Soil‐Test Biological Activity with the Flush of CO 2 : IV. Fall‐Stockpiled Tall Fescue Yield Response to Applied Nitrogen

Core Ideas Soil biological activity is a reliable indicator of soil nitrogen availability. Nitrogen fertilization of fall stockpiled tall fescue should be adjusted based on soil testing. The flush of CO 2 is a robust indicator of soil biological activity. A new paradigm of soil testing based on soil biological activity is possible.Fall stockpiling of tall fescue ( Lolium arundinaceum ) in the southeastern United States is promoted as an ecologically favorable cattle management approach to avoid the financial and environmental burdens of winter hay feeding. We hypothesized that soil N mineralization should be an important factor controlling forage yield response to N fertilizer. We conducted 55 N fertilizer trials in combination with analyses of soil C and N fractions at multiple locations in Georgia, North Carolina, Virginia, and West Virginia during two seasons. Plant‐available N, as a combination of residual inorganic N + mineralizable N at depth of 0 to 10 cm, was significantly negatively related with extent of forage dry matter response to N fertilizer input. Large variations in economically optimum N fertilizer requirement (EONR) occurred among fields, but when several fields were averaged along a gradient of soil biological activity, a strong negative yield response with increasing soil‐test biological activity emerged. With moderate soil‐test biological activity of 200 mg CO 2 –C kg – 1 soil 3 d – 1 , EONR was 20 kg N Mg – 1 forage dry matter (a value similar to current N fertilizer recommendations). However, with progressively greater soil‐test biological activity up to 600 mg CO 2 –C kg – 1 soil 3 d – 1 , EONR declined in a nonlinear manner to near zero. These results illustrate that N fertilizer recommendations for fall stockpiled tall fescue pastures should be a function of soil‐test biological activity as an indicator of biologically active N. Greater economic and environmental sustainability would likely be attainable with a shift to recognizing soil biological activity in an ecologically oriented fertilization paradigm.