Soil Aggregation and Fungal and Bacterial Biomass under Annual and Perennial Cropping Systems

Abstract

Understanding the mechanisms controlling crop effects on soil aggregation is necessary to develop sustainable soil management practices. Changes in soil aggregation, carbohydrates, and fungal vs. bacterial biomass were monitored following a shift from fallow (bare soil) to various cropping systems. Faba bean (Vicia faba L.) and wheat (Triticum aestivum 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 perennial cropping systems. Crops were established in spring 1989, on a silty clay loam (fine, mixed, frigid Typic Dystro‐chrept) and a clay loam (fine‐loamy, mixed, frigid Typic Humaquept) near Québec City (Canada). After three growing seasons, the mean‐weight diameter of water‐stable aggregates (MWD) was higher under reed canarygrass (2.26 mm in the clay loam and 2.45 mm in the silty clay loam) and timothy (2.13 and 2.26 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. Changes in MWD were not correlated with microbial biomass C. Acid‐hydrolyzable carbohydrates correlated with MWD in the silty clay loam (r2 = 0.42**) but the relationship decreased with higher carbohydrate levels in the clay loam (r2 = 0.05). Close correlations were found between MWD and both fungal glucosamine (r2 = 0.68***, soils combined) and bacterial muramic acid (r2 = 0.48***). Changes in MWD were mostly reflected in aggregates >2 mm, and the close relationship with fungal glucosamine suggests that fungi played a dominant role in soil macroaggregation. Measurement of muramic acid and glucosamine is proposed as a potential approach to compare bacterial vs. fungal contributions to soil aggregation.

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

“…Higher plants do not synthesize significant amounts of amino sugars, while bacteria and fungi do (Benzing-Purdie, 1984;Chantigny et al, 1997;Zhang and Amelung, 1996). Glucosamine and muramic acid are of special interest because while glucosamine is found in bacterial and fungal cell walls and the exoskeletons of microarthropods, muramic acid uniquely originates from bacteria (Chantigny et al, 1997).…”

“…Higher plants do not synthesize significant amounts of amino sugars, while bacteria and fungi do (Benzing-Purdie, 1984;Chantigny et al, 1997;Zhang and Amelung, 1996). Glucosamine and muramic acid are of special interest because while glucosamine is found in bacterial and fungal cell walls and the exoskeletons of microarthropods, muramic acid uniquely originates from bacteria (Chantigny et al, 1997). Since the biomass of soil invertebrates is low relative to microbial biomass and the muramic acid to glucosamine ratio is approximately 1 in bacterial cell walls, the ratio of glucosamine to muramic acid can be used to differentiate between fungal and bacterial contributions to MOM (Chantigny et al, 1997;Guggenberger et al, 1999;Six et al, 2001;Zelles, 1988;Simpson et al, 2004).…”

Interpretation of microbial biomass measurements for soil quality assessment in humid temperate regions

“…Some components of the SMB, however, can be more directly involved in aggregation (i.e., fungal hyphae, Tisdall 1996). Studies have illustrated that fungal biomass C can be closely related to aggregation on a range of pastoral and arable soil (Chantigny et al 1996;Beare 1997;Beare et al unpublished, Fig. 6).…”

Optimisation of amino sugar quantification by HPLC in soil and plant hydrolysates

“…Muramic acid occurs exclusively in bacterial cell walls, especially in the murein skeleton of Gram-positive species (Millar and Casida 1970;Kenne and Lindburg 1983). Also, bacteria contain glucosamine in their peptidoglycan cell wall, but only the glucosamine that occurs in excess to muramic acid may be attributed to fungal sources (Chantiny et al 1997;Guggenberger et al 1999;Amelung 2001). Appuhn and Joergensen (2006) determined average conversion factors of 9 to convert fungal glucosamine to fungal C and 45 to convert muramic acid to bacterial C.…”