Glucose additions to aggregates subjected to drying/wetting cycles promote carbon sequestration and aggregate stability

Abstract

Biogeochemical mechanisms at microscale regions within soil macroaggregates strengthen aggregates during repeated DW cycles. Knowledge of additional biogeochemical processes that promote the movement of dissolved organic carbon (DOC) into and throughout soil aggregates and soil aggregate stabilization are essential before we can more accurately predict maximum carbon (C) sequestration by soils subjected to best management practices. We investigated the spatial distribution of 13C-glucose supplied to individual soil macroaggregate surfaces and subjected to multiple drying and wetting (DW) cycles. Subsequent distribution of added glucose-C, CO2 respiration, increased microbial community activity and concomitant changes in soil aggregate stabilization were monitored. Moist macroaggregates were treated with no DW cycles and zero glucose C (Control), 5 DW cycles and zero glucose (DW0G), and 5 DW cycles with additions of 250 μg glucose-13C/g soil during each cycle (DW+G). Repeated additions of glucose-C to aggregate surfaces reduced the mineralization of pre-existing soil C by an average of 45% and established concentric gradients of glucose-derived C. It is concluded these increasing gradients promoted the diffusion of soluble C into interior regions and became less available to microbial respiration. Spatial gradients of glucose-derived C within aggregates influenced a shift in the abundance of unique ribotypes spatially distributed within aggregates. Rapid decreases in the mineralization rates of glucose-C during repeated DW cycles suggested greater C sequestration by either physical restriction of microbes or chemical sorption of new C that diffused into aggregates. Aggregate stability decreased significantly following 2-3 DW cycles, when glucose-C was not added. Additions of glucose-C with each DW cycle maintained soil aggregate stability equal to the moist but not cycled control throughout the 5 DW cycles of this study. These data simulate the strengthening of soil aggregates in no tillage agroecosystems which provides continuous additions of DOC compounds generated by decomposing plant residues on the soil surface, and root exudates and decomposition, as well as the mineralization of POM materials within nondisturbed soil profiles.