Decomposers down-under

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1 What s a Worm Worth? Steve Wratten, Harpinder Sandhu, Sue Unsworth, Jacquie Bay Earthworms, large or small, are all annelids belonging to the class Oligochaeta. New Zealand has native earthworms that belong to the family Megascolecidae. These are the earthworms that you will find in native forests. Those that you commonly find in your garden, city parks, farmlands and orchards are introduced species that belong to the family Lumbricidae. Decomposers down-under Earthworms are decomposers, feeding on dead and decaying plant matter under the soil surface. Different species occupy niches in different sections of the soil profile. Some species are well adapted to live in the leaf litter of the forest floor while others occupy the rich dark topsoil (just below the lead litter) and others the subsoil. No matter what the niche, the earthworm is an essential component of our ecosystems, with a public profile that does little to recognise its true worth. Without decomposers, such as earthworms, bacteria and fungi, the nutrient cycles would come to a standstill. Decomposers play an essential role in unlocking the nutrients held in the tissue of dead organisms. Through the work of the decomposers, cells and tissues are broken down. The decomposer gains the energy it needs to live from the dead tissues. At the same time important organic matter and nutrients such as carbon, nitrogen and phosphorus are released back into the environment., creating in the case of earthworms rich soils that we rely on for horticultural and agricultural productivity. The plough is one of the most ancient and most valuable of man's inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures. Charles Darwin 1881 (p 313) The formation of vegetable mould, through the action of worms. London: John Murray 1

2 What s a Worm Worth? New Zealand spends millions of dollars each year on soil management practices. These are activities that are designed to improve the productivity of soil such as the addition of fertilisers or compost, irrigation or drainage, crop rotation or effluent application. This investigation is best carried out between the months of September and December (Southern Hemisphere), however useful results can be obtained in any season. Aim: a) To investigate the effect of soil management practices on earthworm populations. b) To calculate the economic value of earthworms in terms of the topsoil they produce. Materials: Garden spade, plastic bags, newspaper, ruler, data sheet, electronic balance (2dp), calculator. Method: Make sure that all groups in your class agree on the method so that you can combine the results. 1. Select two or more contrasting sites, where different soil management practices have been in place. Examples of contrasting sites could include: A well maintained garden which has had a lot of organic matter (e.g. compost) added to it. A fence line with no vegetation because it is regularly sprayed with weed killer. A grassy area such as a garden lawn or city park (with permission in all cases!). A farm paddock with a history of fertiliser use. 2. Create a square on your spade: Measure the width of the spade blade and record this in the data sheet (W). Measure this distance up from the bottom of the spade blade. Using a permanent marker pen, draw a line across the top of W. 3. At each of your sites, dig holes the same width as your spade, and as deep as the horizontal red line on your spade. Collect the sample and put it into a labelled bag. 4. Spread the soil from each sample on to a plastic sheet or newspaper. Pull the soil sample apart with your fingers and count the number of juvenile and adult earthworms in each sample. See page 3 for instructions on how to tell the difference between juvenile and adult earthworms. 5. Using an electronic balance to 2 decimal places, measure the total mass of earthworms each sample. This is called the earthworm biomass. 6. Record your measurements in the data sheet on page 4 and then calculate the value of the earthworms based on the total biomass present in your samples. 2

3 Earthworm Identification You will not be able to identify the individual species of earthworm that you find; however you will be able to tell whether the earthworms are juveniles or adults. The adult earthworm has a recognisable saddle or clitellum that is part of the reproductive system, forming a sac or cocoon into which the eggs are deposited. The clitellum is visible in adult earthworms, distinguishing them from the juveniles. Clitellum Adult earthworm showing clitellum The Basis of the Calculations The economic value of earthworms in soil formation will be calculated based on the assumption that: that one tonne of earthworms forms 1000kg of topsoil. the value of farmland includes the contribution to production made by its topsoil. A tonne of topsoil can be purchased from garden centres in Christchurch, New Zealand at a rate of NZ$40 per tonne. Useful Facts: 1 hectare = 10,000 m 2 1 tonne = 1000kg; 1kg = 1,000g 1m = 100cm 1 tonne of earthworms in a hectare creates 1000kg of topsoil/hectare/year Topsoil is worth $40/tonne Topsoil that you can purchase commercially comes from building sites where it is scraped off the ground before construction starts. Often this scraped layer is not just the topsoil, but also contains the less valuable lighter coloured subsoil. The most valuable topsoil is dark in colour and contains a high level of organic matter. The price charged by commercial topsoil suppliers (around $40 per tonne) is probably far less than the real worth of good quality topsoil when we consider how long it takes to produce and the value that this soil adds to the productivity of the land. Some assumptions we ve made: Earthworms are uniformly spread through the topsoil. Earthworms will not be found deeper than a spade depth. 3

4 Results: Complete the results table for each of your samples. To improve the reliability of your data, you should share the results of each group in your class, enabling you to take the average of several samples for each site. Soil Management Histories: Describe (and if possible photograph) your sites. Try to find out as much as possible about the soil management history. An example has been provided for you on page 6. Site Site 1: Site 2: Site 3: Location Aspect Current planting where samples were taken Historical planting Soil management history Information sources Raw Results count and biomass of earthworms at each site Site 1: Site 2: Site 3: Sample 1 (Gp 1) Sample 2 (Gp2) Sample 3 (Gp3) Sample 4 (Gp4) Sample 5 (Gp 5) Count (No.) Biomass (g) Count (No.) Biomass (g) Count (No.) Combined Combined Adult Juv. Adult & Juv Adult Juv. Adult & Juv. Adult Juv. Biomass (g) Combined Adult & Juv. Sample 6 (Gp 6) Sample 7 (Gp 7) Sample 8 (Gp 8) Average (Enter into the table on page 5) 4

5 Calculating the area of each sample dug by the spade If your spade is 17cm (0.17m) wide, the area of a sample hole will be 0.17 x 0.17 = 0.03m 2 (2 significant figures) Spade width (W) = Length of Sample (L) Area of the surface of the sample = W x W = W 2 Because we are assuming that earthworms are not found below approximately 1 spade length, we can assume that using the surface area of the sample (rather than volume) we can estimate the number of earthworms per hectare under the N = Average number of earthworms per sample for each site (from page 4) M = Average total biomass of earthworms per sample for each site (from page 4) A = Area of soil sample in m 2 A = length x width of sample A = W x W H = Number of soil samples that will fit into 1 hectare H = 10,000m 2 A E= Number of earthworms per hectare E = H x N ME = Mass of earthworms per hectare in grams ME = H x M 1 gram of living earthworm biomass makes 1 gram of topsoil per year Therefore how many grams of topsoil do our worms make / hectare? Convert ME from grams to tonnes 1 tonne = 1,000,000 grams ME (grams) 1,000,000 = ME (tonnes) If it costs $40 to buy 1 tonne of topsoil and our earthworms made 1.32 tonnes per hectare how much money are the earthworms making for the farmer? Economic value of earthworms in terms of topsoil production / hectare $ = ME x 40 (1 tonne of topsoil costs $40) g 0.17m x 0.17m = 0.03m 2 10,000m m 2 = 330,000 samples/ hectare 330,000 x 20 = 6,600,000 earthworms / hectare 330,000 x 4.00g = 1,300,000g / hectare If there are 1,320,000g of worms in 1 hectare, they will make 1,300,000 grams of topsoil / year. 1,300,000g / hectare 1,000,000 = 1.3 tonnes / hectare 1.3 tonnes / hectare x $40 = $52 per hectare. The worms in 1 hectare of soil are making $52 worth of topsoil each year. On a 100 hectare farm that is $5,200 worth of topsoil / year. W = Width of Spade Used (cm) Calculations (Use 2 significant figures) Example calculation for 1 site Site 1 Site 2 Site 3 5

6 Example Soil Management Histories: Site Site 1: Community Garden Site 2: School Field, Auckland Grammar School Site 3: Fence edge Location Cathedral Place, Parnell, Auckland Mountain Road, Epsom, Auckland Cathedral Place, Parnell, Auckland Aspect Land slopes towards the east at an angle of approx. 30 degrees. Large oak trees and mixed natives on the northerly border. Open lawn on southerly border. Flat, open on all aspects. No shadows. Fence is alongside car park with narrow strip of lawn (1m wide) between the car park and the fence. Current planting where samples were taken Historical planting Soil management history Information sources Mixed vegetable planting / fruit trees. Samples taken from bean / tomato / spinach mixed beds. Mixed vegetables in rotating crops for the past 4 years. Previous planting - overgrown scrub. Mulched every 4 months Watered during the summer. Drainage laid. Perennial ryegrasses. Sample taken from outside edge of the field. Perennial ryegrasses > 50 years. Watered during summer months. Drainage. Annual fertiliser. Perennial ryegrasses beside concrete block fence. Sample taken from soil immediately adjacent to the fence (in the sprayed zone). Perennial ryegrasses beside concrete block fence. Sprayed with weed killer every 6 months. Community Garden Coordinator School Grounds Manager Grounds Manager Conclusion From the results, what conclusions (if any) can you draw about the effect of soil management practices on earthworm populations and their value in your samples. Discussion Questions: 1. Considering the information you collected about soil management histories at each site, discuss factors that may have contributed to the patterns seen in the data. 2. What does the adult : juvenile ratio at each site suggest about the suitability of the soil at the time of collection for sustaining earthworm populations. How could you use this ratio to track the positive or negative impact of soil management over a period of time. 3. Discuss the validity and reliability of your data. 4. Discuss the value of the environmental service that worms provide in horticultural, agricultural and forestry ecosystems, considering in your discussion soil management and productivity. Acknowledgements: This resource is derived from research on biodiversity at Lincoln University funded by the Foundation for Research, Science and Technology (LINX0303) conducted by Professors Steve Wratten, Ross Cullen and Dr Harpinder Sandhu. The resource was initially developed by Sue Unsworth and Steve Wratten for Science Outreach, Lincoln University and later adapted by Jacquie Bay and Steve Wratten for the BioProtection Research Centre in collaboration with LENScience. Photos and diagrams from istockphoto.com (used under licence), LENScience, Lincoln University, or the public domain. For further information contact: steve.wratten@lincoln.ac.nz sue.unsworth@lincoln.ac.nz j.bay@auckland.ac.nz Copyright Lincoln University,