Carbon and Nitrogen Cycling in High-Elevation Hay Meadows: Understanding Processes for Improved Agroecosystem Productivity

D. Adamson, J. Norton
University of Wyoming
Introduction
Background
  • High-elevation hay meadows provide critical winter forage for livestock producers in the Mountain West1.
  • Flood irrigation continuously saturates the soil for 6-8 weeks in spring and early summer.
  • One harvest following irrigation averages about 2,200 kg/ha.
Meadow Soils
  • Seasonal flooding and short growing season result in low OM decomposition and the development of a 3-5 cm O-horizon.
  • The O-horizon has high N content (1,100 - 2,400 kg N/ha, 0-10 cm)2.
Rationale
  • High N stores in meadow soils are a resource that should be examined to improve sustainability and resiliency of western ranches.
  • Balancing OM sequestration with nutrient mineralization could improve productivity while maintaining ecosystem services.
Objective
  • Describe soil properties and SOM processes that affect N availability after long-term irrigated meadow management.
Methods
                                 Figure 2: Meadow- system study design pairing fertilized and unfertilized meadows with natural rangeland all on the same original soil type.
Design
  • Four meadow systems in SE Wyoming and N Colorado
  • Three study sites per meadow 
    • ​Long-term fertilized, irrigated hay meadow
    • Long-term unfertilized, irrigated hay meadow
    • Unfertilized, natural rangeland
Sampling
  • Spring and summer 2021
  • 0-5 cm (O-horizon)
  • 5-15 cm (A-horizon)
  • 0-10 cm (A-horizon, rangeland only)
Analyses
  • Total organic carbon (TOC) and nitrogen (TN) by combusiton analysis
  • Potentially mineralizable carbon (PMC) and nitrogen (PMN) by aerobic (C) and anaerobic (N) incubation
  • Micrbial biomass carbon (MBC) and nitrogen (MBN) by fumigation
  • ANOVA (R 4.1.1) followed by TukeyHSD for means separation
Results and Discussion
 
Discussion
  • Decades of continuous flood irrigation resulted in the development of an O-horizon in meadows that accumulates nitrogen compared to rangeland soils (Figure 1).
  • The O-horizon significantly affects C and N cycling, as horizon is a highly significant model factor for all soil analyses for both sampling times (Table 1).
  • In meadow soils, the O-horizon stores the majority of both stable and labile C and N (Figure 1, 2, 3). This partitioning of C and N between horizons is promoted by the presence of rhizomatous grasses with shallow root systems in meadows, as opposed to deeper rooted species in rangelands3.
  • Low average forage yields (1,700-2,800 kg/ha) mean N mineralization is low in field conditions or asynchronous to plant uptake.
  • High levels of PMN from the O-horizon in the lab (Figure 2) and microbial biomass (Figure 3) suggest substrate is not limiting microbial activity, but saturated conditions in the field limit mineralization of OM4.
Acknowledgements
This project is funded by USDA-NIFA AFRI Competitive Grants Program. 
We would also like to thank our research team: Dr. Urszula Norton, Dr. Joe Brummer, Dr. Shannon Albeke, Dr. Mike Zhu, Dr. Linda Van Diepen, Brian Sebade, and Rael Otuya.
References
  1. Sketch, M., Dayer , A. A., & Metcalf, A. L. (2020). Western Ranchers’ Perspectives on Enablers and Constraints to Flood Irrigation. Rangeland Ecology and Management , 73 (2), 285 296.
  2. Siemer , E. (1979). Mountain Meadow Management Problems and Research Needs. Proceedings from Management of Intermountain Meadows Meeting, Jackson WY , 1 (1), 176 211.
  3. 4. USDA NRCS. (2013). ‘Garrison’ Creeping Foxtail Alopecurus arundinaceus ). Bridger PMC, Bridger MT.
  4. Cassman , KG, & Munns , DN. (1980). Nitrogen mineralization as affected by soil moisture, temperature, and depth. SSSAJ, 44(6),
    1233-1237.

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