Comparing the Effects of Forage Mix and Nutrient Management on Soil Greenhouse Gas Flux in Semi-arid Improved Pastures

Progress report for GS20-226

Project Type: Graduate Student
Funds awarded in 2020: $16,450.00
Projected End Date: 08/31/2023
Grant Recipient: Texas Tech University
Region: Southern
State: Texas
Graduate Student:
Major Professor:
Lindsey Slaughter
Texas Tech University
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Project Information

Summary:

Researchers and producers are challenged with developing agricultural systems that use limited resources effectively while improving productivity in the face of land degradation and increased climate variability. This is especially critical in semi-arid systems that are vulnerable to soil erosion, nutrient depletion, and extreme water scarcity. A sustainable and economically feasible management option is converting high-input row crop systems to grazed perennial grasslands, which are managed to provide high-quality forage for animal performance while improving soil health and conserving water and nutrient resources through minimized disturbance and continuous soil cover. Enteric methane (CH4) production from cattle is a potential sustainability tradeoff in these systems, but it is likely possible to offset this impact through integrated soil-plant-animal management. Preliminary data for our study indicate that adding legumes can increase soil microbial uptake of CH4 and improve forage quality for livestock in semi-arid pastures, which improves resource efficiency, sustainability, and productivity. Our proposed two-year study will leverage long-term forage manipulations in grazed semi-arid pastures to determine how management regulates soil greenhouse gas (GHG) fluxes. Within this established system, we will investigate how nutrient and forage management increases soil CH4 uptake while improving soil health in semi-arid pastures. The results of our study will ultimately help us create more efficient and resilient semi-arid agricultural systems across the globe.

Project Objectives:
  1. Discover how different sources of soil nitrogen regulate the presence and activity of CH4 cycling soil microbes and GHG soil fluxes.
  2. Quantify how legume density influences GHG flux in established long-term pastures.

 

Research

Materials and methods:

Study Site and Experimental Design

We will leverage an established long-term pasture research site located at the Texas Tech University New Deal Research Farm in New Deal, TX that is supported and maintained by Co-PI Chuck West and has been used in previous SARE research. For this study, we will focus specifically on pasture grass base that was established in 2003, which consisted of WW-B.Dahl Old World Bluestem (OWB, Bothriochloa bladhii  (Retz) S.T. Blake), in six, 2.1-ha pastures across three blocks (two pastures per block, all mapped as Pullman clay loam). Further, 20-m-2 fenced areas were established to provide long-term grazing exclusion treatments (ungrazed) within each pasture. In fall of 2016, alfalfa was interseeded into one OWB-containing pasture of each block, resulting in two pasture treatments replicated across three field blocks: 1) OWB growing alone and 2) a mixture of the OWB and alfalfa. Pastures containing only OWB receive annual fertilization at a rate of 67 kg ha-1 N, with no fertilization of grass + alfalfa pastures. Alfalfa was re-seeded into one block in the fall of 2018 to correct a mediocre establishment. Alfalfa stand cover ranged from approx. 15 to 40% during summer 2019. Irrigation will not exceed 38 mm per month as needed to make up an irrigation + rainfall target of 102 mm per month. Maximum grazing-season irrigation will be 152 mm to make up an irrigation + rainfall target of 408 mm. This regime will deliver about 40% of potential ET in an average year and represent a low irrigation scenario that is realistic for SHP producers transitioning to dryland systems.

Objective 1:

For this objective, we will monitor soil GHG fluxes (CH4, CO2, N2O) in long-term semi-arid pastures containing either 1) warm-season perennial grass OWB alone, or 2) OWB + alfalfa as mixed forage from the established experimental site described above. Although the effect of grazing is not a central component of this objective, we propose to measure GHG flux and soil microbial properties from pasture areas both inside and outside the ungrazed exclosures in each pasture for comparison.

For approximately ten weeks during the grazing season (June—August), we will make weekly soil GHG flux measurements using the static chamber method, following the chamber design and deployment methods and other considerations described in the USDA-ARS GRACEnet Project Protocol for soil GHG flux measurements (Parkin and Venterea, 2010). Chamber bases will be installed in analytical duplicates in ungrazed exclosures (two per pasture), and in four analytical replicates in grazed areas (four per pasture), resulting in 36 total chamber bases for gas measurement. Headspace gas samples collected at 0, 15, and 30-minute intervals with a 30 mL syringe and injected into pre-evacuated glass vials to await analysis. Headspace CH4, CO2, and N2O concentrations (ppm) will be simultaneously measured on a GC-2014 gas chromatograph (Shimadzu Corporation, Kyoto, Japan). Soil temperature and moisture will be monitored weekly during gas flux measurements.

Soil cores (5 cm diameter, 5 cm depth) used to measure soil bulk density, aggregates, routine soil test nutrients, and carbon and nitrogen will be collected once at the beginning of the experiment. Mean weight diameter (MWD) and size distribution of soil aggregates will be determined in air-dried subsamples by passing 200 g of soil through four sieve sizes, separated by dry-sieving on a Ro-Tap Test Sieve Shaker (W.S. Tyler Industrial Group, Mentor, OH). Routine soil test nutrients (including P, K, Ca, Mg, Cu, Na, S, Zn) will be measured via inductively coupled plasma (ICP) analysis in air-dried samples sent to a commercial laboratory. Soil organic carbon (SOC) and total nitrogen (TN) will be measured via high-temperature combustion analysis on a LECO TruSpec CN analyzer (LECO Corporation, St. Joseph, MI). Total soil organic matter (SOM) will also be determined via loss-on-ignition (LOI) at 400°C for 24 hrs. Available NH4+ and NO3 concentrations will be extracted in a 1:10 ratio of soil:2 M KCl and quantified using flow injection analysis on a Lachat QuikChem® 8500 Series 2 (Lachat Instruments, Hach Company, Loveland, CO).

