Leaf Litter Decomposition

Transcription

1 Leaf Litter Decomposition Written by Dr. Karen Kuers, Sewanee: The University of the South and Dr. Jeffrey Simmons, West Virginia Wesleyan College Decomposition of leaf litter is a major source of nutrients in forest ecosystems. As leaves are broken down by insect and microbial decomposers, organically-bound nutrients are released as free ions to the soil solution which are then available for uptake by plants. In this exercise students will measure the rate of decomposition of leaf litter during winter, spring, and summer at contrasting sites using the litterbag technique. Objectives: 1. To compare the rate of leaf litter decomposition between different species at two contrasting sites in a watershed (or other forested location). 2. To compare the rates of leaf litter decomposition among Appalachian watersheds. 3. To learn how to use mesh litterbags to trace litter decomposition rates. I. Background In most forests the major source of nutrients for trees is the process of decomposition. Decomposition refers to the processes that convert dead organic matter into smaller and simpler compounds. The products of complete decomposition are carbon dioxide, water, and inorganic ions (like ammonium, nitrate, phosphate, and sulfate). Decomposition is mainly a biological process carried out by insects, worms, bacteria, and fungi both on the soil surface and in the soil. The rate of decomposition is influenced by many factors. Because decomposition is a biological process carried out primarily by bacteria and fungi, its speed will be affected by temperature and soil moisture. Generally decomposition increases exponentially with temperature; that is, for every 10 degree rise in temperature, decomposition increases by a factor of 2 (reference). Nevertheless, leaf decomposition does occur at a low rate during the winter months even under deep snow (Taylor and Jones 1990). Soil moisture effects are a little more complicated. Decomposition is inhibited in very dry soils because bacteria and fungi dry out. Decomposition is also slow in very wet soils because anaerobic conditions develop in saturated soils. Anaerobic decomposition is less efficient than aerobic and as a result is slower. Decomposition proceeds fastest at intermediate water contents. The quality of the leaves as a food source for microbial decomposers is another important factor. Substrate quality has been defined in many different ways - as the nitrogen concentration (N), as the lignin content, and as the C:N ratio (Moorhead et al. 1999). Researchers have found that decomposition of leaf litter can be predicted by the C:N ratio (Taylor et al. 1989), by the lignin content (Meentemeyer 1978), or by the lignin:nitrogen ratio (Melillo et al. 1982). Basically, high quality leaves (like nutrient-rich alder leaves) will decompose faster than low quality leaves (like nutrient-poor conifer needles). Many studies have shown striking differences in decomposition rates among species (for example, Adams and Angradi 1996; Cornelissen 1996). Substrate quality can even vary within a leaf. Berg and co-workers (Berg and Staaf 1980; McClaugherty and Berg 1987) have shown that in the initial stages (0 to 3 months) of leaf breakdown small soluble carbon molecules, like starches and amino acids, are lost first leaving behind the more recalcitrant molecules like lignin. Decomposition during this first phase is rapid because these molecules are easy to break down and energy rich. The second stage of decomposition - the break down of lignin - is much slower because lignin consists of very large and complex molecules. This rapid initial breakdown followed by a longer period of slow decomposition results in a mass loss curve that resembles an exponential decay curve. 1

2 The decomposition rate constant, k, can be calculated from the decay curve using the following equation: ln ( M 0 / M t ) = k * t where M 0 = mass of litter at time 0, M t = mass of litter at time t, t = time of incubation (usually in years) k = decomposition rate constant. Leaf litter decomposition is most commonly measured using the litterbag technique. A known quantity of leaf litter is placed into a mesh bag, and the bag is then inserted into the litter layer of a forest floor. Bags are harvested at periodic intervals, dried and reweighed to determine the amount of mass lost. By incubating the leaves in situ, they are exposed to the normal fluctuations in temperature and moisture. The mesh bags allow smaller insects as well as microorganisms access to the leaves. In this experiment you will use the litterbag technique to compare the decomposition rates of leaves of two different tree species incubated in two contrasting environments. II. Procedures A. Preparing the Litterbags Collect red maple leaves (Acer rubrum L.) and chestnut oak leaves (Quercus montana Willd.) from a single location after leaf fall in the autumn. (You could substitute a different oak species if chestnut oak is not available in your area, and red maple could be replaced by either dogwood (Cornus florida L.), or yellow-poplar (Liriodendron tulipifera L.) Flatten the leaves in newspaper (you may moisten them with distilled or deionized water to keep them from breaking), and dry them in an oven at 30 ºC for hours. Make the litter bags using standard black fiberglass window screen. This can be purchased in rolls 48 wide (122 cm), which can be easily cut to the final bag size of 20 x 28 cm. The bag edges may be melted closed with a soldering iron, closed using a glue gun, or sewn shut with fishing line. Make three extra travel bags for each species that will be used for field blanks (to correct for leaf loss during travel to and from the site) and for 30 C to 105 C mass correction. You will need 12 replicate bags of leaves of each species at each location in addition to the 3 travel bags. (Total bags = 27 per species for 4 collection dates.) Randomly assign 3 litterbags of each species to be the "travel bags". Assign 3 more to each of the planned collection dates. Remove the petioles from the leaves and weigh out sets of 4-5 g of leaves. Place each sample set in a separate mesh bag labeled with the species, bag number, and topographic position, and record the Initial Mass (30) of each in the appropriate location on the spreadsheet. (Data for the "travel bags" goes in Section I, and for the field samples goes in Section II of the spreadsheet.) B. Installing the Litter Bags Select at least two contrasting sites for placement of the litterbags. Try to install the bags either in early December or early January. At the predetermined field locations, lift off and set to the side the litter from a spot large enough to place each mesh bag. Place the leaf filled mesh bag at the soil surface, pin it in place, replace the removed litter, and mark the location with a flag. Take the three travel bags for each species to the field and back during installation. Dry them to 30ºC for 24 to 48 hours. Determine the mass and record this in Section I of the data spreadsheet as the Final Mass (30). Then dry the travel bags at 105ºC for 48 hours and record this in Section I as the Final Mass (105). Divide the Final Mass (105) by the Final Mass (30) to get the 30ºC to 105ºC conversion factor (CF). Use the 30ºC to 105ºC conversion to adjust the Initial Mass (30) of the each sample to the Initial Mass (105). Calculate the Travel Mass Loss (105) by subtracting the Final Mass (105) from the Initial Mass Loss (105). 2

