Progress report for OS24-175
Project Information
The Virgin Islands faces a complex challenge with the annual influx of Sargassum, necessitating a comprehensive and research-driven solution. The proposed remedy centers on initiating a grant-funded on-farm research project, fostering a direct collaboration between a University of the Virgin Islands researcher (David Hensley, St. Croix) and a local farmer (Laura Martin, St. Thomas). The primary objective is to address uncertainties and risks linked to the use of beach-cast Sargassum in agriculture, with a primary focus on arsenic contamination, composting dynamics, soil type variations, and the promotion of sustainable crop production.
This research endeavor will begin with an analysis of beach-cast Sargassum samples to quantify As levels. This initial step is imperative for establishing a baseline understanding of the As content in Sargassum, though it is expected to comport with recent literature from the Caribbean region and unpublished data collected by University of the Virgin Islands agricultural scientists in 2018 (approximately 50 ppm; Hensley, unpublished data).
In order to gather information about the effect of Sargassum application to the varying soil types of the Virgin Islands, the volcanic-soil hillside farms of St. Thomas will be contrasted with the calcareous, flat plains soils of St. Croix for a composting study. For this, the participation of St. Thomas farmers is crucial, because farms are the likely site of any agricultural composting effort (no centralized or commercial effort to compost Sargassum has so far been discussed in the territory). Pre- and post-soil sampling of As and Na content at depths will permit a characterization of the annual effect of composting unlined piles of Sargassum on the range of soil types of the Virgin Islands to quantify the effects of such a practice on soil health. Though this would primarily benefit farmers to understand the possible effects on soil salinization and As contamination associated with on-site composting, the results would be informative for government authorities who may be considering initiating large-scale dump sites for collected Sargassum from beaches, particularly from valuable tourist beaches.
In tandem with soil analyses, the research proposal extends to the examination of compost derived entirely or primarily from Sargassum, considering its interaction with different soil types, unlike past efforts in other Caribbean islands that consider Sargassum composting as a minor addition to primarily sugarcane bagasse or green-waste composting (Devault et al., 2020). This step is crucial in assessing whether composting effectively reduces As levels to acceptable levels as established by the US EPA or European Union, but also is an important piece of information to determine the assumed fraction of As leached into groundwater stores, since about 80% of As in the original Sargassum input not accounted for in the compost or soil fraction will be assumed to have been leached below the sampling region (since arsine gas volatilization is reported in As-rich composting to be about 20%; Cao et al., 2010). Follow up soil sampling after several months to a year of fallowing will also permit initial estimates on any potential of As build-up in soils if the composting process is repeated in the same location year after year.
Finally, to comprehensively address the potential risks associated with Sargassum use in agriculture, the research project includes studies on crop uptake of As, with a focus on promoting sustainable crop production. The choice of crops will be led by the farmers to reflect the current agricultural reality, while realizing that the choice of species plays a role in how much As is present in the harvested yield (Sahoo and Mukherjee, 2014). By cultivating crops in Sargassum-amended soil and monitoring As levels in the harvested produce, the project aims to quantify the extent of As transfer to food crops while establishing sustainable practices.
Additionally, the research project will address the salinity aspect by assessing the Na content in Sargassum and its potential impact on soil salinity, ensuring that sustainable soil management practices are incorporated into the proposed solution. One important outcome of studying these together could be to emphasize which of the two possible problems with Sargassum use, Na or As, is more immediate and should be emphasized in management.
Recognizing the limited capacity of individual farmers in the Virgin Islands for comprehensive testing, the proposed solution emphasizes a collaborative on-farm approach. The research project will actively engage local farmers, ensuring a direct and hands-on collaboration with a focus on improving the sustainability of crop production while retaining fidelity to actual, realistic agricultural practices in the current Virgin Islands context. This cooperative effort involves the farmers in all phases of the research, fostering knowledge exchange and ensuring that research findings promote sustainable choices for on-farm practices, and improving transparency on the topic.
The insights gained from the research will form the basis for the development of clear and practical guidelines for the safe and sustainable use of Sargassum in Virgin Islands agriculture. These guidelines will encompass recommended application rates, frequency, and best practices to minimize risks and maximize the potential benefits of Sargassum utilization while promoting sustainable crop production. The collaborative nature of the on-farm research ensures that these guidelines are not only scientifically sound but also reflective of the practical realities faced by local farmers, aligning with the broader goal of sustainable agricultural practices in the Virgin Islands.
SPRING 2025 UPDATE:
Project activities have proceeded as described above, with sargassum tissue samples collected in the first year, control soil, soil beneath compost pile, and sargassum compost, all submitted to lab for salinity and arsenic content analysis. The compost has been applied to a vegetable crop which will be harvested for arsenic tissue analysis sometime in Spring 2025. A second year of data collection along the same lines as year 1 will begin over the summer of 2025 to test for arsenic accumulation in the soil and interannual variability.
As described above, the project will comprise essentially three major objectives or phases to provide useful data at all stages of Sargassum application in Virgin Islands agriculture with respect to risks associated with arsenic and sodium: 1) the first objective is to characterize the total As and Na content of raw beach-cast sargassum; 2) the second is to characterize the fate over time of total As and Na in compost piles over an area and its effect on the root zone of the soil underneath (as well as assumed leachate) with varying application rates and on varying soil types; and 3) the third objective is to characterize the fate of total As and Na added to soil from sargassum soil amendment in the root zone, and in plant tissue, including harvested crops intended for human consumption. Project methods are based loosely on the As uptake study of Khan et al. (2010), who studied As uptake of wetland rice in Bangladesh.
