Microbial Proxies for Anoxic Microsites Vary with Management and Partially Explain Soil Carbon Concentration

Abstract

Anoxic microsites are potentially important but unresolved contributors to soil organic carbon (C) storage. How anoxic microsites vary with soil management and the degree to which anoxic microsites contribute to soil C stabilization remain unknown. Sampling from four long-term agricultural experiments in the central United States, we examined how anoxic microsites varied with management (e.g., cultivation, tillage, and manure amendments) and whether anoxic microsites determine soil C concentration in surface (0-15 cm) soils. We used a novel approach to track anaerobe habitat space and, hence, anoxic microsites using DNA copies of anaerobic functional genes over a confined volume of soil. No-till practices inconsistently increased anoxic microsite extent compared to conventionally tilled soils, and within one site organic matter amendments increased anaerobe abundance in no-till soils. Across all long-term tillage trials, uncultivated soils had ∼2-4 times more copies of anaerobic functional genes than their cropland counterparts. Finally, anaerobe abundance was positively correlated to soil C concentration. Even when accounting for other soil C protection mechanisms, anaerobe abundance, our proxy for anoxic microsites, explained 41% of the variance and 5% of the unique variance in soil C concentration in cropland soils, making anoxic microsites the strongest management-responsive predictor of soil C concentration. Our results suggest that careful management of anoxic microsites may be a promising strategy to increase soil C storage within agricultural soils.

Keywords: anoxic microsites; carbon; oxygen; redox; soil organic carbon; soils.

Figure 1

Schematic showing the sampling design. Soil is conceptualized as a mix of present or past anaerobe habitat space and nonanaerobe habitat space, which may be void of microorganisms or consistently populated by aerobic organisms. With our sampling design, we volume average anaerobic habitat space by collecting set-volume composite soil samples. The proportion of anaerobe vs non-anaerobe habitat space persists through each step of the analysis and is ultimately reflected in the droplet digital PCR (ddPCR) results.

Figure 2

Absolute abundance of anaerobe target genes in unamended conventional till (CT), no-till (NT), and uncultivated (UN) soils. (a) Total anaerobe abundance per treatment; the stacked bar representation of panel b. (b) Abundance of individual target genes across sites and tillage practices. Bar height represents the mean (n = 2–3) and error bars represent the standard error of each mean. Letters denote differences among CT, NT, and UN soils within a single site.

Figure 3

Anaerobe abundance with manure amendments across conventional till (CT) and no-till (NT) treatments in Carrington, ND. U = Unamended. A = Amended with steer manure. Bar height represents the mean (n = 2–3), and error bars represent the standard error of each mean. Stars represent significant differences via t test between amended and unamended soils.

Figure 4

Linear regression between anaerobe abundance and organic C concentration. Panels depict (a) the entire data set (n = 47), (b) Carrington, ND (n = 18), (c) Novelty, MO (n = 7), (d) Crossville, AL (n = 14), and (e) Wooster, OH (n = 8). Gray line represents the best fit line for each panel of data.

Figure 5

Variance partitioning analysis for soil carbon concentration (%) across (a) all sites (n = 47) and (b) solely cropland sites (n = 39). Predictor variables included in each grouping are listed under each group’s title. SSA = specific surface area; SRO = estimated short-range order mineral content (acid ammonium extractable Fe, Al, and Mn); ag. use = agricultural use (i.e., cropland or uncultivated); till = no-till, minimum till, or conventional till; MAP = mean annual precipitation.