Soil samples to a depth of 5 cm will be collected in each pasture from the area between adjacent PVC collars weekly in each of the treatment areas, corresponding with one of the GHG flux measurements. We will measure soil moisture and temperature at the time of soil and GHG collection, and soil pH and EC will be subsequently measured in 1:1 soil: water extracts in the laboratory. To assess microbial communities, we will characterize microbial biomass and community structure via ester-linked fatty acid methyl ester (EL-FAME) analysis (Schutter and Dick, 2000).

In soil samples collected at the end of this study, we will extract DNA from each sample for use in Illumina-based next-generation metagenomic sequencing and quantitative PCR (qPCR) assays targeting the mcrA gene for methanogenesis (Steinberg and Regan 2009; Narihiro and Sekiguchi, 2011) and the pmoA gene for methane oxidation (Kolb et al. 2003; Zeng et al., 2019) to determine the CH4-cycling potential of microbial communities in soil samples due to forage treatments.

Objective 2:

In pastures used for Objective 1 that include grass mixed with alfalfa, we identify field areas that consistently contain a gradient of alfalfa densities ranging from 0% cover to 100% cover from which to measure GHG fluxes and soil microbial properties described above.  We identify four (e.g., 0-25%, 25-50%, 50-75%, 75-100%) legume density ranges to use for this study. This adds four chamber bases for gas measurement to the total from objective 1 for each of the three OWB+alfalfa pastures, resulting in a total of 48 chamber bases (36 in objective 1 + 12 in Objective 2) for gas measurement in the overall study.

Outreach

We will host pasture walks, publish updates in the fact sheets and newsletter, and present results at the annual “Water College” hosted by the Texas Alliance for Water Conservation (TAWC), directed by our collaborators Dr. Chuck West (retired) and Dr. Krishna Jagadish. In addition, we will communicate the results of each objective as published manuscripts in peer-reviewed scientific journals.

Research results and discussion:

The data collected so far show that inclusion of legumes (alfalfa) creates a stronger methane sink while also minimizing nitrous oxide emissions from soil compared to fertilized grass-only (old-world bluestem) pastures, and that this difference is greatest during the summer months following fertilization of the grass-only pasture in May (Figures 1 and 2). Because few differences in methane or nitrous oxide emissions were observed between different alfalfa densities within grass-legume pastures, this effect is likely due more to presence or absence of nitrogen fertilizer than to presence or absence of alfalfa (Figures 1 and 2). We have also observed significantly higher CO2 emissions from grass-legume pastures, but only in the late spring months due to greater vegetative biomass and earlier greenup of alfalfa than of the warm-season grasses (Figure 3). Because we were not able to include the 2020 summer season at the start of the project due to the COVID-19 pandemic, we have requested extending the deadline by one year (12 months) to give us more time to complete the microbial sequencing work for the project and continue collecting gas samples throughout the 2022 field season. After completing the 2022 field season measurements and soil collections, completion of the microbial sequencing work would require several months to collect, manage, and interpret the large amount of data produced.

Soil greenhouse gas flux from semi-arid pastures

Participation Summary

Educational & Outreach Activities

5 Webinars / talks / presentations

Participation Summary:

Education/outreach description:

Project is ongoing. Presentations communicating this research have been delivered to producers and researchers at regional conferences and meetings (e.g., Texas Soil Survey and Land Resource Workshop in College Station, TX; Sustainable Dairy Research Meeting in Amarillo, TX in February 2022), local meetings (e.g., Annual PSS Student Research Symposium; Texas Tech University Graduate Research Symposium in May 2022), and international meetings (e.g., virtual poster presentation for the 2022 World Congress of Soil Science in Glasgow, Scotland in July/August 2022). Further presentations are planned for the 2022 ASA-CSSA-SSSA International Annual Meeting in Baltimore, MD this November.

Project Outcomes

1 Grant received that built upon this project
1 New working collaboration
Project outcomes:

Project is ongoing. Our work so far demonstrates that not only can producers improve their climate footprint through reduced fertilizer application and water savings, but mixed legume-grass forages can help mitigate soil greenhouse gas emissions in semi-arid pasture soils.

Knowledge Gained:

Project is ongoing. Throughout this project, we have gained a better understanding of the differences and practicality of managing nitrogen in semi-arid pastures through either nitrogen fertilizer or legume addition. Previous work from collaborators in the same pastures has shown that adding legumes instead of fertilizing grass-only pastures can provide a good source of protein for grazing cattle and improve animal weight gain. Our work shows that mixed alfalfa and warm-season old world bluestem forage can also help mitigate soil greenhouse gas emissions (CH4 and N2O) from semi-arid pastures compared to managing grass-only stands with fertilizer. However, working in the fields and communicating with pasture managers and producers has also reinforced our appreciation for the additional benefits of legume inclusion to sustainability and agricultural production such as through greater presence of pollinators and wildlife (e.g., the threatened Texas horned lizard) in the mixed forage pastures than in grass-only pastures or adjacent row crop systems.

Recommendations:

Project is ongoing.

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.