Bacterial and Fungal Cell-Wall Residues in Conventional and No-Tillage Agroecosystems

Agricultural management practices have been shown to influence the decomposer community in soils, with no-tillage (NT) systems favoring fungi as compared with conventional tillage (CT) systems. In this study, we examined six North American agroecosystems with respect to the effects of NT vs. CT management systems on the accrual of microbial cell-wall residues in surface soil. We used total amino sugar contents to estimate living and decomposing microbial cell-wall mass in soil and the contents of glucosamine and muramic acid to separate fungal and bacterial contributions to microbial-derived soil organic matter (SOM). Compared with estimates of glucosamine and muramic acid present in living biomass of fungi and bacteria, total concentrations of these compounds (745–2076 mg glucosamine kg⁻¹ soil and 37–79 mg muramic acid kg⁻¹ soil) were larger by factors of 54 to 745 and 26 to 82, respectively. 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. This coincided with higher values for fungal biomass, particulate organic matter carbon (POM-C), mean weight diameter of water-stable aggregates (MWD), and total organic carbon (TOC). Analysis of aggregate-size classes showed that the additional glucosamine accumulated in >53-mm aggregates but not in smaller particles. The enrichment of SOM in fungal-derived glucosamine suggests 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 soil concurrent with an increase in aggregate stability. The other soils, having a lower clay plus silt content, exhibited no significant differences in 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 for NT to sequester microbial-derived SOM is lower because of limited physical stabilization.