Effects of cover crop functional traits and termination timing on nitrous oxide emissions and soil nitrogen dynamics in a humid temperate crop rotation

Agricultural soils are the largest anthropogenic source of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance. Emissions of N2O primarily result from microbial processes such as nitrification and denitrification, which are strongly influenced by soil nitrogen (N) availability, moisture, and carbon (C) inputs. In the context of climate change mitigation and sustainable nutrient management, reducing N2O emissions from croplands remains a major challenge. Cover crops (CCs) are widely promoted as a key component of sustainable agriculture due to their ability to improve soil structure, reduce erosion, enhance soil organic matter, and retain nutrients. They are also expected to mitigate N2O emissions by scavenging residual N and supporting soil health. However, research findings on their effectiveness remain inconsistent. Reported outcomes vary depending on species traits, biomass production, site conditions, and management practices, particularly termination timing and residue management. This thesis investigates how CC functional traits and termination timing influence N2O emissions, soil mineral nitrogen (SMN) dynamics, and soil organic carbon (SOC) sequestration in humid temperate agroecosystems. Through multi-site and multi-year field experiments, the work evaluates short and medium-term biogeochemical responses to different CC species across soils of varying texture. The studies combine field-based N2O flux measurements, SMN analysis, microbial profiling, and long-term SOC modeling. The first study assessed three winter CC species: winter rye as a frost-tolerant grass, saia oat as a frost-sensitive grass, and spring vetch as a frost-sensitive legume, in comparison to a bare fallow control across four site-years. Non-legume CC species reduced SMN levels during autumn and winter and thereby lowered nitrate leaching risk. However, frost-sensitive species triggered substantial N2O emissions during freeze-thaw periods, and high-biomass CCs such as rye resulted in the highest N2O emissions following incorporation into moist soils. Despite this, rye also showed the greatest potential for C sequestration and mitigation of indirect N2O emissions by reducing nitrate leaching risk. These results highlight the trade-offs between N retention, direct and indirect N2O emissions, and long-term SOC storage, emphasizing the importance of balanced and species-informed CC strategies. The second study focused on the effect of termination timing using oil radish as a cruciferous winter CC. Early autumn termination was compared with spring termination and a fallow control across five site-years. Although CCs reduced SMN prior to winter, early termination significantly increased winter N2O emissions in fine-textured loamy soils due to anaerobic decomposition of residues. This effect was not observed in sandy soils, where nitrate leaching was likely the dominant N loss pathway. Early termination also increased SMN availability in spring in loamy soils, which may benefit early crop growth and reduce fertilizer requirements. In contrast, late termination reduced winter N2O emissions but may shift the risk to the spring period, especially under fertilized conditions or where pre-emptive N uptake by CCs reduces SMN availability for the subsequent crop. Microbial analyses revealed increased abundance of genes involved in nitrification and denitrification under early termination during winter, although gene abundance did not correlate strongly with measured N2O fluxes. Overall, the results show that the environmental performance of CCs is highly context specific, shaped by interactions among soil properties, climate, CC functional traits, and management practices. Methodologically, the thesis demonstrates the value of integrating field data, molecular analyses, and modeling to assess the environmental trade-offs of CC-based N management. Limitations include the use of non-continuous gas sampling, limited temporal resolution of microbial measurements, the absence of direct C measurements, and incomplete annual N2O budgets across all site-years in the termination study. This work shows that CCs can contribute to climate-smart N management and C sequestration when adapted to local conditions. It calls for systems-based approaches that integrate agronomy, soil microbiology, and environmental modeling. Site-specific guidance for CC selection and termination will be essential to optimize N retention, minimize GHG emissions, and realize the multifunctional benefits of CCs in temperate cropping systems.

2025-08-18