Objectives 1 and 2 of the project will be carried out, at a minimum, at two sites, one on the island of St. Thomas (rocky, steep, volcanic-origin soils) and one on the island of St. Croix (sedimentary, gentle, marine/calcareous-origin soils). For the first objective, during the first year of the project (summer 2024), beach-cast Sargassum will be collected from the beaches of St. Thomas and St. Croix, to be tested for (at a minimum) As and Na. This is expected to align with published literature on the range of As content in pelagic Sargassum species S. natans and S. fluitans found on Caribbean beaches, but will also be necessary for characterizations in Objective 2, and will permit comparisons between island and between years of the project (sampling will also occur during the summertime Sargassum season in Year 2, 2025). For all samples (tissue, soil, and compost), samples will be collected and processed before being shipped to laboratories to perform chemical analysis of total As species and Na content.
For the second objective, compost bins will be constructed at both study locations (on-farm St. Thomas and St. Croix) where harvested Sargassum will be piled for aerobic composting. In keeping with typical practice, the Sargassum will be rinsed with freshwater before composting to remove excess surficial Na (this will also be done for samples sent for chemical analysis in Objective 1). Samples will be taken and dried to estimate the total dry weight of the entire pile of raw Sargassum. Prior to the deposition of Sargassum, soil samples at 3 depths will be collected at the location of the intended compost pile to characterize initial soil chemical conditions, porosity, particle size distribution, and so on.
Sargassum will be left, with periodic turning, to compost for the next 3-4 months, the typical time period required for aerobic composting in the region and in line with reported Sargassum composting in the literature (Devault et al., 2020). Finished compost will be sampled to estimate total finished product dry weight and for chemical analysis (total As species and Na content). After removal of compost from the pile location, soil will be sampled at 3 depths below the pile to determine chemical changes in terms of As and Na. A simple physical model of As distribution throughout this profile and the remaining As in the compost will be used to determine what fraction, if any, is assumed to have been leached beyond the sampling domain. Results can be compared across both island/soil sites, and at different rates of composting (i.e., depth of pile). This process will be repeated the following year (2025) in the same locations, with soil pre-sampling in the same locations to estimate the fate over time of soil As, and allowing an estimate of the potential for As accumulation in the soil. The same analysis will be carried out for Na to determine any soil salinization effect.
For the third objective, in Year 1, when finished Sargassum compost has been collected, it will be applied by St. Thomas farmers as a soil amendment with measured rates to annual crops of their choosing for the winter growing season. Soil will be sampled and chemically tested prior to amendment, and at the time of harvest, to detect change over time. Plant tissues (i.e., root, stem, leaf, and fruit) will be sampled and chemically analyzed for total As, which will include harvested yield. This will permit an As-balance similar to that in Objective 2 that allows for assumed leachate, and also provides a final estimate of any potential health risk posed by As content in harvested crops from one year of Sargassum compost use at the measured As content rates. These rates, both for compost and crop tissues, can be compared to safe limits for As established by public authorities. Though only crops selected by the farmers will be tested, this is considered to be beneficial since it will provide data on crops actually normally grown by farmers in the Virgin Islands, and will provide a needed starting point to determine the feasibility, if any, of safely using Sargassum compost in local sustainable agriculture.
Cooperators
- - Producer
Research
The methods primarily involve harvesting beach-cast sargassum (the most practical method for an individual farmer to access the sargassum). We first test the salinity and arsenic content of the raw sargassum to confirm its arsenic content is as expected for the species and in the Caribbean region. We then compost the sargassum in a compost bin, but first test soil samples from the site of the compost bin and a control location before applying any sargassum. After composting has completed, we re-test the soil beneath the pile and at the control site for arsenic content, and test the compost itself for arsenic. We apply the compost as a soil amendment and mulch to a vegetable crop chosen by the farmer, and test the arsenic content of the harvested crop after harvest for arsenic. After harvest, we test the soil where the crop was planted for arsenic. Finally, we repeat everything a second year to measure year-on-year effects.
Results are still in analysis phase - arsenic and salinity results have just come back from lab. But early results indicate that, as expected the compost material is high in arsenic, and some quantity of arsenic has been adsorbed to surface soil beneath the compost pile. Food crop arsenic uptake is not yet known, pending tissue harvest and analysis. Year-on-year additive effects not yet known - pending second year of composting in the same location.
Educational & Outreach Activities
Participation Summary:
Because very little existing data for the risk of As in the proposed agricultural application currently exists, and none exists for the Virgin Islands specifically, limited targeted outreach activity will take place in Year 1 while the initial data is still being collected, but the Year 2 Sargassum beach collection and composting effort will be the subject of at least one on-farm field day for local producers who can be shown the methods in use for Sargassum composting, and also received information about the initial results of soil testing and crop testing for As and Na. This will provide information to the farmers to understand the methods that were used to produce the results that have been shared with them, as well as provide hands-on context for whatever the final conclusions of the research project will be.
The summer after the project concludes (2026) will provide an opportunity for the results to be shared with the wider Caribbean agricultural community at the annual conference of the Caribbean Food Crops Society, held in July every year (this international travel to be supported by other funds). This meeting comprises producers, representatives of national governments within the Caribbean region and Caribbean agricultural agencies, as well as agricultural scientists working in research and extension institutions throughout the Caribbean region, including land-grant universities from the US and its territories that are part of the Southern SARE region (University of Florida, University of Puerto Rico, and University of the Virgin Islands). This forum is expected to alert the primary target audience for this research apart from Virgin Islands farmers to the findings and potential warnings or benefits to be made regarding the use of Sargassum in Caribbean agriculture. Significant interest at these meetings in Sargassum has been evident for years, so this presentation is expected to be noteworthy and of significant value to the region. In addition, a scientific manuscript will be developed and published as a result of this work, as will an informal extension document to be released by the University to the farming community at the conclusion of the work.