Lecture 3&4: Terrestrial Carbon Process I. Photosynthesis and respiration (revisit) II. Carbon Stocks and Fluxes in Terrestrial Ecosystems III. Terrestrial Ecosystems A. Ecosystem Concept B. Ecosystem Carbon Balance (GPP, NPP, NEP, NBP) VI. Missing carbon sinks?

Lecture 3: Photosynthesis and Respiration 1. What is the Photosynthesis? 2. Where the Photosynthesis Occurs? 3. How Photosynthesis works? – Key Processes 4. Alternative Photosynthesis Pathway Photosynthesis Overview

1. What is the Photosynthesis? Photosynthesis is the biogeochemical process by which Plants cover atmospheric CO2 to carbon productions that results in plant growth  is the conversion of light-energy to chemical-energy via the chloroplasts  generates 170 billion metric tons of sugar annually, which enters our biosphere We express this combined knowledge as the photosynthetic equation: 6CO H 2 O + light energy ---> C 6 H 12 O 6 + 6O 2 + 6H 2 O (or CO 2 + 2H 2 O + light energy ---> CH 2 O+ O 2 + H 2 O)

Photosynthesis takes place within leaf (or green stem) cells containing chloroplasts e.g. each cell contains 40 to 50 chloroplasts a chloroplast has dozens of the thylakoids and millions of pigment molecules each thylakoid contains thousands of photosystems 2. Where the Photosynthesis Occurs?

Photosynthetic Overview A. There are three basic steps in photosynthesis: (1) Light Reactions - energy capture (2) Dark Reactions - fixation of carbon (3) Pigment Regeneration electron replacement from the splitting of H 2 O in oxygenic photosynthesis. 3. How Photosynthesis works? – key processes

C3 photosynthesis ( three-carbon sugars) – C3 plant

(2) Crassulacean Acid Metabolism (CAM) (1) C 4 Photosynthesis 4. Alternative Photosynthetic Pathways Some plants (e.g., corn,sugarcane, and many tropical grass) begin the photosynthetic process by producing a four-carbon compounds. Plants of this type are called C 4 plants (e.g. live in hot and dry environments and are able to photosynthesize at low CO 2 concentration) -Like C4 plant, CAM is an adaptation to hot dry environments -CAM plats utilize both the C 3 and the C 4 pathways of photosynthesis -CO 2 is fixed by PEP (phosphhoenolpyruvate) carboxylase at night to form malic acid which is then is stored in the vacuole of the cell.

Respiration Overview 1. Autotrophic Respiration (Plant Respiration) A. Maintenance Respiration B. Growth Respiration 2. Heterotrophic Respiration (Soil Respiration)

1.Autotrophic Respiration (R a ) R a involves the oxidation of organic substances to CO 2 and water: CH 2 O+ O 2 ---> CO 2 + 2H 2 O +energy Total autotrophic respiration (R a ) consists of two major components: (1) Maintenance Respiration (R m ) (2) Growth Respiration (R s ) (1)Maintenance Respiration: the basal rate of metabolism, includes the energy expended on ion uptake and transfer within plants.

(a) The temperature is most important environmental factor affecting maintenance respiration Protein synthesis rates increase exponentially with increasing temperature (T) (b) The T dependence of R m is expressed in term of the Q 10, or change in rate with a 10ºC rise in temperature: R m (T) = R 0 Q 10 [(T-T0)/10] Where: R 0 is the basal respiration rate at T= 0ºC (or ref. T) Q 10 represents the change in the rate of respiration for 10ºC change in T (about )

(c) using plant nitrogen content Micheal Ryan (1991) derived an empirical relationship based on a wide variety of species and plant tissues: R m = N Where R m is maintenance respiration (moles C per hour), and N is plant nitrogen in moles of N. (see Ryan 1991, Ecological Applications, 1: ). Most of the organic N in plants is in protein and about 60% R m supports protein repair and replacement

(2) Growth Respiration (Construction respiration) (a) R g includes the carbon cost of synthesizing new tissue from glucose and minerals. (b) R g for various tissues differ, depending on the biochemical pathways involved. e.g.: 1 g lignin requires 2.5g of glucose 1 g needles require 1.28 g of glucose 1 g roots require 1.2 g of glucose (C) Growth respiration can be estimated based on construction cost

Estimating Growth respiration (R g ) Growth respiration is generally considered to be independent of temperature and is proportional to GPP: Where r g,i is a growth respiration coefficient for plant component i, and r a,i is the carbon allocation fraction for plant component i. Ryan (1991) used 25% for overall and root growth respiration coefficient. In the TEM (Mellilo et al. 1993), 20% of GPP is applied to growth respiration.

2. Heterotrophic Respiration (R h ) 1) Concept The form of respiration by which organic matter is converted back into CO 2, mainly by soil micro-organisms. 2) Modeling R h Soil respiration (total soil CO 2 flux) including: root respiration and decomposition litter decomposition soil organic matter decomposition a.Linear/nonlinear models b.Moisture and temperature combined models

Linear/nonlinear model Peng and Apps (2000) used the 150 sites published by Raich and Schlesinger (1992) to develop TPAET models for estimating soil respiration (SR) at global scale Group A: Major Natural Ecosystems (n=117) SR = 7.6 exp (0.029T) P AET (R 2 = 0.70) Group B: Croplands (n=19) SR = 0.66P AET – 7.11T– 468 (R 2 = 0.71) Group C: Wetlands (e.g. bogs, mires, and marshes) (n=14) SR = AET – 0.023P T–140 (R 2 = 0.74) Where T is mean annual temperature; P is annual precipitation; AET is actual evapotranspiration (Ref. Peng, C.H. and M.J. Apps J. of Environmental Sciences, 12: )

Moisture and temperature combined models In CENTURY (Parton et al, 1993), the rate of soil C decomposition for each pool (R i ) is expressed as: R i = K i C i M d T d Where i refers to different carbon pools, C i is the C stock for a particular pool, K i is the maximum decomposition rate, and M d and T d are the effects of soil moisture and temperature on decomposition, which are calculated as: M d = 1/ (1+ 4e –6Ws) T d = e 0.07Ts where T s is the soil temperature (ºC ) and W s is the soil water content (%).

Factors affecting respiration 1.Tissue age 2.Temperature 3.Moisture (water stress) 4.Atmospheric CO 2 5.Air pollution (ozone, sulphur dioxide) Reading Reference: Ryan, M Effects of climate change on plant respiration. Ecological Application, 1(2):