Recipient Organization
TEXAS A&M UNIVERSITY
750 AGRONOMY RD STE 2701
COLLEGE STATION,TX 77843-0001
Performing Department
Ecosystem Science & Management
Non Technical Summary
Woody plant encroachment into grass-dominated ecosystems has been a globally significant land-cover change during the last century, and has been documented in grasslands and savannas of North America, South America, Africa, Europe, and Asia. This dramatic land cover change appears to be driven largely by livestock grazing and the suppresion of natural fire regimes. Both woody encroachment and livestock grazing have strong potential to influence the storage of key soil nutrients, including carbon, nitrogen, and phosphorus. In addition, grazing and woody encroachment may also modify the biodiversity and functional attributes of the bacterial and fungal communities present in the soil environment. Thus, woody encroachment and livestock grazing may interact to modify nutrient cycles, the availability of limiting nutrients, and the structure and function of soil microbial communities at scales ranging from the ecosystem to the globe, and potentially influence the sustainability of livestock production systems. To better understand how woody encroachment and livestock grazing may interact to modify soil nutrient storage and cycling, we will quantify the magnitude of changes in soil carbon, nitrogen, and phosphorus storage in soils following woody encroachment in both grazed and ungrazed ecosystems. In these same study areas, we will also examine the impact of woody encroachment, grazing, and their interaction on the structure and function of soil bacterial and fungal communities. Results of this study will afford a unique opportunity to explore potential interactions between grazing and woody plant encroachment on belowground portions of the ecosystem that are critical for soil nutrient storage and availability. At present, these interactions remain largely unexplored and limit our understanding of the structure and function of grasslands, savannas, and other dryland ecosystem types. Results should also contribute to the development of more science-based methodologies for managing livestock grazing in these ecosystems that will help ensure the long-term sustainability of ecosystem services provided by these ecosystems.
Animal Health Component
10%
Research Effort Categories
Basic
85%
Applied
10%
Developmental
5%
Goals / Objectives
Arid and semiarid ecosystems such as deserts, grasslands, savannas, and dry woodlands cover approximately 40% of the Earth's surface, conduct 41% of terrestrial net primary productivity, and store 37% of soil organic carbon. Drylands also play a key role in the provision of ecosystem services, including the maintenance of biodiversity, hydrologic processes, biogeochemical functions, and the climate system. These ecosystems are also sensitive to natural and anthropogenic forcing factors such as fire, herbivory, and drought, causing them to be structurally and functionally dynamic. Woody plant encroachment into these ecosystems is a globally extensive land cover change that has been occurring during the past 150 yrs., and rates of increase in woody cover range from 0.1 to 2.3% yr-1 in grasslands and savannas throughout the world. This important vegetation change is likely driven by several potentially interacting local and global phenomena, including reduced fire frequencies, chronic livestock grazing, rising atmospheric CO2 concentrations, and climate change. The purpose of this research is to evaluate the long-term impacts of woody plant encroachment, herbivory, and their interaction on the biogeochemistry of soil C, N, and P, and on soil microbial community structure and function in juniper-oak savannas of the southern Great Plains. We will test the hypotheses that: (1) Long-term livestock grazing will reduce stores of soil C, N, and P; (2) Portions of the landscape dominated by woody plants will have larger pool sizes of soil organic C, total N, and total P than grass-dominated areas; and (3) Plant community composition and herbivory will interact to influence the structure, function, and size of the soil microbial compartment. This proposed research will be part of the Savanna Long-Term Research and Education Initiative in the Department of Ecosystem Science and Management at Texas A&M University. This program is aimed at understanding ecosystem function, dynamics, and processes within the grasslands and savannas of central Texas. This large interdisciplinary effort should generate new and significant ecological insights regarding the structure and function of savanna landscapes that will help landowners and land managers to develop proactive management practices that will help ensure the sustainability of ecosystem services in this region.
