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A Comparative Field Study of Four Soil Respiration Systems

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A Comparative Field Study of Four Soil Respiration Systems Earth System Science Group Internship Earth System Science A comparative field study of four soil respiration systems E. Vermue Supervisors: 811125880110 Ing. J. Elbers Dr. M.R. Hoosbeek January 2008 Preface Preface The fulfillment of this internship was the last step in completing the requirements for my master degrees. From June to December 2007 this study on the comparison of different soil respiration systems was performed at Alterra, ESS-CC. I would like to thank Dirk de Kramer for his help during the nightly measurements and Bert Heusinkveld for calculating and providing the eddy correlation data. I also like to thank Eef Velthorst with his help during the fieldwork and to get started with the ADC soil respiration system. Last I want to thank Jan Elbers and Marcel Hoosbeek for their supervision and for answering all my questions. i A comparative field study of four soil respiration systems ii A comparative field study of four soil respiration systems Table of contents Table of contents 1 Introduction................................................................................................................. 1 2 Materials and Methods................................................................................................ 5 2.1 Site description.................................................................................................... 5 2.2 Soil respiration systems ...................................................................................... 6 System 1: Automatic open chamber system ............................................................... 6 System 2: EGM-1/4 by PP systems ............................................................................ 6 System 3: Two chamber ADC .................................................................................... 7 System 4: Eddy correlation system............................................................................. 7 2.3 Complementary measurements........................................................................... 8 Soil moisture ............................................................................................................... 8 Soil temperature.......................................................................................................... 8 2.4 Methods............................................................................................................... 8 3 Results....................................................................................................................... 11 3.1 Open chamber system....................................................................................... 11 3.2 Chamber scale: Open chamber and EGM......................................................... 16 3.3 Chamber scale: Open chamber and ADC ......................................................... 21 3.4 Field scale: Open chamber vs EC ..................................................................... 24 3.5 Modeling soil temperature response curves...................................................... 26 4 Discussion................................................................................................................. 33 5 Conclusions............................................................................................................... 35 References......................................................................................................................... 37 Appendix 1 Response curve fitting............................................................................... 40 iii A comparative field study of four soil respiration systems iv A comparative field study of four soil respiration systems Introduction 1 Introduction One of the major terrestrial ecosystem fluxes of the global carbon cycle is formed by soil respiration. Soil respiration is the flux of CO 2 from the soil surface to the atmosphere and consists of heterotrophic respiration and autotrophic root respiration (Han et al., 2007). The global soil respiration flux is estimated at 80.4 Pg C yr -1 and is therefore one of the major fluxes of the global carbon cycle (Raich et al., 2002). For comparison; anthropogenic fossil fuel combustion accounts for a flux of 5.5 Gt C yr -1. Small changes in the soil respiration flux can cause large perturbations on the global C cycle. Therefore detailed knowledge on soil respiration and the driving environmental factors is required. Figure 1-1 presents the in- and outputs concerning soil CO 2 effluxes. However, soil respiration measurements form a major source of uncertainty, not in the last place because of the lack of a reference method (Raich & Schlesinger, 1992 in Jensen et al., 1996). Soil respiration thus consists of soil CO 2 production and transport of CO 2 to the atmosphere (gaseous diffusion and mass flow). The concentration gradient between the soil and atmosphere drives the diffusion of CO 2 to the atmosphere. Mass flow is defined as the pumping of air by atmospheric pressure fluctuations on turbulent scales (Kimball and Lemon, 1971 in Janssens et al., 2000). Several environmental parameters can influence soil respiration (Han et al., 2007): soil temperature, soil moisture, root biomass, NPP, litter input, microbial population, root nitrogen concentrations, soil texture, substrate quantity and substrate quality, see also Figure 1-1. Of these parameters soil temperature and soil moisture are the main influencing factors. Furthermore management practices, like ploughing, irrigation and the addition of fertilizer, can have considerable consequences for the soil CO 2 efflux. Soil respiration measurements show considerable temporal and spatial variation (Rayment & Jarvis, 1997). The temporal variation is mainly driven by soil temperature and soil moisture, which control the CO2 production by affecting terrestrial ecosystem productivity and the rate of SOM decomposition (Wiseman and Seiler, 2004). Other factors, like soil porosity, water content and the turtuosity of the soil affect the diffusion of soil CO 2 to the atmosphere (Simunek and Suarez, 1993 in Jensen et al., 1996). Spatial variations can occur over small distances. Rayment and Jarvis (1997) found that soil respiration varied from 0.25 µmol m-2 s-1 to 3.6 µmol m-2 s-1 over a few meters for a black spruce forest. Han et al. (2007) indicated that within an agricultural ecosystem consisting of maize the spatial variation of soil respiration is correlated with the amount of root biomass i.e. distance from the plant, soil temperature and NPP. 1 A comparative field study of four soil respirati on systems Introduction Figure 1-1 A conceptual model of the in- and outputs of the soil CO 2 effux (from Ryan & Law, 2005). Accurate measurement of CO 2 effluxes is difficult. An appropriate method does not alter the soil respiratory activity and furthermore the concentration and pressure gradient and the air motion near the soil surface should resemble the ambient situation. The lack of a standard reference methodology for the measurement of soil respiration has lead to the use of several types of methods, which are listed below. Each method has its dis- and advantages. Between the different methods often large differences are observed, but these are often strongly correlated (Janssens et al., 2000). Different soil respiration measurement techniques are: 1. Eddy covariance techniques 2. Measurement of [CO 2] in soil profile 3. Static systems 4. Dynamic systems 1. Eddy covariance technique The eddy covariance technique is based on the concept that gas transport is accomplished by eddies that displace air parcels from the soil to the measurement height (Mosier, 1990 in Janssens et al., 2000). This vertical flux is measured at a reference level, which represents the CO 2 efflux from the soil. One of the major advantages is the soil system remaining undisturbed, while measurements can be done continuously. A second advantage is the measurement of soil CO 2 effluxes from a larger surface area, thus representing (part of) the spatial variability (Janssens et al., 2000). However, for correct measurements of the CO 2 effluxes certain conditions have to be met. It is questioned if these conditions prevail continuously throughout 2 A comparative field study of four soil respirati on systems Introduction the measurement period, but often they are assumed to occur (Rayment & Jarvis, 1997). The assumed requirements are: ••• Level and homogeneous upwind fetch. • Zero mean vertical windspeed, steady-state conditions. • Absence of sources and sinks between soil and the sensor (like canopy or vegetation cover) 2. Measurement of CO 2 in the soil profile A second technique for measuring CO 2 fluxes is the measurement of CO 2 in the soil profile. This method is less frequently used due to its practical limitations. To determine the diffusivity of CO 2 knowledge on gas diffusivities is required (Rayment & Jarvis, 1997). However, soil is a very heterogeneous substrate resulting in a highly spatially variable CO 2 diffusion coefficient, making accurate estimates difficult. Furthermore gas sampling with a syringe can cause an overestimation of the CO 2 concentrations as also gas is sucked from other parts of the soil system, due to the suction (Le Dantec et al., 1999) . 3. Static systems In static chambers a volume of atmosphere above the soil surface is sealed for an extended period, preferably 24 hours, to allow gasses to accumulate to a concentration that can be determined by an alkali solution, like soda-lime. The alkali solution traps
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