Xin Xi March 13, 2008

Basis 1. Photosynthesis (gross photosynthesis minus photorespiration) C3/C4/ CAM (Crassulacean Acid Metabolism) 2. Ecosystem Respiration Autotrophic (Plant) Respiration Heterotrophic (Soil) Respiration 3. Terrestrial carbon budget concepts Gross Primary Production (GPP) Net Primary Production (NPP) Net Ecosystem Production/Exchange (NEP/NEE) Net Biome Production/Exchange (NBP/NBE)

Rubisco Solar visible radiation  temporary chemical energy (ATP+NADPH)  permanent chemical energy (sugar)

Acronyms Mes: mesophyll cell; BS: bundle sheath cell; Rmi: mitochondrial respiration; C3 Ps: C3 carbon fixation (CO2+RuBP  2C3); C4 Ps: C4 carbon fixation (CO2+PEP  C4 acid);

C4 vs. C3: (1). C4 acid is moved to bundle sheath cells and releases CO2 around Rubisco, thus increasing the CO2 concentration and fixation rate; Photorespiration is low; high efficiency of Rubisco reduces the demand for Nitrogen, and makes the plants survive in warm-dry environment. (2). PEP captures CO2 and increases the CO2 concentration gradient between external air and inside leaf, thus enhancing CO2 absorption with smaller stomata and less water loss. (3). For each CO2 fixation, 2 ATPs are needed to generate one PEP, causing a 30% increase in energy requirement. C4 vs. CAM: (1). CAM fix CO2 in C4 acid at night and release it at daytime for the normal C3 photosynthesis pathway. So it can close the stomata at daytime avoiding water loss in very dry environment (e.g, pineapple). (2). CAM has a temporal separation of C3 and C4 fixation, while C4 has a spatial separation of C3 and C4 fixation between bundle sheath and mesophyll cells. NOTE: plants can adapt to the environment and change their photosynthesis paths.

2. Respiration Ecosystem Res. (Reco) = Autotrophic Res. (Ra) + Heterotrophic Res. (Rh) Autotrophic (Plant) Respiration: carbon lost by plant itself (biomass above and below ground) CH2O + O2  CO2 + H2O + energy Maintenance respiration: basal rate of plant metabolism; Temperature Growth respiration: use carbon to synthesize new tissues Heterotrophic (Soil) Respiration: carbon lost by animals (herbivore) and soil micro-organisms, which convert organic matter back to CO2.

3. Terrestrial carbon budget concepts Gross Primary Production (GPP): the total amount of carbon fixed by photosynthesis in an ecosystem. total global GPP estimate: 120 Gt C per yr Net Primary Production (NPP): the net production of organic carbon by plants in an ecosystem; equals GPP minus the amount of carbon consumed by plants in autotrophic respiration, Ra NPP = GPP – Ra total global NPP estimate: 60 Gt C per yr

Net Ecosystem Production/Exchange (NEP/NEE): the net accumulation of carbon by the ecosystem; equals NPP minus carbon loss due to heterotrophic respiration, Rh: NEE = NEP = NPP – Rh or NEE = GPP – Reco total global NEE estimate: 10 Gt C per yr Net Biome Production/Exchange (NBP/NBE): the net production of organic matter in the ecosystem after episodic carbon loss due to non-respiratory processes (natural or anthropogenic disturbance), e.g., fire, insect, harvest, clearance, etc NBE = NBP = NEE – non-respiratory C loss total global NBE estimate: 0.2GtC per yr ( ) ; 1.4GtC per yr ( )

Background Previous studies: The ecosystem can switch from a net sink in non-drought year to a net source in drought year (Meyers, 2001) Carbon assimilation is also closely related to the length of dry season (House&Hall, 2001) or the timing of rain events (Hunt etal, 2004) …… Study aim: To use CO2 covariance measurements in two contrasting hydrological years (dry vs. normal) to study how climate variability (rainfall) affect the interannual and seasonal change in NEE (GPP – Reco). Study region: a grazed Mediterranean grassland in south Portugal Mediterranean grasslands are typically dominated by C3 grasses and legumes, which are active in wet period and terminated by the start of dry season. C4 plant (warm-season perennial grass) can invade into these community, especially when climate warms. Most of the precipitation occurs in autumn, with a negative anomaly in late winter and spring.

Data analysis: Field measurements: CO2 flux; wind velocity; sonic temperature; air temperature; relative humidity; photosynthetic photon flux density (PPFD); radiation flux; soil temperature; soil volumetric water content; precipitation; LAI CO2 flux (half-hourly): time mean variance between fluctuations in vertical wind speed and CO2 concentration. Negative NEE value means net carbon sink. The data is further processed in quality control (filtering), gap-filling and flux-partitioning procedure. See paper for details. The partitioning of NEE into GPP and Reco is based on Reichstein et al, 2005

Hyperbola fitting of NEE and PPFD (photosynthetic photon flux density): Fitting between night-time Reco (or night-time NEE) and soil temperature: The temperature sensitivity coefficient: Water-use efficiency: the ratio of daily-integrated GPP to the daily-integrated ET, on a daily basis. Light-use efficiency: the ratio of daily-integrated GPP to the daily-integrated PPFD, on a daily basis.

