Soil microbial community size and composition changes along a tanglehead (Heteropogon contortus) gradient in a semiarid region

Authors: GRACE, JOSHUA - Texas A&M University; Acosta-Martinez, Veronica; RIDEOUT-HAZAK, SANDRA - Texas A&M University; STANKO, RANDY - Texas A&M University; ORTEGA, ALFONSO - Texas A&M University; WESTER, DAVID - Texas A&M University

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2019
Publication Date: 2/28/2019
Citation: Grace, J., Acosta Martinez, V., Rideout-Hazak, S., Stanko, R., Ortega, A., Wester, D. 2019. Soil microbial community size and composition changes along a tanglehead (Heteropogon contortus) gradient in a semiarid region. Applied Soil Ecology. 138:37-46.

Interpretive Summary: Invasive plants are a serious concern in many rangelands in North America because they can out compete more desirable range species for soil water, thus leading to degradation of the range for ruminant production. Little is known, however, on how invasive plants affect soil microbial associations, which are so important in the overall ecosystem functioning including soil and water conservation. Tanglehead is an invasive grass that is rapidly increasing throughout the Coastal Sand Plain in southern Texas. Scientists from Texas A&M in Kingsville, and USDA-ARS in Lubbock, Texas examined the response of soil microbial community size and structure over a 3-year period (2013-2015) along a tanglehead invasion gradient represented by sites with native plant communities (no tanglehead present), tanglehead-native mixtures, and tanglehead-dominated plant communities. Soil microbial communities were higher in sites with native vegetation, however, they had lower saprophytic fungi:bacteria ratios than invaded sites. Overall, invaded species of tanglehead change soil microbial communities compared to soils with native vegetation, but at what point these communities change during invasion is not clear, and how these changes relate competition among plant species for soil water is unclear.

Technical Abstract: Invasive plants are a serious concern throughout many rangelands in North America because of their detrimental effects on vegetation community composition, diversity and wildlife habitat. Growing evidence shows that invasive plants affect ecosystem processes such as energy and nutrient cycling and soil microbial associations. Tanglehead (Heteropogon contortus [L.] P. Beauv. ex Roem and Schult.) is a C4, perennial caespitose grass that is rapidly increasing throughout the Coastal Sand Plain in southern Texas. In a field study, we examined the response of soil microbial community size and structure over a 3-year period (2013–2015) along a tanglehead invasion gradient represented by the natives-without-tanglehead plant communities (< 15% tanglehead), natives- with-tanglehead mixtures (15–70% tanglehead), and tanglehead-dominated (> 70% tanglehead) plant communities. Soil microbial size [microbial biomass carbon (MBC), nitrogen (MBN), and total fatty acid methyl esters (FAME)] differed between natives-without-tanglehead and tanglehead-dominated sites in 2013 and 2015 (dry years). MBC averaged ~200 mg kg-1 soil in natives-without-tanglehead sites but<70 mg kg-1 soil in invaded sites, and total FAME averaged 36 and 41 nmol g-1 soil in native-without-tanglehead and invaded sites, respectively, in 2013 and 44 and 55 nmol g-1 in 2015. Invaded sites had higher saprophytic fungi:bacteria ratios than sites with<15% tanglehead (0.64 vs. 0.53) over all 3 years. We conducted a separate inoculation experiment where tanglehead was planted in native soils that lacked tanglehead to investigate effects of tanglehead inoculation on soil microbial community dynamics; introduction of tanglehead to native soils did not change microbial community structure, size, or relative abundance of key microbial FAME indicators over the course of 18 months. Field study results revealed different soil microbial communities associated with tanglehead, but at what point these communities change during invasion is not clear, and how these effects are related to overall habitat degradation associated with this invasive plant requires further study.