3 C. Site Characterization: To monitor soil temperature place the sensor of one or more temperature loggers (HOBO or other variety) within the litter layer at each of the locations. Set the logger to record temperatures at hourly intervals for the duration of the experiment. (Alternately someone could be assigned to collect the temperature of the interface of the soil and forest floor at weekly intervals at each site.) Soil samples can be collected at each sampling location to characterize and compare the sites. Use a hand shovel and collect a cylinder of mineral soil to a 10 cm depth at 3 spots at each of the 2 locations. (Bulb planters can also be used to extract a cylinder of soil.) Place each sample in a separate ziplock bag, label it with the date and site location, and return the samples to the lab. In the lab, remove as many rocks and roots as possible, record the fresh weight to the nearest 0.1 g, and place the samples in the drying oven at 105 until dry. (It may be best to wait a week.) Record the dry mass to the nearest 0.1 g, and calculate the moisture content of the soil. (See the "Analysis" section for the formula). If possible, analyze a soil sample from each location for ph, organic matter, and texture class. (Soil ph kits are available from biological supply companies, at most large home improvement centers, and online.) Collect a sample of the forest floor (soil organic horizon (O i /O e /O a ) by using a wooden square or other straight edge to mark a 25 x 25 cm square in the forest floor at each location. Cut along the straight edge and collect all of the forest floor within the square down to the beginning of the mineral soil layer. Place the sample in a brown bag labeled with the watershed, location, and date, and dry at 30ºC. Separate the sample into leaves, fruit, and woody material and determine the dry mass of each. Calculate the average mass of material on the forest floor (per hectare) by dividing the mass of the materials by the size (in hectares) of the 25 x 25 cm sample plot. (See Analysis section for conversion factors). Other Options: If you have GPS units available, and access to ArcView or similar mapping software, record the GPS coordinates (longitude, latitude, elevation), slope, and aspect of each site. If you have topographic maps, you can try to find the location of the plots on your map. You may also want to inventory the woody vegetation at each site to compare species composition. D. Collecting the Litter Bags Leaf litterbags should be collected every 2 months for 8 months. (An alternate plan is to collect samples for a full year, in which case you will want to install additional samples.) On each sample date retrieve 3 mesh bags of each species at each location, brush off all external leaves, and slide each mesh bag into a separate clean brown bag for transport back to the lab. To quantify differences in soil moisture, repeat the soil sampling procedure at each sample date. Again collect three cylinders of soil at each site, sort out the rocks and roots, weigh them fresh, dry them at 105 C, and reweigh them to get oven-dry mass. Calculate soil moisture (in g water / g dry soil). When you return to the lab, place the litterbags in the drying oven at 30ºC and dry them to a constant weight (2 to 4 days). Gently remove the contents of each mesh bag and place them on a clean tray or clean paper. Using gloves remove roots, soil, insects, and other materials that have entered the mesh bag. Weigh, and record the Final Mass (30) of the leaf material. Use the average 30ºC to 105ºC CF you obtained earlier for each species and calculate the Initial Mass (105) and the Final Mass (105) for each sample. At the final collection period remove the temperature datalogger from the field and download the data. (It would be better to download the data at each sampling period if possible). Calculate the Mass Loss (g) by subtracting the Final Mass (105) and the average Travel Loss (105) from the Initial Mass (105) for each sample. Calculate the Mass Loss % and the Remaining Mass %. Record the total number of days the samples were in the woods, and calculate the average Daily Decomposition %, and the Decay Constant (k) for each species, at each site, for each collection period. (The formulae for these values can be found in the Calculation and Analyses section.) 3

