and wheat (Triticum aesl ivum L.) were used as annual cropping systems, while alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), bromegrass (Bromus inermis L.), and reed canarygrass (Phalaris arundinacea L.) were used as peren nial cropping systems. Crops were established in spring 1989, on a sil ty clay loam (fine, mixed, frigid Typic Dystro-chrept) and a clay loam (fine-loamy, mixed, frigid Typic Humaquept) near Quebec City (Canada) . After three growing seasons, the mean-weight diameter of water-stabl e aggregates (MWD) was higher under reed canarygrass (2.26 mm in the c lay loam and 2.45 mm in the silty clay loam) and timothy (2.13 and 2.2 6 mm) than under faba bean (1.92 and 1.89 mm) or wheat (2.06 and 1.57 mm). Intermediate values were found under alfalfa and bromegrass. Chan ges in MWD were not correlated with microbial biomass C. Acid-hydrolyz able carbohydrates correlated with MWD in the silty clay loam (r(2) = 0.42*) but the relationship decreased with higher carbohydrate levels in the clay loam (r(2) = 0.05), Close correlations were found between MWD and both fungal glucosamine (r(2) = 0.68**, soils combined) and bacterial muramic acid (r(2) = 0.48**). Changes in MWD were mostly re flected in aggregates >2 mm, and the close relationship with fungal gl ucosamine suggests that fungi played a dominant role in soil macroaggr egation. Measurement of muramic acid and glucosamine is proposed as a potential approach to compare bacterial vs. fungal contributions to so il aggregation.