Scientists conduct investigations for all kinds of reasons. They may want to explore new ideas, gather evidence or prove or disprove previous results. Although scientists must follow certain methods to ensure their results are fair and accurate, there are many ways they can conduct an investigation. Two aspects scientists need to consider are how big their investigation will be (small scale versus large scale) and how long it will run (short term versus long term).
Small-scale and large-scale investigations
To gain a better understanding of the concept of scale, consider an episode of MythBusters – the TV programme that uses science to test the validity of urban myths and internet videos. The MythBusters team often starts by building a small-scale model to test a myth. This allows them to gather data about the way their model works. If the small-scale investigation answers their questions, they go on to build a full-scale model. They do this to ensure the investigation is more realistic and the results are more accurate.
Dr Nicole Schon from AgResearch follows the same principle. She wants to know more about the role anecic earthworms play in carbon storage in pasture soils. To do this, Nicole puts carbon sources (plant litter and dung) on the soil surface. She uses differences in carbon isotopes to track its incorporation and distribution within the soil profile. (At an atomic level, carbon can have different forms with different atomic masses.)
Nicole’s investigation involves two components. The first is a smaller lab-based investigation set to run for a year. For this, Nicole established a number of microcosms – buckets of soil that simulate earthworms’ usual environments. Nicole is able to manipulate variables (such as the type of earthworm species) and make precise measurements of changes to carbon within the soil in each bucket.
Running alongside the microcosm experiments is a longer and larger-scale field investigation. Nicole has a number of field sites – intensively farmed pastures – where anecic earthworms were absent. She has introduced anecic blackhead earthworms (Aporrectodea longa) to these sites and allowed them to become established. Nicole periodically applies carbon sources (dung) to the soil surface. She uses carbon isotopes to assess the changes in carbon by removing a core of soil, analysing the contents and correlating it with the abundance of the anecic earthworms. The larger field investigations provide real-life conditions, and Nicole hopes they will confirm the microcosm experiments. The field sites will also provide information on how the A. longa species becomes established and how it spreads. If they are found to incorporate large amounts of carbon, the goal will be to introduce A. longa earthworms to pastures throughout the country.
Short-term and long-term investigations
Just as scientific investigations range in size, they also vary in the time it takes to run them. Some investigations run for a short time. Others can go for years – or even decades!
Dr Trish Fraser, a soil scientist with Plant & Food Research, has been involved with both short-term and long-term investigations. In one short-term investigation, Trish studied earthworm-burrowing behaviour using PVC cylinders 30 cm in diameter and 50 cm in length. She used soil from a nearby pasture to match the cylinders with field conditions. Trish added three different species of earthworms at rates commonly found in agricultural soils and sealed the cylinders with fine gauze to keep the earthworms in place and prevent others from entering. The cylinders were buried with the soil surfaces level with the surrounding soil and exposed to the weather. After 6 months, Trish removed the cylinders to study the macropores created by earthworm burrowing.
Trish tapped into a long-term pasture trial to aid her research into the effects of land management on the composition and size of earthworm populations. The Winchmore Irrigation Research Station began experimental fertiliser treatments in 1952, and the land has been continuously monitored ever since. Trish had access to Winchmore pastureland, a nearby wilderness area covered with native grasses and herbs and a site under intensive cultivation for 11 years. She excavated soil samples from all three sites to a depth of 25 cm and hand sorted the soil to collect the earthworms, which were then identified, counted and weighed. Trish’s research showed that earthworm numbers were lower under cropped land than either wilderness or improved pasture sites. This is primarily due to differences in organic matter input and rates of organic matter turnover. The cropped land had a lower supply of organic litter. The fertilised pastures had greater grass growth, more animal excreta and, consequently, more food for the earthworms.
Nature of science
Scientists often control conditions in order to focus on the effect of a single variable. This is often easier to do when conducting a short-term lab-based investigation. Field investigations using real-world conditions are then carried out to validate or verify lab results.
In this video, Dr Ravi Gooneratne describes how he uses controls in his earthworm/polluted soil experiments.