4 III. Calculations and Analyses A. Calculations [Note: All masses should be in grams] Litter: (Used to calculate % decomposition and species decay constants.) Initial Mass (30) = Initial air-dry or 30 C mass Initial Mass (105) = Sample dried to 105 C or calculated from Initial Mass (30) * CF CF = factor used to calculate 105 C mass of a sample dried at 30 C Measured Mass 105 C / Mass 30 C Final Mass (30) = Mass of field sample dried to 30 C Final Mass (105) = 105 C mass of a sample dried to 30 C Final Mass (30) * CF % Mass Loss (105) = (Initial Mass (105) - Final Mass (105)) / Initial Mass (105) * 100 % Mass Remaining (105) = % Mass Loss (105) Daily Decomposition Rate = % Mass Loss (105) / Length of Incubation (in days) Decay constant (k) = -( ln ( % Mass Remaining (105) / 100% ) ) / (t) in years t (in years) = days / 365 Soil Moisture: (Used to compare the average soil moisture between sites) g water per g dry soil = (Wet soil mass (g) - dry soil mass (g)) / dry soil mass (g) Forest Floor: (Used to compare the amount of litter produced by the trees at the two sites.) 25 x 25 cm plot in sq m = (25 cm x 25 cm) / 10,000 sq cm 10,000 sq meters = 1 hectare B. Analyses (Sample questions for you to consider). Biological Considerations 1. After completing the spread sheet, and calculating the decay constant for the litterbags at each site, look to see if there were differences between the two species that you studied. Which species decomposed slower - red maple or chestnut oak? Explain why. (Hint: you may need to read some of the literature on litter decomposition for this one. See the bibliography for some possible sources). 2. How did soil moisture, temperature, forest floor, and/or tree species compare at the two sites? Was decomposition slower at one site, or was it the same? Does the difference correlate with any environmental differences? Explain how the environmental variables may have influenced decomposition rates of the species. 4

5 3. Compare your results with results from the other CAWS watersheds or other published studies (See Blair and Crossley 1988, Blair et al. 1990). Can you explain some of the differences in decomposition rate among the sites? Was one species always slower to decompose than the other? Was one site (wet or dry) always slower to decompose than the other? What site variables might explain these differences? 4. How would you expect wood decay rates to compare to that of leaves? (See Abbot and Crossley, 1982, for wood decay constants for chestnut oak.) 5. Could the design of the bags have influenced the rate of the leaf decomposition in the bags compared to that of the surrounding forest floor? Experimental Design and Statistical Considerations 1. What are the experimental treatments for the experiment at your watershed? 2. Write out the null and working hypotheses for this experiment. 3. Determine the appropriate statistical test to use for this experiment. Then perform that test on the Decomposition Rates. Create a table containing the results of the statistical test (means, standard deviations, number of replicates, p-values). IV. Bibliography and Links Abbot, D.T., and Crossley, D.A. Jr Woody litter decomposition following clear-cutting. Ecology. 63 (1): Adams, M.B. and T.R. Angradi Decomposition and nutrient dynamics of hardwood leaf litter in the Fernow whole-watershed acidification experiment. Forest Ecology and Management. 83: Berg, B., and H. Staaf Decomposition rate and chemical changes in decomposing needle litter in Scots pine. In T. Persson, (ed.) Structure and function of northern coniferous forests. Ecological Bulletins-NFR. 32: Blair, J.M., aned Crossley, D.A., Jr Litter decomposition, nitrogen dynamics and litter microarthropods in a southern Appalachian hardwood forest 8 years following clearcutting. Journal of Applied Ecology. 25: Blair, J.M., Parmelee, R.W., and Beare, M.H Decay rates, nitrogen fluxes, and decomposer communities of single- and mixed species foliar litter. Ecology. 71(5): Cornelissen, J.H.C An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types. Journal of Ecology. 84: McClaugherty, C.A. and B. Berg Celluolose, lignin and nitrogen concentrations as rate regulating factors in late stages of forest litter decomposition. Pedobiologia. 30: Meentemeyer, V Macroclimate and lignin control of litter decomposition rates. Ecology 59: Melillo, J.M., J.D. Aber, and J.F. Muratore Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology. 63: Moorhead, D.L., W.S. Currie, E.B. Rastetter, W.J. Parton, and M.E. Harmon Climate and litter quality controls on decomposition: An analysis of modeling approaches. Global Biogeochemical Cycles. 13(2): Taylor, B.R., and H.G. Jones Litter decomposition under snow cover in a balsam fir forest. Canadian Journal of Forest Research. 68: