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Building Legacy Together: Our Communities’ Journey of Strength and Resilience

With support from USDA’s National Institute of Food and Agriculture, Land-grant Universities in the Pacific Islands carry out innovative education, applied research and related community development programs that create stronger, more resilient Pacific Islander communities. Learn about the innovative research being conducted at the University of Guam.

This information originally was published in the 2021 Impact Report for the Western Pacific Tropical Research Center, College of Natural and Applied Sciences, University of Guam. It is reprinted here with permission.

Agroforestry Management System for Protecting Corals

Soil sedimentation causes serious damage to corals on southern Guam’s fringing reefs. In watersheds in the red clay hills of the southern villages, large amounts of sediment move downstream into the ocean with plumes of sediment extending out quite a distance from shore during heavy rain events. Soil scientists have been experimenting using novel agroforestry techniques to slow down the runoff and mitigate erosion and sedimentation from agricultural practices and other sources.

Graduate student Daniel Encio plants vetiver hedgerows as one type of six different plot treatments to  study erosion and sedimentation. Credit: Western Pacific Tropical Research Center
Graduate student Daniel Encio plants vetiver hedgerows as one type of six different plot
treatments to study erosion and sedimentation. Credit: Western Pacific Tropical Research Center

Agroforestry Management System for Protecting Corals

Using several types of plantings in sloping conditions at the Ija Research & Education Center, experimental plots were designed so that runoff would be funneled through weirs at the end of the plots and collected in storage tanks. Sediment levels could then be measured for each plot treatment, which included three vetiver grass treatments with different numbers of hedgerows and agroforestry plots with plantings of two tree species, noni and eggfruit. Additional plots were striped of vegetation to resemble the badlands in the area or not manipulated at all allowing vegetation that grows naturally in the surrounding savanna.

The results showed that vetiver grass is highly effective in controlling soil runoff, which suggests that farmers in southern Guam may wish to plant borders of vetiver grass along the slope-side of their plots. Farmers can also utilize sloping lands to grow crops in between rows of vetiver and the rows of trees. Farmers who wish to grow on lands with sloping contours can safely plant their crops using vetiver and agroforestry techniques and know that a heavy rain will not fill the rivers, streams and bays with sediment.

Symptoms of southern bacterial wilt in a tomato plant caused by Ralstonia solanacearum. Credit: Western Pacific Tropical Research Center
Symptoms of southern bacterial wilt in a tomato plant caused by Ralstonia solanacearum. Credit: Western Pacific Tropical Research Center

In Search of Bacterial Wilt

An extensive search on Guam for one of the most severe soil-borne plant bacteria complex is underway. Ralstonia solanacearum species complex (RSSC) comprise a group of plant pathogenic bacteria that are found in the soil and infect plants through root wounds such as those caused by cultivation or pests. Once inside the plant host, RSSC has an affinity for the vascular system where it multiplies rapidly, filling the xylem with bacterial cells and slime. This leads to the wilting of the host and ultimately to its death.

Though known to be present on Guam since the 1970s and occasionally impacting tomato and eggplant production, its presence was never considered a serious threat to any of Guam’s plants until 2011, when it was discovered in Casuarina equisetifolia, locally known as gågu in Chamoru and in English as ironwood.

With the need to understand the full impact of bacterial wilt on Guam, NIFA’s Hatch program funded a two-year project searching for bacterial wilt among Guam’s agricultural, landscape and forest plant species. Scientists began testing plants for RSSC using a rapid Ralstonia solanacearum specific immunostrip tests. By September 2021, they had performed 382 tests of 78 different plant species on Guam, including native and ornamental species. Of these 78 species, two have been tentatively identified as new RSSC host reports for Guam: Dracaena marginata L. (dragontree) and Colocasia esculenta L. (taro).

Aerial capture of biochar field trials on porous limestone soils at the Yigo Research & Education Center in northern Guam. Credit: Western Pacific Tropical Research Center
Aerial capture of biochar field trials on porous limestone soils at the Yigo Research & Education Center
in northern Guam. Credit: Western Pacific Tropical Research Center

Biochar: The Benefits

The pre-contact indigenous people of the Amazon are credited with being one of the first to use charred organic substances to enrich the nutrient-lacking soil common in the tropics. Today, researchers are investigating the use of biochar as a soil amendment and as a strategy for sequestering carbon.

