COS 93-3 - Influence of legume inclusion on greenhouse gas fluxes from pasture systems in the Southern High Plains of Texas

Abstract: Semi-arid grassland ecosystems such as the Southern High Plains (SHP) of Texas hold enormous potential for providing a variety of ecosystem services that support both agricultural and natural ecosystem productivity in the face of water and nutrient scarcity. Although these systems cover about thirty percent of the arable land area in North America, agricultural productivity is severely limited by water scarcity and degraded soils. The main source of irrigation from the Ogallala Aquifer is increasingly depleted. To sustain their productivity, many growers in this region are transitioning from row crops to less soil-disturbing and more water-efficient perennial forages integrated with livestock. A significant trade-off to this conversion is the higher climate footprint of these systems due to release of enteric methane (CH₄) from cattle grazing. However, some studies have shown that incorporating legumes to these systems can potentially reduce soil CH₄ emissions in addition to minimizing the need for external inputs like nitrogen (N) fertilizer. The objective of this study was to quantify how legume (Alfalfa, Medicago sativa L.) presence and density affect soil greenhouse gas fluxes in established long-term pastures (WW-B.Dahl Old World bluestem) as compared to N-fertilized monoculture pastures. Soil greenhouse gas (GHG) samples (CO₂, CH₄, N₂O) were collected using static chamber methods from the field on a bi-weekly basis during the growing season, and on monthly basis for rest of the year. Soil samples were collected adjacent to each gas flux chamber and analysed for extracellular enzyme activities, microbial community size and structure (EL-FAMEs), and inorganic nitrogen in soil. The grass-legume pastures released less N₂O and consumed more CH₄ in the soil than the grass monoculture systems. Because CH₄ and NH₄⁺ are competing substrates for CH₄-oxidizing monooxygenase enzymes in soil, higher availability of inorganic nitrogen in soil likely contributed to less CH₄ consumption and more N₂O emission from soils. EL-FAME analysis showed a higher soil microbial biomass in the grass-legume systems as compared to grass only systems which could be due to more labile sources of soil organic carbon (C), lower C:N ratio of plant residues, and more diverse substrates for the microbes. Additional ongoing soil analyses and qPCR for methanogenesis and methane oxidation genes, and 16s rRNA sequencing will further provide a better understanding of the above- & belowground components interactively regulate the climate footprint of these systems.