Current Research

Woody plant encroachment into grasslands: Impacts on landscape-scale 3-dimensional spatial patterns of soil C, N, and P storage.
Stable isotope partnership for ecology, environment, and energy research
Hydrologic influences on soil organic carbon loss monitoring using stable isotopes.
Identifying sources of N2O production in agroecosystems: A step towards more effective mitigation
Analysis of dietary overlap among cattle, deer, and nilgai using stable isotopes.


http://www.nsf.gov/images/logos/nsf1.gifWoody plant encroachment into grasslands:  Impacts on landscape-scale 3-dimensional spatial patterns of soil C, N, and P storage and dynamics.

2016-2018.  NSF Ecosystem Studies Program, Dissertation Research, DEB-1600790  (T. Boutton and Y. Zhou).

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Modification of soil nutrient pool sizes following woody plant proliferation has long been of interest to grassland, savanna, and desert ecologists. However, most previous studies examining nutrient dynamics following woody encroachment have been confined to small spatial scales, limited to the uppermost portions of the soil profile, and focused primarily on C and/or N.  This research will quantify soil organic C, total N, and total P throughout the entire soil profile in order to make the first assessment of landscape-scale C:N:P soil stoichiometry following woody plant encroachment into grassland. Specific objectives are: (1) Examine whether vegetation cover change alters the 3-dimensional spatial patterns of soil C, N, and P storage at the landscape scale; and (2) Test whether soil P scales isometrically with respect to C and N, and whether these isometric patterns change with soil depth in N-fixer encroached systems. Nutrient stores will be quantified in spatially-specific soil cores taken to a depth of 1.2 meters in a subtropical savanna landscape in southern Texas where N-fixing woody plants have encroached into grasslands during the past century.  Results will offer new perceptions on the effects of woody encroachment on interactions between C, N, and P cycles in arid and semi-arid ecosystems across the globe, and enhance our ability to represent these interactions in linked biogeochemistry-climate models.


Box_stackStable isotope partnership for ecology, environment, and energy research

2016-2018.  Texas A&M University Research Development Fund  (E. Grossman, B. Roark, T. Boutton, N. Slowey, J. West, A. Hyodo)

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This project funds the acquisition of two major instruments: a gas chromatograph combustion-isotope ratio mass spectrometer system (GC-C-MS-IRMS), and a clumped isotope mass spectrometer  (CIMS). These instruments provide unique analytical capabilities at the forefront of ecology and the biogeosciences.  These capabilities will be developed jointly through the partnership of the Stable Isotopes for Biosphere Science (SIBS) Laboratory in the College of Agriculture and Life Sciences, and the Stable Isotope Geosciences Facility (SIGF) in the College of Geosciences. The GC-C-MS-IRMS instrument enables simultaneous separation and identification of specific biochemical compounds using gas chromatography/quadrupole mass spectrometry, followed by determination of their isotopic composition (15N, 13C, 2H) through a combustion or pyrolysis interface to an IRMS.  This system will provide cutting-edge methods to track sources and fates of specific biochemical compounds (e.g., alkanes, lipids, amino acids) through the biosphere, geosphere, hydrosphere, and atmosphere, expanding  interpretations of bulk isotopic measurement. This instrument will be housed in the SIBS Laboratory.  The clumped isotope mass spectrometer (CIMS) measures the concentration of moleDSCN0280cules with two rare isotopes, for example, 13C18O16O/12C16O2. This ratio in carbonate rocks and minerals is temperature dependent, providing a geothermometer for the pore-filling carbonate cements that occlude porosity and permeability in carbonate petroleum reservoirs. Moreover, with burial of carbonate rocks, these clumped isotope signatures reset and provide burial temperatures essential for petroleum exploration.  Clumped isotopes are also an emerging methodology in paleoclimate studies in soil science, geology, and oceanography. In addition, this instrumentation would also provide triple oxygen isotope analysis (18O/16O, 17O/16O), an emerging tracer for hydrologic and atmospheric research.  The CIMS will be housed at SIGF.  Together, these two new instruments will catalyze new research ventures in the biogeosciences.


HyUSDA logodrologic influences on soil organic carbon loss monitoring using stable isotopes.

2012-2016. USDA-NIFA-CBG Program, 2011-38821-30970 (I. Ahmed, T.W. Boutton, K.B. Strom).

research_clip_image004_0003The thematic focus of this USDA-NIFA-CBG collaborative applied research program is sustainable integrated monitoring of soil organic carbon (SOC) and total nitrogen (TN) loss from the natural, agricultural, and urban ecosystems that constitute the Buffalo Bayou watershed on the west side of Houston, TX.  To accomplish this, we will use state-of-the-art stable isotope tracer methods under uncertain hydrologic influences.  In this study, SOC and TN loss and water runoff will be monitored at the watershed scale using natural rainfall events.  We will measure total organic carbon/nitrogen concentrations (TOC/TN) and their stable isotopic composition (δ13C, δ15N) in soils and streamflow in different land cover/land-use types within the largely urban Buffalo Bayou watershed, which is notably affected by soil erosion and deposition.  This proposal emphasizes an interdisciplinary research framework to uncover practical yet scientifically sound methods to monitor SOC/TN fluxes in watersheds.  Our research team includes expertise in hydrology, biogeochemistry, stable isotope chemistry, and modeling.  Specific objectives are to: (1) Create statistical models to assess the researchrelationship between rainfall-runoff and SOC/TN release during soil erosion in space and time; (2) Capture the episodic nature of rainfall events and their role in the spatial distribution of SOC/TN loss from erosion; (3) Employ stable isotope fingerprinting (source and quantity) of SOC/TN to examine how various types of erosion processes common in a heterogeneous watershed affect SOC/TN loss from different land cover/land-use categories; (4) Create an integrated watershed-scale statistical soil loss monitoring model driven by uncertainty in hydrologic inputs, and spatial and temporal correlation of flow and stable isotope composition; and (5) Create an integrated decision support system (DSS) for sustainable management of SOC/TN under hydrologic uncertainty to assist end users with maintaining ecosystem services and environmental quality in the natural, agricultural, and urban ecosystems that comprise our watersheds.


