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

Progress report for GNE21-250

Project Type: Graduate Student
Funds awarded in 2021: $12,662.00
Projected End Date: 07/31/2022
Grant Recipient: University of Vermont
Region: Northeast
State: Vermont
Graduate Student:
Faculty Advisor:
Dr. Heather Darby
University of Vermont Extension
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Project Information

Project Objectives:

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.

Introduction:

Inorganic nitrogen is important for maintaining yields, and agricultural management practices can influence its loss or retention. One important nitrogen loss pathway from agricultural soils is nitrous oxide (N2O) emissions. With a warming potential 265 times that of carbon dioxide over 100 years, N2O contributes strongly to climate change (IPCC, 2014) and its atmospheric concentration has been increasing since the Industrial Revolution (Prinn et al., 2018; Tian et al., 2020). In the U.S., soil management accounts for about 78% of N2O emissions, and agricultural activities as a whole are the largest contributor of N2O emissions globally (U.S. EPA, n.d.; Jia et al., 2019). The primary source of agricultural N2O is thought to be denitrification, a process that converts nitrate (NO3-) to N2O and N2. Denitrification is enhanced by high soil moisture, high temperature, low soil oxygen, and high NO3- and carbon availability (Wallenstein et al., 2006; Butterbach-Bahl et al., 2013; Xue et al., 2013). Thus, management practices that alter these conditions can have large impacts on agricultural emissions. Indeed, agriculture has great potential to reduce its impact on, and even mitigate, climate change via soil management practices (Griscom et al., 2017).

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 N2O emissions. In particular, BMPs that are adopted for their contributions to soil health, runoff prevention, and/or carbon sequestration can produce greater (e.g., Duncan et al., 2017) or lower (e.g., Gregorich et al., 2008) N2O emissions. A better understanding of how and why BMPs impact N2O emissions is thus required to adequately account for their contributions to or mitigation of climate change.

In the Northeast and across the United States, no-till is a BMP that can increase soil health (Nunes et al., 2018) and reduce farming costs (Creech, 2017). While the USDA Natural Resources Conservation Service incentivizes no-till because it reduces greenhouse gases (USDA NRCS, n.d.), there is also evidence that no-till can produce more N2O emissions than conventional tillage in certain conditions (Rochette, 2008; Rochette et al., 2008). Similarly, manure injection, a BMP for manure application that can be used in no-till systems, provides agronomic and environmental benefits, including nitrogen retention and reduced manure runoff (Maguire et al., 2011). However, manure injection can also produce more N2O emissions than surface application of manure (Dittmer et al., 2020; Duncan et al., 2017; Rodhe et al., 2006). Thus, the purpose of this project is to examine the factors that influence 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, as well as a better understanding of the mechanisms driving agricultural N2O emissions.

Research

Materials and methods:

Experiments 1 and 2: Site Description (All Objectives)

Soil will be collected from an ongoing trial established in 2011 at Borderview Farm in Alburgh, VT (45.005° lat., −73.308° long.). The objective of the trial is to assess the impact of corn cropping systems on overall soil health and crop productivity, and practices used in the trial include conventional tillage, cover cropping, and long-term no-till. Each treatment plot is 6 m x 15.2 m (20 ft x 50 ft) and is replicated four times. Soil within this trial is an Armenia silt loam with a 0-2 percent slope. In 2020, the no-till corn plots had an average of 3.72% organic matter, 2.49% total carbon, 0.265% total nitrogen, and aggregate stability of 61.5%. Additional soil analysis will be performed in 2021 to obtain updated results. Fertilizer is applied annually during corn sowing at a rate of 200 lbs per acre and as topdress about a month after planting at the same rate. Chemical weed control is also applied annually to the no-till plots shortly after planting.

All soil for this project will be collected from the no-till corn plots. In August 2021, soil will be collected from a 0.25 m2 area to a 20 cm depth in each no-till corn replicate, and soil collected from all four replicates will be processed in the lab when preparing the column experiments.

