Soil‐Test Biological Activity with the Flush of CO2: III. Corn Yield Responses to Applied Nitrogen

Core Ideas Relative corn yield was associated with residual inorganic and mineralizable N. Soil‐test biological activity was a good surrogate for predicting N availability. Economically optimum N fertilizer requirement could be adjusted with soil testing. Corn (Zea mays L.) is an important cereal grain in many states and typically receives large N fertilizer inputs, irrespective of historical management. Tailoring N inputs to soil‐specific conditions would help to increase efficiency of N use and avoid environmental contamination. A total of 47 trials were conducted on research stations and private farms in four different regions of North Carolina and Virginia from 2014 to 2016 to associate soil N availability with yield response to sidedress N application. Corn grain yield was 10.6 ± 3.4 Mg ha–¹ on 36 sites and silage yield was 44.6 ± 8.2 Mg ha–¹ on 11 sites. There was positive association between relative yield (i.e., yield without sidedress N application divided by yield with full fertilization) and levels of both plant available N (residual inorganic N + net N mineralization during 24 d) and the flush of CO₂ following rewetting of dried soil during 3 d. Economically optimum N fertilizer requirement (EONR) at sidedress declined with increasing level of plant available N and soil‐test biological activity (i.e., the flush of CO₂). The scalable N factor for production at EONR declined from 20 kg N Mg–¹ of grain (i.e., 1.1 lb N bu–¹) with no soil biological activity to no N required with soil‐test biological activity of 600 mg CO₂–C kg–¹ in a 3‐d period (depth of 0–10 cm). The flush of CO₂ when determined in spring at or prior to planting corn was considered an ideal soil‐test indicator of soil biological activity due to its simple, rapid, and reliable characteristics related to potential soil N mineralization and corn yield responses to applied N fertilizer.