Trade-offs in greenhouse gas emissions across a liming-induced gradient of soil pH: Role of microbial structure and functioning

Liming is a common agricultural practice to improve crop yields by raising soil pH. Liming also modulates the production and consumption of greenhouse gases (GHGs) in soils, but the direction and strength of such effects are largely unknown. Since lime application is not a dichotomous variable (application or not), but rather can be dosed according to crop requirements, a critical research gap is whether liming effects on GHG emissions are linear or follow alternate relationships. Based on two-years of data from a long-term field-liming experiment (initiated in 1942), we show that the relationship between an established soil pH gradient (pH of 3.77, 4.92, 6.39, and 6.84 in 1 M KCl extracts) and the emission of GHGs deviates from linearity. Liming increased carbon dioxide (CO2) fluxes from ecosystem respiration, but there were no differences between limed treatments. The increased respiration was due to higher plant carbon (C) inputs and root respiration, enhanced decomposition activity (beta-glucosidase) and increased abundance of specific microbial groups (e.g., cellulose-decomposing bacteria). Liming decreased nitrous oxide (N2O) emissions due to increased N2O reduction via denitrification and by promoting plant growth. Compared with native acidic soil, lime application stimulated methane (CH4) oxidation, but the highest liming rate was least stimulatory. This aligned with the soil CH4 oxidation potential, which was modified by liming-induced changes in soil microbial structure as indicated by phospholipid fatty acid analyses. There were no differences in soil organic C (SOC) content between liming levels, apparently because increased ecosystem respiration rates compensated for higher plant C inputs from limed plots. Overall, our results reveal a fundamental ecological trade-off: applying lime at a rate that maximizes crop yield with low N2O emissions may increase CO2 emissions (not compensated by higher SOC in a sandy soil) and decrease CH4 oxidation compared to liming applications below this level.