Experiment 1: Experimental Setup (Objectives 1, 2, and 3)

Soil will be air dried and then sieved into four aggregate size categories: 2-4 mm (largest macroaggregates), 0.25-2 mm (smaller macroaggregates), 0.02-0.25 mm (mesoaggregates), and <0.02 mm (microaggregates). Aggregates will be carefully packed into PVC cores (6 cm o.d., 5.5 cm i.d., 20 cm long) at a consistent bulk density across all replicates and treatments. There will be eight replicates of each aggregate size category, four to be used with the manure injection treatment and four to be left unamended as controls. Eight additional PVC cores will be packed with 2-4 mm aggregates that are homogenized, i.e., their structure will be destroyed, before the addition of manure (4 cores) or being left unamended as controls (4 cores). Thus, in total, there will be 40 cores used for GHG measurements. All soil cores will be maintained at field capacity and stored at 20 degrees C in incubation chambers throughout the course of the experiment.

The bottom of each soil core will be closed using an air-tight PVC cap. Each core will be placed in a glass Ball jar (1.9 L, 24 cm tall) and covered with perforated polyethylene film to minimize water loss and allow oxygen exchange between gas measurements. Soil cores will be weighed every few days throughout the experiment to monitor water content, and water will be added as needed to maintain constant soil moisture.

Liquid dairy cattle manure will be collected from Borderview Farm and analyzed for macronutrient (including total nitrogen, ammonium, and organic nitrogen) and micronutrient content at the Agricultural & Environmental Testing Laboratory at UVM. Manure will be stored at -12 degrees C until it is added to the soil cores, at which point it will be thawed to room temperature before injection in the lab. To simulate manure injection, a sterilized metal stake will be used to slice a slot (1 cm wide, 5.5 cm long, 15 cm deep) through the center of each soil column. Manure will then be added to each core using a syringe to place the manure along the entire depth and length of the injection slot. Manure will be applied to the cores at a rate proportional to 74,832 L/ha (8,000 gal/acre), the typical target rate for manure application used at Borderview Farm in corn cropping systems (H. Darby, personal communication, April 21, 2021).

Experiment 1: GHG Flux Measurements (Objectives 1 and 2)

GHG measurements will be taken for 56 days following manure injection. Measurements will be taken every day for the first 14 days post-injection, followed by every other day for 14 days, and finally once every 7 days for the remainder of the experiment. At the time of sampling, an air-tight lid fitted with a butyl rubber septum will be placed on each jar, and gas samples (10 mL) will be collected with polypropylene syringes through the septum at time 0, 15, 30, and 45 minutes after chamber lid deployment. Samples will be immediately stored for analysis in 6 mL pre-evacuated vials fit with butyl rubber septa. Between gas sampling at each 15-minute interval, jars will be kept in the incubation chamber to maintain a constant soil temperature. N2O, CO2, and CH4 fluxes will be measured by a GC-2014 Gas Chromatograph (GC) (Shimadzu Instruments, Kyoto, Japan) equipped with a flame ionization detector (FID), electron capture detector (ECD), and a Hayesep N 80/100 Mesh 1/8in X 1.5M stainless steel pre-conditioned column.

 Experiment 1: Soil Analysis (Objective 3)

Initial inorganic nitrogen (nitrate, NO3-, and ammonium, NH4+) concentration will be determined for all treatments before manure injection and again at the end of the experiment. To extract NO3- and NH4+, 5 g of each soil sample will be combined with 50 mL of 2 M KCl solution in a sample cup. The sample cups will be shaken for one hour, and then the solution will be filtered into 20 mL scintillation vials. The vials will be stored in a freezer (-12 degrees C) for later analysis using the colorimetric nitrogen protocol, which determines soil NO3- and NH4+ concentrations.

Similarly, total carbon and nitrogen of all soil cores will be measured pre-injection and at the end of the experiment. For each core, 10 g of soil will be dried for 48 hours at 60 degrees C, and total carbon and nitrogen will then be measured by combustion using a LECO CHN628 (LECO Corporation, St. Joseph, MI).

Experiment 2: Experimental Setup (Objective 1)

Soil will be prepared using the same methods as those in Experiment 1, although soil cores will be a fraction of the length (5 cm) as those used in Experiment 1 because manure injection will not be applied to the treatments. As in Experiment 1, there will be eight replicates of each aggregate size category, four to be used with acetylene inhibition of nitrous oxide reductase and four to be used as a reference. Eight additional PVC cores will be packed with 2-4 mm aggregates that are homogenized, i.e., their structure will be destroyed, before the addition of acetylene (4 cores) or being left unamended (4 cores). Thus, in total, there will be 40 cores used for determination of potential denitrification activity.

