Click here to return to main page

A Comparison of Soils from Two Diverse Wetland Areas

in the Tempisque Watershed

By Dee Ann Wilson, Michelle Breinholt , Christopher Soldat and Ken Harasty.

Abstract

In conducting this study our objectives were to determine the effects of human alteration and interaction on soils. This study compares a rice field to the soil of an existing wetland. Our comparison was based on a short chemical analysis of soil and a survey of existing macroinvertebrates. We found that there were no significant differences between the two study sites.

Introduction: Recent information says that, “ between one-half and one-third of the land surface (of the Earth) has been transformed by human interaction.” (Vitousek, Mooney, Lubchenco and Melillo, 1997). Human agricultural activities in Guanacaste province Costa Rica have altered the primary forest. Initially forests were cleared for cattle production and grazing. Subsequently pastureland has been converted to rice production. In rice production, herbicides and fertilizers are used, the fields are flooded two to three times and the water is allowed to slowly drain. Farmers in Guanacaste yield two crops a year.

We believed that natural wetland soil would contain greater population numbers and diversity of macroinvertebrates than the cultivated soil of a rice field. We have witnessed similar bird species feeding in both rice field and wetlands. Many of these species sieve macroinvertebrates from the soil with their specialized beaks.

This study is a comparison of the soils of a nearby rice field to a protected wetland area inside the Palo Verde National Park. We were interested in determining if rice production and the use of land-applied chemicals has altered the soils from the wetlands inside a protected park.

Materials and Methods:

Collection materials- PVC soil collection pipe, tape measure, bucket, six ziplock bags, rubber boots/hip waders, GPS location device, digital camera and a permanent marker.

Data sampling materials- balance beam, sieve, bowls, forceps, probes, wash bottles, hand lenses, plastic vials, soil test kit, hammer, petri dishes, and filter paper.

• Two study sites were selected: a cultivated rice field, which was between crops, and a protected RAMSAR-designated wetland area. The rice field was approximately 10 hectares in full sun and was under water. A canal system and other rice fields surrounded it. There were numerous species of wading birds, such as great egrets, snowy egrets, black-neck stilt, and wood storks. The RAMSAR-designated wetland area was full of dense plant material, which made it difficult to determine its size from our sample site. Much of the bird population was not visible from our study site because of the vegetation. This wetland site was very diverse in terms of its plant communities. (see attachment for map of area)

Collection Procedures:

• A transect tape measure was set to be approximately perpendicular to the bank.
• The soil sampler (8.5 cm inside diameter) PVC pipe was inserted four centimeters into the bottom. The pipe was twisted and slowly raised to the surface into a horizontal position.
• A collection bowl was placed underneath the pipe and the material was extruded into the bowl.
• A labeled plastic bag was used to separate the samples. Each bag was labeled with its site and distance from the bank.

Soil Chemistry Procedures:

• After returning from the field a 50-gram sample was placed on filter paper and set into a labeled petri dish to dry.
• Later the dry materials were pulverized with a hammer and extraneous materials were removed.
• The following chemical analysis was performed according to their printed protocols in the LaMotte Soil Test kit: pH, phosphorus, potassium and nitrogen.

Diversity and Population Count Procedures:

• Wet soil samples were placed into bowls and Macroinvertebrates were separated and then counted.

Results:

Data Table 1

Rice Sample	Species name And number	Species name And number
Rice- 1 meter	65 bloodworms	1 mollusk species
Rice- 2 meters	220 bloodworms
Rice- 3 meters	144 bloodworms
total	429 bloodworms	1 mollusk species

Average 430/3= 143 per sample

GPS reading: 10 degrees 25’ N, 85 degrees 18’ W

Data Table 2

Wetland sample	Species name And number	Species name And number
Wetland- 1 meter	120 bloodworms	2 arthropods
Wetland- 2 meters	44 bloodworms	1 arthropods
Wetland- 3 meters	8 bloodworms	1 arthropods
total	172 bloodworms	4 arthropods

Average 172/3= 57 per sample

GPS reading: 10 degrees 20.92’N, 85 degrees 6’ W

Data Table 3

Sample	pH	nitrogen	phosphorus	potassium
Rice field	7	no	high	Medium low
Wetlands	7	no	high	high

These tables indicated that the rice fields had a higher average number of macro-invertebrates than the wetland field. The chemical analysis of the soils proved to be identical except for a small difference in the potassium level.

Summary:

Our conclusion indicates that there is no significant difference in diversity between the soil samples at the two tested sites. The higher number of organisms in the rice field sample would seem to show that human impact has not unusually altered that environment. Chemical composition tests do not show any unusual levels of soil parameters either. We feel that future study should include a larger number of related sample sites. We feel that it is important to utilize soil chemistry test equipment, which provide qualitative as well as quantitative results. Also, we feel that it would be important to continue to investigate the quality of life by including microorganisms in the next investigation as well.

Educational Applications:

Most classrooms can identify wetlands within their own watersheds, which are close to human use areas such as farms, housing developments or commercial interests. These areas in students’ home communities are excellent study areas. They provide a natural area where students can begin inquiry activities to determine the connections between human use practices and the natural landscape. Students can begin to collect data to help determine facts concerning environmental issues. It is a good learning experience which helps integrate school and learning into a more authentic experience.

Click here to return to main page