The LI-8100 土壤碳通量测量系统 Biosciences Why is Soil

The LI-8100 土壤碳通量测量系统 Biosciences

Why is Soil CO 2 Flux Important? • The soil carbon pool is large, 2 x atm, 3 x land plants, 0. 3 x all fossil fuel reserves (Schlesinger, 1991) • The amount of carbon released is large, carbon flux from soil is 10 x that of combined fossil fuel & deforestation sources (Pacala, S. W. et. al. 2001) • Soil respiration is a large part of total ecosystem respiration, 92± 15% in a beech forest (Longdoz, et al. , 2000) Biosciences

Where does soil CO 2 come from? • Respiration from roots & microorganisms as they metabolize substrates from the plant • Respiration from microorganisms breaking down soil organic matter and plant litter • Respiration of soil fauna • Action of water on inorganic carbonates in the soil Biosciences

What factors affect soil CO 2 production? Ecosystem Productivity, Temperature and Moisture • Carbohydrate supply to roots • Nutrient transport in the soil • Metabolic activity of plants and microorganisms • Microorganism population size Biosciences

What affects soil CO 2 movement? Diffusion through a porous medium – Porosity of the soil – CO 2 concentration gradients – Pressure (Bulk flow transport >> diffusion transport) – Moisture (soil porosity, volume displacement) – Wind (boundary layer conductance, pressure) Biosciences

Soil CO 2 flux is quite variable Root distributions vary Litter distributions vary Animal populations vary Soil structure varies Diffusion of CO 2 from the soil surface varies strongly in space and time. Biosciences

How is CO 2 flux from the soil measured? • • Eddy covariance at night over plant canopies Eddy covariance over “bare soil” Gradient methods in the soil Chamber methods Flow through steady state system Draw-down transient: LI-6200, LI-6400 Transient: most home-made systems, LI-8100 Biosciences

Chamber Based Soil CO 2 Flux Measurements Biosciences

Measurement Requirements 1. Adequate sampling Sampling with enough replication Spatial variability Sampling over time Temporal variability Biosciences

Measurement Requirements 2. Minimum disturbance Minimize collar movement Prevent pressure gradients & pulses Moisture at ambient Temperature at ambient 3. Good mixing Biosciences

Measurement Requirements 4. Measure the rate when the chamber CO 2 concentration is near ambient. CO 2 flux rate (CO 2 soil - CO 2 chamber) For the chamber rate to = the ambient rate, CO 2 soil must remain ~ constant CO 2 chamber = CO 2 air Biosciences

LI-8100 Automated Soil CO 2 Flux System Biosciences

Measurement Requirements 1. Adequate sampling Sampling with enough replication Rapid measurements with the Survey Chamber Sampling over time Automatic sampling over time at one spot with the Long Term Chamber Biosciences

Survey and Long Term Chambers Biosciences

Measurement Requirements 2. Minimum disturbance Minimize collar movement Prevent pressure gradients & pulses Moisture at ambient Temperature at ambient 3. Good mixing Biosciences

Survey & Long-term Chambers Biosciences

Minimize Pressure Perturbations: Insensitive to Wind Direction Biosciences

Measurement Requirements 2. Minimum disturbance Minimize collar movement Prevent pressure gradients & pulses Moisture at ambient Temperature at ambient 3. Good mixing Biosciences

Minimize Changes in Moisture & Temperature Biosciences

Minimize Temperature Perturbations: Careful Choice of Materials and Coatings Biosciences

Measurement Requirements 2. Minimum disturbance • • Minimize collar movement Prevent pressure gradients & pulses – No fans • • Moisture at ambient Temperature at ambient 3. Good mixing Biosciences

Good Mixing Survey Chamber Mixing Manifold Provides Good Mixing with Minimal Perturbations Biosciences Hemispherical Shape of the Long Term Chamber is Optimal for Good Mixing

Measurement Requirements 4. Measure the rate when the chamber CO 2 concentration is near ambient. CO 2 flux rate (CO 2 soil - CO 2 chamber) For the chamber rate to = the ambient rate, CO 2 soil must remain ~ constant CO 2 chamber = CO 2 air Biosciences

Problem CO 2 chamber must increase to make the measurement How do we resolve this problem? Biosciences

chamber CO 2 LI-6400 Method measure scrub + CO 2 ambient time - This has become a widely accepted method. Biosciences

LI-8100 Method • Chemicals are inappropriate for long-term field measurements • No chemicals • No CO 2 draw-down Biosciences

LI-8100 Method Chamber closes and CO 2 begins to increase Need a way to estimate the initial rate of increase before CO 2 builds up in the chamber Initial rate cannot be measured It must be estimated mathematically Biosciences

C’(water corrected CO 2) Model: C'(t) = Cx’ + (C 0’- Cx’) e-a (t-t 0) Biosciences

Equations Slope at any time t where Slope at t = t 0 Biosciences

Equations Compute CO 2 flux rate from the slope of water corrected CO 2 at the ambient concentration. Biosciences

Summary of Calculations Compute initial values Compute regression of water-corrected CO 2 against time Starts after the dead band Fits C'(t) = Cx' + (C 0'- Cx') e-a (t-t 0) Compute the slope of the regression at the ambient (initial) CO 2 concentration Biosciences

LI-6400 and LI-8100 Comparison Data Biosciences

Long Term and Survey Chamber Measurements Biosciences

LI-8100 Palm Software Instrument Status Screen – Instrument sn and software versions – Battery Power – Available Memory – Current Date and Time Biosciences

One Measurement, Three Observations Biosciences

LI-8100 Palm Software In Measurement Protocol, User defines. . . – Dead band for all observations – Number of observations in each measurement – Time interval between observations – Number of measurements to be made – Time interval between measurements Biosciences

LI-8100 Palm Software Monitor Screen Allows You To: – View any 5 variables simultaneously and in real time – Monitor Measurement Progress Biosciences

LI-8100 Palm Software Summary Page: – Includes Calculated Fluxes, Averages and Statistical Information at the end of an Observation Biosciences

Why the LI-8100? • Minimizes Perturbations – Maximizes Data Quality • Rugged, portable, weather tight, low power • Survey or long-term unattended measurements • Laptop or Palm PDA • Wireless communication • Powerful software and analytical tools Biosciences

LI-8100 Automated Soil CO 2 Flux System A dedicated soil CO 2 flux system Fast Repeatable Convenient Accurate Measurements Biosciences