Unlike in Experiment 1, the soil cores used in Experiment 2 will be perforated with small holes (0.4 mm in diameter) along the length of each core to allow for acetylene gas to permeate the entire core. Each core will be placed in a glass Ball jar (500 mL, 12 cm tall) and covered with an air-tight lid fitted with a butyl rubber septum.

Experiment 2: Potential Denitrification Activity Methods and Measurements (Objective 1)

Denitrification activity will be measured by blocking the activity of nitrous oxide reductase using acetylene, as described by Tiedje (1994). Nitrous oxide reductase is an enzyme that catalyzes the conversion of N2O to N2, the latter of which is difficult to measure. By blocking nitrous oxide reductase, this method prevents the formation of N2 and instead favors the production of N2O, thus allowing for the quantification of potential denitrification activity by measuring N2O fluxes. This activity describes potential denitrification, since ideal conditions for denitrification are created in the lab by adding required substrates and creating an anaerobic environment. A nutrient solution consisting of 1 mM potassium nitrate (KNO3) and 1 mM glucose will be added to each core. After substrate addition, each jar will be repetitively evacuated and flushed with N2 gas four times to create a low-oxygen atmosphere that inhibits nitrification (an aerobic process). Acetylene gas will then be added to half of the jars, creating a 10% acetylene atmosphere. The remaining jars will have an N2 atmosphere without acetylene. Gas samples (10 mL) will be collected with polypropylene syringes through the septum at time 0, 30, 60, 90 and 120 minutes after acetylene addition. Samples will be immediately stored for analysis in 6 mL pre-evacuated vials fit with butyl rubber septa. All soil cores will be maintained at field capacity and stored at 20 degrees C in an incubation chamber between gas measurements. The incubation will be completed after two hours. As for Experiment 1, N2O fluxes will be measured by a GC-2014 Gas Chromatograph (GC) (Shimadzu Instruments, Kyoto, Japan) equipped with a flame ionization detector (FID), electron capture detector (ECD), and a Hayesep N 80/100 Mesh 1/8in X 1.5M stainless steel pre-conditioned column. Potential denitrification activity will be calculated as micrograms N2O-N g dry soil-1.

Experiments 1 and 2: Data Analysis (All Objectives)

Differences in GHG fluxes and in denitrification potential between aggregate size classes as well as between macroaggregates and homogenized soil will be analyzed using a one-way analysis of variance test in R (R Core Team, 2020). This data analysis will determine whether there are significant differences in GHG production, and in particular N2O emissions, between aggregate size classes in response to manure injection, as well as whether macroaggregates produce significantly different emissions compared to non-aggregated soil. This data analysis will also determine the extent to which denitrification potential can explain any measured differences in N2O emissions across treatments.

Participation Summary
1 Farmer participating in research

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

This project is part of Brickman’s master’s thesis in the Rubenstein School of Environment and Natural Resources at the University of Vermont. Brickman’s thesis will be publicly available online through UVM, and Brickman will submit project results to peer-reviewed journals for publication.

Part one of Brickman’s thesis focuses on GHG emissions in response to different nitrogen application methods and treatments on hayfields in the Northeast. Because the objectives of this project relate to the goals of the field trial, Brickman will combine findings from both thesis sections when preparing outreach materials. Outreach materials will include a farmer friendly research report, a factsheet, and a webinar, and in such materials, findings from this project may be combined with the results from other projects in Vermont or the Northeast that focus on climate change mitigation from agricultural practices. All educational materials will be shared on the UVM Extension Northwest Crops and Soils Program website (www.uvm.edu/extension/nwcrops). The webinar will be archived and posted on the program’s YouTube channel (https://www.youtube.com/user/cropsoilsvteam) for further access by stakeholders.

Brickman will also share results from this project with UVM Extension staff, who will use the findings to inform their work on GHG emission reduction from BMPs in the Northeast. In particular, findings will be incorporated into outreach plans targeted at Vermont’s dairy industry as part of UVM’s ongoing dairy farming research for the USDA Northeast Climate Hub. They will also be shared with UVM Extension’s Farming & Climate Change Program for integration with farmer outreach focused on climate change mitigation practices. Furthermore, Brickman will share project results through UVM Extension outreach events, including the “Annual Crops and Soils Field Day” at Borderview Farm that attracts over 250 attendees each year. Finally, Brickman will reach out to local and regional farmer and advocacy organizations, including the Vermont Healthy Soils Coalition, the Northeast Organic Farming Association, and the Northeast Healthy Soil Network.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.