Net ecosystem carbon budget and net greenhouse gas emissions under diverse crop rotation using cover crops compared to a conventional crop rotation

Introducing cover crops and diversifying annual crop rotations can provide additional carbon (C) input to soils and are practices that have been promoted as climate change mitigation measures. However, year-round field studies are needed to evaluate the net effect on carbon dioxide (CO2) uptake, including consideration of carbon removal from the field during harvest depending on the crop grown to diversify the rotation. In addition, evaluation of net greenhouse gas (GHG) emissions requires accounting for changes in soil nitrous oxide (N2O) and CO2 emissions from production of farm inputs and field operations in the altered crop rotation. In this study, our objectives were to evaluate the annual dynamics of CO2 exchange and to estimate net GHG emissions for a diverse (DIV) corn-soybean-winter wheat crop rotation that included cover crops and a conventional (CONV) corn-soybean-soybean crop rotation. The year-round net ecosystem exchange (NEE) of CO2 on two side-by-side 8-ha fields (one under DIV and one under CONV management) was measured using the eddy covariance method over three years (2018-2021) in Ontario, Canada. Results showed that DIV rotation significantly increased cumulative NEE, gross primary production (GPP), and ecosystem respiration (Re) -578, -3750 and 3170 g C m(-2), respectively, compared to the CONV (-501, -3361, 2859 g C m(-2)) over three years. Overall, the DIV rotation resulted in a 15 % increase in NEE compared to CONV. Accounting for the grain C removed during harvest, on an annual basis, DIV and CONV corn and soybean fields were C sources. However, net ecosystem carbon budget (NECB) was a sink (-44 g C m(-2)) for DIV winter wheat + cover crops if only grain was removed and reversed to a C source (197 g C m(-2)) with straw + grain removal. The summed NECB across the three study years revealed that both fields were C sources (NECB > 0). Similarly, the DIV rotation was a net GHG source compared to CONV for straw + grain removal but had similar net GHG emission to CONV when only wheat grain was removed. From the present study, DIV did not reduce the net GHG emissions in the short term (3 years) as expected, but long-term measurements are needed to confirm this trend. Further multi-year eddy covariance and soil carbon stock measurements should be done to design effective diversified crop rotations.