Fig. 5: Carbon limits growth in Regime II and is released from the soil matrix.
From: Functional regimes define soil microbiome response to environmental change
a, Fold change in nitrate reduction rate (without chloramphenicol/chloramphenicol-treated) as a function of the amount of NaOH added to the soil (n = 470 samples of <30 mM OH−, <25 mM H+; 28 data points with fold change >5.5 were excluded for clarity of visualization). A linear increase in fold change from 0 mM to 25 mM NaOH corresponds to the nutrient-limiting regime (Regime II, light blue region), and suggests that \(\widetilde{C}(0)\) increases with increasing NaOH. b, Cartoon illustrating the hypothesized mechanism of nutrient release. Adding NaOH results in the release of anionic nutrients (magenta spheres, Case 2) from soil particles (brown region), whereas the addition of HCl would release cationic nutrients (grey spheres, Case 1). Microorganisms access nutrients in pore water but not those adsorbed to soil particles. Added OH− ions decrease the number of anion exchange sites, releasing anionic nutrients. Na+ ions stabilize the released anions (Extended Data Fig. 4j and Supplementary Information). Data in a and model in b suggest that growth-limiting nutrients are anionic (negatively charged). c, Scatter plot of model-inferred available limiting nutrient (\(\widetilde{C}(0)\)) with measured WSOC (endpoint WSOC, Methods) (n = 222 samples of 0–25 mM OH− in soil samples 1–12). d,e, Amendment experiments for soil in the nutrient-limiting regime at an unperturbed pH (5.4). d, Nitrate dynamics with different glucose amendments. Dots represent data and lines show model predictions. e, Nitrate reduction rates after amending soils with different concentrations of nutrients. Dots represent mean rates, estimated by linear regression on triplicates, and error bars indicate s.d. Carbon amendments (succinate, acetate and glucose) increased nitrate reduction rates, but ammonium, sulfate and phosphate did not. Model predictions are shown for 1 < rA/rC < 3 (shaded region) with a line for rA/rC = 2 (best fit). See also Extended Data Figs. 2g,h and 4.
