Quantifying How Soil Aggregate Size Impacts Nitrous Oxide Emissions from Manure Injection

View the project final report

Commodities

Practices

• Crop Production: fertilizers, no-till, nutrient cycling, nutrient management, organic fertilizers
• Education and Training: display, extension, on-farm/ranch research
• Natural Resources/Environment: other
• Production Systems: integrated crop and livestock systems
• Soil Management: nutrient mineralization, organic matter, soil analysis, soil microbiology, soil quality/health

Agricultural management practices have been successfully used to promote soil health and to achieve multiple positive environmental outcomes, such as improving water quality. However, the climate change mitigation impacts of these best management practices (BMPs) are less well known, particularly with regard to their impacts on nitrous oxide (N₂O) emissions, a powerful greenhouse gas linked to agricultural soil and nutrient management. In the Northeast and across the United States, no-till is a BMP that can increase soil health and aggregation, but there is also evidence that no-till can produce more N₂O emissions than conventional tillage in certain conditions. Similarly, manure injection, a BMP for manure application that can be used in no-till systems, provides agronomic and environmental benefits, but can also produce more N₂O emissions than surface application of manure. Thus, the purpose of this project is to examine the factors that control N₂O emissions from agricultural soils when no-till and manure injection are combined. In particular, this project will quantify how enhanced soil health, via changes in soil aggregate size, impacts N₂O emissions. Overall, the proposed research will contribute to a broader understanding of the benefits and tradeoffs of combinations of BMPs, and it will provide insight into the mechanisms driving agricultural N₂O emissions. Results will be shared with farmers and the extension community to support advancement of environmental and agronomic goals.

This project will be conducted as two experiments to make each part more feasible.

Experiment 1: Measuring N₂O fluxes in response to manure injection into soils of different aggregate size classes as well as into homogenized (non-aggregated) soil

Objective 1: Quantify N₂O emissions in response to manure injection into soils of different aggregate size classes.

Hypothesis: N₂O emissions will be greatest from the largest soil aggregates and decrease as aggregate size also decreases because larger aggregates will be sufficiently aerobic (oxygenated) to prevent complete denitrification (which requires anaerobic, or oxygen depleted, conditions) of N₂O to N₂, while smaller, more anaerobic aggregates will favor complete denitrification of NO₃^- to N₂ and thus produce fewer N₂O emissions.

Objective 2: Quantify the role of aggregates in producing N₂O emissions after manure injection by comparing N₂O fluxes from intact macroaggregates to N₂O fluxes from homogenized soil (i.e., no aggregates).

Hypothesis: N₂O fluxes will be higher from intact macroaggregates than homogenized soil because the intra-aggregate pore space within macroaggregates will serve as important anaerobic microsites for N₂O generation.

Objective 3: Quantify net nitrogen mineralization in response to manure injection into soils of different aggregate size classes.

Hypothesis: There will be no difference in ammonium (NH₄⁺) concentrations across treatments, but NO₃^- concentrations will be higher in soil with larger aggregates, as denitrification (which reduces NO₃^- to N₂ and N₂O) rates will be lower in these more oxygenated soils.

Experiment 2: Measuring potential denitrification activity of soils of different aggregate size classes as well as homogenized (non-aggregated) soil

Objective 1: Quantify potential denitrification activity of aggregates of different size classes and of homogenized soil.

Hypothesis: There will be greater potential denitrification activity in the smaller aggregates due to their smaller intra-aggregate pore space limiting oxygen diffusion into the aggregate and thus favoring anaerobic conditions. This potential will decrease with increasing aggregate size.