Carbon-rich biochar is derived from plants or other organic materials that have been burned under the complete absence (pyrolysis) or partial absence (gasification) of oxygen. Soil scientists have been conducting field experiments with biochar as a soil amendment to study the efficacy of the use of biochar to sequester carbon and as an alternative to inorganic fertilizer for crops.

Researchers conducted field trials on the porous limestone soils of northern Guam at the Yigo Research and Education Center. The study plots included several treatments, including compost, biochar, biochar/compost mix and synthetic fertilizers.

Scientists performed crop yield, soil microbial activities and soil nutrient analyses, and monitored for the presence of crop diseases throughout the planting season. The Yigo plots were then sent North Carolina and analyzed by USDA for the presence of microorganisms. Soil microorganisms such as bacteria and fungi are important in facilitating nutrient availability to plants. Carbon dioxide emissions were measured for each of the plot treatments using a CO2 capturing device.

Researchers found that the application of biochar not only improved soil quality, it increased the carbon content of the soil due to its ability to sequester carbon. This indicates biochar can be successfully used to keep carbon in the soil, thereby reducing the amount of carbon emissions caused by soil disturbances from industrial farming and deforestation. Additionally, crop yields from the open-pollinated sweet corn variety grown in biochar plots increased by 40% compared to control plots.

Native sea grapes (Caulpera racemose) are considered a potential candidate for polyculture with shrimp or other marine species. Credit: Western Pacific Tropical Research Center
Native sea grapes (Caulpera racemose) are considered a potential candidate for polyculture with shrimp or other marine species. Credit: Western Pacific Tropical Research Center

Decoupled Aquaponics Systems and Polyculture Possibilities of Sea Grapes and Shrimp

Aquaculture is one of the fastest growing food sectors in the world, but the U.S. aquaculture sector has experienced slower growth at 1% annually. The U.S. seafood market is dominated by imports, which is comparable to Guam, where most food is imported, with only 10% of food consumed on the island produced locally. The complexity of the regulatory environment, foreign competition, social licensing issues, lack of investment, market uncertainty, diseases and low-profit margins are some of the factors behind limited growth in U.S. aquaculture.

Guam’s aquaculture remains small-scale with many risks from production to market. Therefore, it is necessary for the island to develop and promote local aquaculture by addressing the challenges in order to limit the effects of food insecurity and limited food options on families and communities. Scientists are working to evaluate and mitigate risks related to production, disease, health management and markets.

Infectious disease outbreaks are the most catastrophic threats to the aquaculture industry. Health management via the establishment of high health stock, and implementation of systematic prevention and control of diseases are fundamental to sustainable aquaculture development. For 14 years, scientists have complied with effective biosecurity measures and maintained health management standards to produce high health status stock at the Guam Aquaculture Development and Training Center.

Strict health surveillance and monitoring regimes have been actively in place for the facility and its operations. Both shrimp and prawn stocks remained specific pathogen-free (SPF) from the many viruses that plague the industry. By far, this is a much more comprehensive SPF list than the World Organization for Animal Health list, which includes all significant pathogens, both known and emerging.

Schematic implementation for a decoupled aquaponic system. Credit: Western Pacific Tropical Research Center
Schematic implementation for a decoupled aquaponic system. Credit: Western Pacific Tropical Research Center

As for shrimp and prawn selective breeding efforts, scientists continuously select for fast growth strains in Guam’s environment, work to maintain genetic diversity and minimize inbreeding of the existing stock population of giant freshwater prawn and whiteleg shrimp, both of which are commercially important species worldwide. Experiments are underway to evaluate the co-culturing of these species for the best combination in terms of production performance. Native sea grapes also are on the list to be considered as a potential candidate for polyculture with shrimp or other marine species.

Researchers have initiated a multistate project to investigate ways to expand and diversify U.S. aquaculture production by combining the analyses on consumers’ preferences and production challenges and opportunities. They are evaluating production and costs for a commercial decoupled aquaponics system. Traditional aquaponics systems were arranged in a single-process loop that directs nutrient-rich water from fish to plants and back. Given the differing specific plant and fish nutrients and environmental requirements, such systems presented a compromise to the ideal conditions for rearing of both, thus reducing the efficiency and productivity of the systems. More recent designs that allow for decoupling of units provide a more finely tuned regulation of the processed water in each of the respective units while also allowing for better recycling of nutrients from sludge. A decoupled aquaponic system has the potential to be a nearly zero-discharge system.

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