Mean air temperature = 14.7°C (2005); 14.5°C (2006) 2005 dry2006 normal drought Solar shortwave and air temp. Results

Total rainfall: 364mm (2005); 751mm (2006) averaged LAI (growth period): 0.4 (2005); 2.5 (2006); LAI reduction by grazing: 0.8 in winter and less in peak growth; C3 senescence at 132 DOY Clay layer C3 senescence 132 Growth of C4 with rainfall Cutting grazing

Daily-integrated NEE, GPP and Reco Germination&growth due to rainfall drought Reco > GPP 1.Birch effect: immediate carbon release from water-stressed soil due to rainfall, because of the quick activation of soil microbial respiration. 2.In 2005, Reco is larger than GPP with time and NEE is slightly positive, showing that the warm- season C4 plant fails to compensate carbon loss via plant and microbial respiration. 3.In 2006, quick growth of C4 plant due to strong rainfall (163 DOY) switch the ecosystem into a net carbon sink (negative NEE). C3 senescence Birch Effect grazing sourcesink

1.GPP positively correlates with precipitation. 2.GPP also positively correlates with ecosystem respiration Reco. 3.However, precipitation has a larger impact on GPP than Reco, so it’s a net carbon source in 2005 (49gCm -2 ) and a net carbon sink in 2006 (-190gCm -2 ). 4.This contrast study supports that the total amount of precipitation is the main factor in determining the interannual variation of NEE.

1.Water-use efficiency: reaches maximum in early winter with small ET, and minimum in late summer with large ET. 2.Light-use efficiency: with similar shortwave flux, LUE depends on the difference of photosynthesis rate, which in turn is determined by many factors, e.g., air temperature, humidity, wind speed, soil water, nitrogen, CO2 concentration, diffuse radiation fraction, etc. (Gu et al, 2002)

NEE - PPFD 1.The soil moisture and LAI are the main factors in determining the inter-annual variability. 2.More than 90% of the variations of NEE is explained by the changes in PPFD (quadratic polynomial fitting). 3.The low NEE in the late winter (DOY70-79) in 2005 results from a reduction of photosynthesis due to midday stomatal closure at high irradiance, temperature and vapor pressure deficit (VPD). Large rainfall& LAI C4 regrowth in summer drought

NEE/GPP - LAI 1.GPP and NEE responses linearly to changes in LAI, which explains 77% and 88% of the variations in NEE and GPP. Similar results are found on other studies.

Reco - soil temperature 1.Reco is affected by temperature, as well as soil moisture and substrate availability. 2.With similar LAI and soil moisture, the temperature sensitivity coefficient (Q) decreases from 2.3 in the growing season to 1.22 in the dry summer. Q=2.3 Q=1.22

Reco - GPP 1.Daily Reco is strongly correlated with GPP, which explained 85% of variations in Reco in the growing season, and 77% in the dry summer with only C4 grass. 2.This shows that that the canopy photosynthesis is the best indicator of Reco by controlling the substrate availability for autotrophic and heterotrophic respirations. growing season dry summer

Grazing - NEE 1.Grazing reduces LAI, thus affecting NEE. 2.The grazing on DOY349 reduces NEE from about 9 (DOY347) to 6 (DOY350) umolCO2m -2 s The irregular pattern of NEE on DOY 349 is caused by sheep respiration. 4.Although grazing has a negative effect on NEE in short term, its impact on NEE relative to non- grazed region is unknown. This depends on how grazing affects the processes that control Reco and GPP.

Conclusions 1.In the dry year with negative rainfall anomaly in winter and early spring, the maximum NEE is about half of that in normal year. The light-use and water-use efficiencies are also reduced by about 50%. 2.After the senescence of C3 grasses, the regrowth of C4 grass accounts for 23% and 21% of the GPP in the dry and normal years, respectively. But, C4 grass essentially converts the ecosystem into a net carbon sink in the normal year, after its fast growth due to the strong rainfall. 3.For the two contrasting years, total precipitation is the determinant of the interannual variability of NEE. The grassland loses 49gCm -2 in the dry year and captures 190gCm -2 in the normal year. 4.For this grassland, NEE and GPP are strongly related to PPFD in short- term time scales, and LAI in long-term time scales. The variation of Reco is mainly controlled by canopy photosynthesis. 5.Grazing event reduces the development of LAI and biomass, and negatively affect NEE.

Critical thinking Can the measurements from a single flux tower represent the entire study region?