Mechanical termination of a perennial grain crop minimally impacts soil structure, carbon and carbon dioxide emissions

Introduction: Mechanical termination of crops can negatively affect soil biological, chemical, and structural characteristics. Perennial crops do not require annual termination and can improve these same soil characteristics, which has catalysed interest in the development of new perennial crops. Advanced lines of the perennial grass intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey; IWG) have been bred for increased seed size and marketed as Kernza (R) perennial grain, but little is known about how this new crop can be terminated for subsequent annual crop production in rotations that enhance agricultural productivity and environmental sustainability. Materials and Methods: Five methods of terminating IWG were tested in Minnesota, USA. Treatments included mechanical tillage using a chisel plow (CHI), undercutter (UND), and disc (DSC), along with no-till treatments of glyphosate (GLY) and a repeated-mowing control (CTRL). Treatment effects on IWG mortality, soil carbon dioxide (CO2) emissions, bulk density, aggregate stability, soil carbon stocks and soybean yield were measured. Results: Daily CO2 fluxes differed by treatment (p < 0.05) on only one of 19 sample dates, and cumulative soil CO2 emissions over the course of the growing season did not differ across treatments. Bulk density decreased relative to baseline in all treatments except CTRL. Aggregate stability remained unchanged in all treatments except CTRL, which increased from the baseline. Soil carbon stocks did not change in any treatment. Soybean yield was highest in GLY but was not significantly different from CHI or UND. Conclusions: Soil structure, soil carbon stocks and soil CO2 emissions were unaffected by tillage and no-till IWG termination treatments. However, tillage followed by preplanting harrowing proved ineffective at terminating IWG and required subsequent summer herbicide applications. Therefore, additional tillage events may be required to fully terminate IWG when herbicide use is prohibited.