Harvard University Soil Respiration and Decomposition Lab Report

Soil Respiration and Decomposition This lab is a modification of Simmons, JA. 2009. Decomposition and soil CO2 emission. Teaching issues and Experiments in Ecology: volume 6. http://tiee.esa.org/ Introduction Plants get water and nutrients from soil, and the inherent characteristics of the soil in combination with environmental factors determine soil fertility. Soils are complex and dynamic ecosystems with communities of organisms. Like all ecosystems, they have a food web that may include bacteria, fungi, algae, protists, insects, worms, plant roots, and burrowing animals. Soils also carry out essential ecosystem functions like water storage and filtration and, perhaps most importantly, decomposition. Decomposition in soils is a key ecosystem function that in part determines the productivity and health of the plants growing there. Decomposers feed on dead organic matter and in the process break it down into its simplest components: carbon dioxide, water and nutrients. The process of decomposition releases large quantities of essential nutrients to the soil solution, thereby making them available to plant roots. In northern hardwood forests, for example, about 85% of a tree’s nitrogen comes from decomposition (Bormann and Likens 1979). Thus, if decomposition of a forest is impaired by drought, acid rain, or some other stress, the vegetation may experience nutrient deficiencies. Decomposition is an important part of the global carbon cycle. The carbon cycle is the cyclical movement of carbon atoms from the atmosphere to the biosphere-lithosphere and back to the atmosphere. In the atmosphere, most carbon is in the form of carbon dioxide gas. Through the process of photosynthesis, some of that carbon is converted into organic carbon in organic matter or biomass. Plants and animals perform cellular respiration and convert a small percentage of that organic carbon back to CO2. A larger portion of the organic carbon in plants is transferred to the soil when plants shed their leaves or when they die. Decomposers then begin their work of breaking down the organic matter. Some of the organic carbon in the organic matter is converted into CO2 which is released into the soil pore spaces leading to relatively high concentrations of CO2 compared to the atmosphere. The difference in concentration causes CO2 to diffuse from the soil to the atmosphere (Fig. 1). Decomposition is not the only source of CO2 in soil. In a forest or grassland ecosystem, plant roots and associated mycorrhizal fungi are abundant in the soil, and the root and fungal cells perform cellular respiration, metabolizing carbohydrates that are sent down from the leaves. This CO2 is released to the soil and can be responsible for anywhere between 0 and 60% of a soil’s CO2 emission (Fig. 1). Total Soil CO2 Efflux (TScer) Soil boundary layer CO2 Decomposition of litter and soil organic matter Sources of CO2 Root Root maintenance costs growth and fast decomposition Aboveground Litter Rhizosphere Liter Unprotected soil carbon Live root and rhizosphere carbon Belowground processes Protected soil carbon Leaching and/or erosion Figure 1. Components of CO2 efflux from forest soils (TScer). TScer from the soil boundary layer to the atmosphere equals CO2 production from roots, rhizosphere heterotrophs, litter, and soil heterotrophs when steady state conditions are approached. Abnormal turbulence at the soil surface can produce TScer which exceeds the rate of CO2 production by the component processes. The dashed line from the surface litter layer indicates a dynamic process highly dependent on litter water content. from Hanson et al. 2000 Several environmental factors impact the rates of decomposition and root respiration and, therefore, the rate of CO2 emission from soils. Since plant roots and decomposers such as bacteria and fungi are ectotherms, their metabolic rates are influenced by soil temperature. Soil moisture (how much of the pore space is filled with water) and pH also affect the metabolic activity of microbes and plant roots. In addition, soil moisture and soil texture the size distribution of soil particles) determine how fast CO2 molecules can diffuse to the soil surface. Thus, soil temperature, moisture, pH, density, and texture all influence CO2 emission rates (Hanson et al. 1993). Methods-Data Collection characteristics and compare the CO2 emission rates of those sites. Your team will select sites in different microhabitats or with different environmental As a team, decide on a comparison to make. Suggested comparisons include: under conifers vs. under hardwoods, sun vs. shade, north slope vs. south slope, forest vs. field, with leaf layer vs. without leaf layer (i.e., the layer of dead leaf litter on the soil surface removed). Have your comparison approved by your lab instructor. Measure and record CO2 emission rate at 5 places within each of your selected microhabitats. To take a reading of CO2 emission rate: 1. Choose a location to measure CO2 emissions. Prepare the location by pinching off or clipping at ground level any green vegetation that would be within the sampling chamber. Remove any twigs or rocks that would prevent a good seal between the sampling chamber and the ground. 2. Position the sampling chamber with the large opening on the ground. Insert the CO2 sensor into the small opening at the top of the chamber and attach the sensor wire to channel 1 of the LabQuest meter. 3. Allow the sampling chamber to equilibrate for 1-2 minutes. 4. During the equilibration, adjust the LabQuest meter if necessary: a. Turn on the meter by pushing the button with the red power symbol on top. b. Set the units for CO2 to ppm (parts per million) by tapping on the units on the screen and selecting “Change Units”. C. Set the Mode to Time Based, the Rate to 0.25 samples/s, and the Duration to 300.0 s. Tap the box on the screen to change any of these values. d. Tap the graph icon at the top of screen to bring up the graph view. 5. Start recording by tapping the green arrow on the lower left side. The CO2 level will be measured and recorded every 4 seconds for 5 minutes. 6. After 5 minutes, record the CO2 emission rate: a. Tap Analyze, then Curve Fit. b. Tap the box next to CO2 and choose Linear from the Fit Equation menu. C. The slope m is the rate of CO2 emission in ppm/s. Record m and tap OK. LAB REPORT Your lab report should include the following sections: I. Introduction A. describe the environmental variable tested and why this variable might affect soil respiration or decomposition B. predict the relative CO2 emission rates of your two study groups II. Results A. properly formatted graph 1. unpaired samples: present the mean values for CO2 emission rate in a bar graph with error bars or show data in a boxplot 2. paired samples: present the mean difference in CO2 emission rate in a bar graph with error bars or show differences in a boxplot B. a sentence or two that describes the trend shown in the graph (either that the rates are equal or that one rate is greater than the other) – present your t, df, and p values in parentheses at the end of the sentence to support what you say III. Discussion A. discuss the results with reference to the prediction you made in I-B B. describe additional experiments we could do to learn more about soil respiration and decomposition

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