Soil texture controls nitrous oxide emissions more than biochar or basalt amendment in two incubated Ultisols

Agricultural soils release carbon dioxide and nitrous oxide. Nitrous oxide carries most of the climate cost per unit mass. Biochar and basalt rock dust are promoted to store carbon and lower these emissions, but their effects vary widely and their dependence on soil texture is poorly resolved. We incubated two contrasting Ultisols, a fine soil (Clay) and a coarse sandy soil (Sand). Each was amended with two biochars, basalt rock dust, and a basalt and softwood combination, all at 4 percent by weight. Carbon dioxide and nitrous oxide were measured continuously with a custom built automated LI-COR system across an 84-day incubation and a nitrogen application. Soil texture, not amendment, controlled the gas balance. The Clay soil released more carbon dioxide and was a strong nitrous oxide source, while Sand emitted little. Amendment had no significant effect on cumulative nitrous oxide. The single amendment effect was a lower carbon dioxide flux under softwood biochar after fertilization. Soil extraction explained the contrast: Clay held abundant nitrate, the substrate for denitrification, while Sand held mostly ammonium. After fertilization, nitrous oxide from Clay was about three times the carbon dioxide flux on a carbon dioxide equivalent basis, an emission factor near 3.6% of applied nitrogen. Because the carbon dioxide is respired in a closed incubation with no plant uptake, this compares two flux terms rather than a net greenhouse gas balance. Soil texture, through its control of water-filled pore space and nitrate, decides whether biochar or basalt can shift these fluxes.