Bacterial and fungal cell-wall residues in conventional and no-tillage agroecosystems

Agricultural management practices have been shown to influence the decompos er community in soils, with no-tillage (NT) systems favoring fungi as compa red with conventional tillage (CT) systems. In this study, we examined six North American agroecosystems with respect to the effects of NT vs. CT mana gement systems on the accrual of microbial cell-wall residues in surface so il. We used total amino sugar contents to estimate living and decomposing m icrobial cell-wall mass in soil and the contents of glucosamine and muramic acid to separate fungal and bacterial contributions to microbial-derived s oil organic matter (SOM). Compared with estimates of glucosamine and murami c acid present in living biomass of fungi and bacteria, total concentration s of these compounds (745-2076 mg glucosamine kg(-1) soil and 37-79 mg mura mic acid kg(-1) soil) were larger by factors of 54 to 745 and 26 to 82, res pectively. At three sites, the ratios of glucosamine to muramic acid in NT soils (32.0, 30.0, 42.2) significantly exceeded those in the respective CT soils (18.8, 22.1, 23.0) because of a higher enrichment of glucosamine. Thi s coincided with higher values for fungal biomass, particulate organic matt er carbon (POMC), mean weight diameter of water-stable aggregates (MWD), an d total organic carbon (TOC). Analysis of aggregate-size classes showed tha t the additional glucosamine accumulated in >53-mm aggregates but not in sm aller particles. The enrichment of SOM in fungal-derived glucosamine sugges ts that the accrual of hyphal cell-wall residues is an important process in the three NT agroecosystems which leads to higher SOM storage in surface s oil concurrent with an increase in aggregate stability. The other soils, ha ving a lower clay plus silt content, exhibited no significant differences i n POM-C, MWD, and total amino sugars between NT and CT management systems. We suggest that at lower clay plus silt contents the beneficial potential f or NT to sequester microbial-derived SOM is lower because of limited physic al stabilization.