Soil‐Test Biological Activity with the Flush of CO2: IV. Fall‐Stockpiled Tall Fescue Yield Response to Applied Nitrogen

Abstract

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 CO2 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 CO2–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 CO2–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.