Soil‐Test Biological Activity with the Flush of CO2: II. Greenhouse Growth Bioassay from Soils in Corn Production

Core Ideas Grass growth in the greenhouse was dependent on soil nitrogen mineralization. Soil‐test biological activity was a valuable indicator of nitrogen mineralization. Biological activity, residual inorganic nitrogen, and total nitrogen were most important. Soil nitrogen (N) mineralization is variably affected by management and edaphic conditions. A routine soil test that reflects both soil biological activity and N mineralization could improve predictions for N fertilizer recommendations to cereal grains on different soil types and landscape settings. We collected soils from 47 corn production fields in North Carolina and Virginia at depths of 0 to 10, 10 to 20, and 20 to 30 cm and evaluated soil C and N characteristics in association with sorghum‐sudangrass [Sorghum bicolor (L.) Moench ssp. Drummondii] dry matter production and N uptake during 6 to 8 wk of growth in the greenhouse. Plant dry matter and N uptake were strongly associated, as expected. Plant available N (sum of net N mineralization during 24 d of aerobic incubation + residual inorganic N) had the strongest association with plant dry matter production (r² = 0.76) and N uptake (r² = 0.85). However, the flush of CO₂ during a 0‐ to 3‐d period following rewetting of dried soil was nearly equally effective at r² = 0.74 and r² = 0.76, respectively. Multiple regression models with 4 ± 2 additional variables led to r² = 0.88 ± 0.10 among different separations of data based on depth, region, and soil textural class. We suggest the optimum combination of variables to predict soil N availability would be the flush of CO₂, residual inorganic N, and total soil N concentration, as they balance relevant scientific information with limited soil‐testing resources (time and labor). We demonstrated that the flush of CO₂ was a rapid and reliable indicator of soil N availability.