Project Methods
Research will be conducted at the Texas A&M AgriLife Sonora Research Station, a 1400 ha facility located 35 km S of Sonora, TX, USA, near the western edge of the Edwards Plateau. Climate is dry-subhumid with a mean annual temperature of 17.9° C, and mean annual precipitation of 586 mm. Soils within the areas sampled in this study are silty clays or stony clays of the Tarrant series (clayey-skeletal, smectitic, thermic Lithic Calciustolls) which lie atop indurated limestone bedrock from the Lower Cretaceous (USDA/NRCS 2015). Soils are generally shallow (<15 cm), although deeper patches may develop where the limestone is fractured. Current vegetation in the study area consists of savannas and woodlands. Over the past century, J. ashei appears to have increased markedly in grasslands and open parklands throughout the Edwards Plateau region. Analyses of recent aerial photographs (1985, 1996, 2004, and 2014) of the Sonora Research Station indicate that woody plant cover has increased from 25% in 1985 to 33% in 2014. The area that now comprises the Sonora Research Station was grazed heavily by livestock (cattle, sheep, and goats) at a stocking rate of 2 ha/animal unit/yr (1 animal unit = 12 kg per day dry biomass requirement) from the time of European settlement around 1880 until 1948. At that time, several long term grazing treatments and grazing exclosures were established and remained in place at the time of this study (~68 years). To test our hypotheses, we will sample three portions of the Sonora research site with contrasting grazing histories: (1) a control site protected from livestock grazing, (2) a moderately grazed site, and (3) a heavily grazed site. The control site (11 ha) has been protected from grazing by domestic herbivores since 1948, but has remained accessible to wild herbivore species, mainly white-tailed deer (Odocoileus virginianus) and axis deer (Axis axis). The moderately grazed site (24 ha) was part of a 4-pasture, 3-herd rotational grazing system that has been grazed by various combinations of cattle, sheep, and goats since 1948 at stocking rates of 6-8 ha/animal unit/yr. The heavily grazed site (7 ha) has been grazed continuously at stocking rates of 2-5 ha/animal unit/yr. The ratio of cattle:sheep:goats in the grazed areas has been approximately 60:20:20 since 1948, although cattle (but not sheep and goats) have been excluded from the moderately grazed site for the past 10 years. Neither the controls nor the grazing treatments have burned since the treatments were initiated in 1948. Between the 1880's and 1948, fires were unlikely due to high animal stocking rates that kept plant fuel loads at a minimum. Within each grazing treatment, 15 sites will be selected for plant, litter, and soil sampling. These sites consist of portions of the landscape dominated by: (a) open grasslands (n = 5), (b) juniper woodlands (n = 5), and (c) oak mottes (n = 5). At each sampling site, three surface litter samples (each taken within separate 0.1 m2 quadrats) will be collected within an area of approximately 4 m2 and then pooled and mixed to create one litter sample per site. Within each of the same quadrats where litter samples will be collected, soil samples will be collected to a depth of 10 cm, pooled and mixed to create a single sample, frozen within 5 hours of collection, and stored for subsequent analyses. Due to the presence of limestone parent material at or near the surface, it is challenging to collect soil from depths >10 cm. However, we will use a backhoe to excavate some trenches to deeper depths in order to access pockets of deeper soil that might not otherwise be accessible using a soil core or shovel. Litter samples will be cleaned gently with distilled water, dried, weighed to compute litter mass, pulverized, and weighed into tin capsules for elemental analyses. Air-dried soil samples will be passed through a 2-mm sieve to remove coarse organic fragments and rock fragments. An aliquot of sieved soil will be used to determine soil texture by the hydrometer method. Soil pH will be determined on a 1:2 (soil: 0.01M CaCl2) mixture using a using an Accumet Basic pH meter. An aliquot of sieved soil will be dried at 60° C for 48 h and pulverized in a centrifugal mill. For each pulverized soil sample, one aliquot will be weighed into a tin combustion capsule (5 x 7 mm) for analysis of total N. A separate aliquot of each soil sample will be weighed into a silver capsule (5 x 7 mm), treated with HCl vapor in a desiccator to remove carbonates for 8 h, dried, and analyzed for organic C concentration. Litter and soil samples will be analyzed for concentrations of organic C and total N using a Costech ECS 4010 elemental analyzer. Total P concentrations in litter and soils will be determined by lithium metaborate fusion coupled with the molybdenum blue colorimetry method. The concentration of P will be measured on a Spectronic 20D+ spectrophotometer and referenced with a standard curve of potassium phosphate solutions (KH2PO4 at 0, 0.125, 0.25, 0.375, 0.5, and 0.625 μg P/mL) verified against NIST SRM 2709a (San Joaquin soil). Carbon, N, and P stocks in litter will be computed as the product of litter dry mass (g m-2) and concentrations of C, N, and P (g g-1) in that litter, respectively. An aliquot of each soil sample will be extracted to obtain soil DNA for community analysis using MoBio Power Soil DNA extraction kits. The DNA will be subjected to a phylogenetic survey by analyzing the 16S ribosomal subunit to characterize prokaryotes using primers 515F (5'-GTGCCAGCMGCCGCGGTAA-3') and 806R (5'-GGACTACHVGGGTWTCTAAT-3'), and the ITS regions using primers ITS1 (5′-TCCGTAGGTGAACCTGCGG-3') and ITS2 (5'-GCTGCGTTCTTCATCGATGC-3') to characterize the fungi. Following library preparation, synthesis based sequencing will be performed using an Illumina MiSeq platform according to the manufacturer's protocol which will yield paired-end reads of 367 ± 39 bp for 16S and 423 ± 64 for ITS. Raw illumine reads will be deposited in NCBI's sequence read archive. Reads will be analyzed using the QIIME 1.9.1 pipeline. Resulting 16S and ITS OTU tables will be used to make predictions for bacterial metabolism using PICRUSt and to classify fungi into ecological guilds using FUNGuild. Each response variable will be analyzed by two-way ANOVA to assess the effects of grazing treatment (control, moderate, heavy), vegetation type (oak, juniper, grassland), and their interactions. When differences are significant, the Tukey-Kramer method will be utilized to assess posthoc contrasts with significance inferred at p ≤ 0.05. R Non-metric multidimensional scaling (NMDS) based on normalized Bray-Curtis distance matrices will be used to evaluate statistical community structure for bacteria and fungi. Differences in microbial community functions among treatments will be evaluated using response ratios.