Nitrous oxide (N2O) emissions from transitioning organic grain production systems: quantification and driving factors

Organic agriculture is growing worldwide due to its environmental benefits, economic potential, and positive implications for societal well-being. Organic agriculture practices through organic inputs can provide soil health benefits such as carbon (C) sequestration, improved soil biological activity, improved soil structure, and increased aggregate stability. However, uncertainty in N₂O emissions from organic agriculture systems may offset the C sequestration benefits. This study aimed to determine the cropping systems with N₂O emissions mitigation potential when transitioning to organic grain rotation systems in warm, humid climates. Four cropping systems with different tillage regimes, cover crop compositions, and PL application strategies were employed in a three-year soybean-wheat-corn rotation. From each cropping system, extensive soil sampling from 0–15 cm during the main crop growing season was done to measure the soil mineral nitrogen (N) (NH₄-N and NO₃-N). Similarly, gas samples from each system during the main crop season were collected using a static gas chamber to measure greenhouse gases [GHGs – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)]. High pulses of mineral N were observed following the application of organic inputs, with a high N surplus across all cropping systems. Peak N₂O fluxes were observed corresponding to high pulses of mineral N. Cumulative N₂O emissions were highest from the corn phase of rotation, likely attributed to the high rate of PL application and high tillage intensity. Despite similar N input in three of the four cropping systems, significantly low N₂O emissions from the cropping systems that received the least tillage intensity suggest that tillage reduction could potentially reduce N₂O emissions. While comparing the effect of cover crops and PL on N₂O emissions, we found that PL was the major contributor to N₂O emissions, with denitrification being the major microbial process controlling peak N₂O emissions. Global warming potential (GWP) was calculated based on the change in soil organic C (SOC) stock, CH₄, and N₂O, and negative GWP was observed across all cropping systems. Negative GWP estimation across all cropping systems suggests that organic crop rotation with high organic inputs could potentially sequester C. However, a long-term study is essential to provide deep insight into the effect of organic crop rotation on C sequestration.