agrilifeIdentifying sources of N2O production in agroecosystems:  A step towards more effective mitigation.

                                        2013-2016.  Texas A&M AgriLife Air Quality Research Program (J. West, T.W. Boutton, F.M. Hons, J. Sparks).

DSCN0267Nitrous oxide (N2O) is among the most powerful greenhouse gases, with a heat-holding capacity 300x greater than CO2 on a per molecule basis, and an atmospheric residence time >100 yrs.  Its concentration is now increasing exponentially due to human industrial and agricultural activities. In agricultural lands, such as croplands and rangelands, N2O is produced by soil microorganisms during the processes of nitrification and denitrification.  Because the conditions and management activities that favor nitrification versus denitrification are significantly different and quite complex, it is necessary to be able to quantify and distinguish these two N2O sources in order to develop more effective strategies to mitigate the production rates associated with each source. Although croplands and rangelands strongly dominate the land cover of Texas, our understanding of both the identity and rates of the soil processes that produce N2O in these systems remains limited.  We will employ a powerful new “isotopomer” methodology that allows the separation of specific soil microbial processes (nitrification vs. denitrification) responsible for N2O production in two important agricultural systems in Texas. This unique approach relies on the fact that N2O produced by nitrification has a significantly different intramolecular arrangement of natural N and O isotopes compared to N2O derived from denitrification. The N2O molecule has a linear structure of the form Nβ=Nα=O which also equilibrates with Nβ≡Nα−O.  The basis for the proposed research rests on the fact that microbially-mediated reactions involved in denitrification result in 15N enrichment at the central Nα relative to the terminal Nβ, while reactions during nitrification result in smaller 15N discrimination between the central Nα and the terminal Nβ. Thus, by determining the intramolecular isotopic composition of N2O, the relative contributions of nitrification vs. denitrification to the soil N2O flux can be fingerprinted and quantified. We will integrate a PreCon/GasBench system with one of our isotope ratio mass spectrometers, develop sample preparation protocols, and then produce initial assessments of N2O sources in: (a) a long-term (> 30 year) agricultural experiment studying crop rotation, tillage and fertilizer effects on soil processes, and (b) a rangeland ecosystem with well-described changes in N cycling driven by invasion of N-fixing tree legumes.


East Wildlife foundationAnalysis of dietary overlap among cattle, deer, and nilgai using stable isotopes.

2013-2016.  East Wildlife Foundation (T.E. Fulbright, A. Ortega-Santos, D.G. Hewitt, T.W. Boutton,  S.Hines, K. Gann, R. DeYoung).

Understanding interactions between wildlife, livestock, and their environment in rangeland ecosystems is essential fordeveloping sustainable conservation and management practices.  One of the most important deermechanisms by which these herbivores might interact is through overlap in their choices of food plants.  The purpose of this study is to estimate seasonal food habits of cattle, white-tailed deer, and nilgai in order to evaluate dietary niche overlap between these native and introduced herbivore species which often coexist in the grasslands and savannas of southern Texas.  Stable carbon isotope analyses of animal dung and body tissues will be utilized to estimate the relative proportions of C3 plants (woody plants and forbs) vs. C4 (grasses) and CAM plants (cacti) in the diets of these herbivores.  Further clarification of animal food habits will be sought in the δ15N,  δ18O, and  δ2H values of plant and animal tissues.  Research will be conducted on the Santa Rosa, El Saus, and Buena Vista Ranches in southern Texas.  cattleTo characterize the natural isotopic composition of potential food items for these herbivores, plant tissue samples will be collected seasonally from the dominant grasses, forbs, cacti, and shrubs that cover the study areas.  Simultaneously, dung, blood, and hair samples will be collected from coexisting cattle, deer, and nilgai.  Both plant and animal tissues will be subjected to δ13C, δ15N, δ18O, and  δ2H analyses  in the Stable Isotopes for Biosphere Science Laboratory at Texas A&M University.  Animal diets will be derived from the isotopic composition of their tissues using Stable Isotope Sourcing Using Sampling (SISUS), a Bayesian model for source partitioning using stable isotopes.  Theresults of this study will:  (a) reveal the extent to which cattle, deer, and nilgai overlap in their dietary preferences in space and time; (b) provide fundamental biological information about these herbivores that should enhance our ability to manage them in an ecologically and economically sustainable manner;and (c) enhance our ability toutilize stable isotopes for quantifying the food habits of large herbivores.

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