Nitrogen and CO₂ Manipulation Experiments

Braunschweig

Contact:  Hans Weigel

Email: hans.weigel@fal.de

Site Location:  Braunschweig, Germany

Latitude:  52.3 decimal degrees

Longitude:  10.43 decimal degrees

Mean Annual Temperature:  8.8 degrees Celsius  

Mean Annual Precipitation: 618 mm 

Biome:  Agricultural Crops

Experimental Manipulation: 

- ambient air and elevation of atmospheric CO₂ concentration to a target concentration of 550 ppm

- CO₂ is isotopically labelled (depleted in ¹³C)

- Split-plot design. addition of 100% and 50% N supply, fertilizer input on average 150 kg N/ha^-1a^-1 with N=100% supply

Technology:  FACE system engineered by Brookhaven National Laboratory, New York, USA (Lewin et al, 1992, Crit. Rev. Plant Sci. 11: 135-141

Start Date:  September 1999 

End Date:   August 2005

Key Research Questions:

- to evaluate C turnover at different scales in time and space within one representative arable agroecosystem

- to assess and quantify atmosphere-plant-soil exchange of relevant C species

- to assess and quantify atmosphere-plant-soil exchange of nutrient elements and water under current and future atmospheric CO₂ concentrations

- to improve the assessment of effects of changing atmospheric CO₂ concentrations on crop growth

- to link soil biological processes to the CO₂ exchange at the atmosphere-plant-interface

Key Findings:

Web Site: 

Braunschweig Publications

Cedar Creek

Contact:  Dan Bahauddin

Email: bahau001@gold.tc.umn.edu

Site Location:  Minnesota, USA

Latitude:  45.4 decimal degrees

Longitude:  -93.2 decimal degrees

Mean Annual Temperature:  

Mean Annual Precipitation:  

Biome:  Grassland

Experimental Manipulation:  550 ppm  

Technology:

Start Date: 

End Date:   

Key Research Questions:

Key Findings:

Web Site: 

Cedar Creek Publications

Duke Forest FACE Experiment: Forest Atmosphere Carbon Transfer and Storage (FACTS-I)

Contact:  Ram Oren

Email:  ramoren@duke.edu

Site Location:  Chapel Hill, North Carolina, USA

Latitude:  35.97 decimal degrees

Longitude:  -79.08 decimal degrees

Mean Annual Temperature:  15.5 degrees Celsius

Mean Annual Precipitation:  1117.6 mm

Biome:  Loblolly Pine (Pinus taeda) plantation in an area of Pine,  Pine-Hardwood and Oak-Hickory upland hardwoods area.

Experimental Manipulation:  Elevated [CO₂], elevated soil N, elevated [CO₂] + soil N

CO₂ enrichment: ambient [CO₂] plus 200 µmol mol^-1 N enrichment: 11.2 g N m^-2 y^-

Application Frequency:  5.6 g N m^-2 y^-1applied in March; 5.6 g N m^-2 y^-1applied in April

Technology:  BNL-designed FACE equipment - the system provides spatial and temporal control of [CO₂]. Each system uses feedback control technology to control [CO₂] in a 30 m diameter forest plot that is over 16 m tall, while monitoring the 3D plot volume to characterize the whole-stand CO₂ regime achieved during enrichment.

Start Date:  Prototype: June 1994; additional 3 plots: August, 1996

End Date:  Ongoing

Key Research Questions:  The project’s goal is to improve our understanding of the effect of spatial variation in nitrogen availability (both native and imposed) and temporal variation in climate (specifically in temperature, growing season length, and soil moisture) on the quantity of the extra carbon fixed under elevated CO₂ (e-[CO₂]) and its allocation among pools of differing longevity. The specific objectives are to quantify (1) how e-[CO₂] affects C assimilation under varying levels of nitrogen and water supply, (2) how the partitioning of extra photosynthate produced under e-[CO₂] is affected by varying nitrogen and water supply, (3) whether e-[CO₂] affects the residence time of C within different pools, and (4) the effect of e-[CO₂] on spatial and temporal variation in soil-N cycling.

Key Findings:  

Aboveground Physiology:

1. We continue to find photosynthetic enhancement to long-term elevated CO2 exposure in the upper canopy of Pinus taeda. Enhancement was > 30% across seasons and needle age classes, with an effect of N on the magnitude of this enhancement. Photosynthetic capacity per unit N of leaves growing in long-term elevated CO2 was reduced in one-year-old pine needles, evidence of significant downregulation in elevated CO2 in recent years (2004-06). However, N addition restored photosynthetic capacity and the relationship between Vcmax and N, even in elevated CO2. The data show that photosynthetic downregulation occurs in long-term elevated CO2 and is alleviated by N addition (Crous et al., in review).

2. Nitrogen fertilization significantly increased Nmass of Liquidambar styraciflua (sweetgum) sun leaves by 35% in the control CO2 plots and by 7% in the elevated CO2 plots. Fertilization stimulated Narea by about 9% regardless of CO2 treatment. Elevated CO2 stimulated photosynthesis (Asat) by 144% in the unfertilized plots and by 71% in fertilized treatments.

3. Averaged across four understory hardwoods species (Acer rubrum, Carya glabra, Ulmus alata, and Liquidambar styraciflua), foliar N concentrations on a mass basis (Nmass) and on an area basis (Narea) both increased by 17% in response to N fertilization. Elevated CO2 stimulated light-saturated photosynthesis by 41% when averaged across all species and N fertilization stimulated photosynthesis by 13%.  The effects of CO2 and N fertilization on photosynthesis were additive; we found no interactions between CO2 and N on photosynthesis of these tree species. Elevated CO2 had no significant effect on photosynthetic capacity, but nitrogen fertilization stimulated photosynthetic capacity a small amount by increasing carboxylation efficiency by 5% and electron transport efficiency by 3%. 

Aboveground Production and Vegetation Dynamics:

4. Leaf area index increased with elevated CO2, nutrient addition, and even more where supplies of both resources increased. The variation in annual aboveground net primary production was entirely dependent on the variation leaf area index, varying spatially with native nitrogen availability and inter-annually with water availability in addition to treatment effects. The fraction of production partitioned to wood decreased with leaf area index and increased with CO2 at a given leaf area index, but because CO2 caused an increase in leaf area index, the fraction was similar under ambient and elevated CO2 (McCarthy et al. 2006). 

5. Elevated CO2 increased the basal area increment (BAI) of trees by 13–27% (Moore et al. 2006). In most years, exposure to elevated CO2 increased the growth rate but not the duration of the active growth period.  The interannual variation in the relative enhancement of BAI caused by elevated CO2 was strongly related to temperature and rainfall, and was greatest in years with high vapor pressure deficit. There was no evidence of a systematic reduction in the stimulation of growth during the first 8 years of this experiment, suggesting that the hypothesized limitation of the CO2 response caused by nitrogen availability has yet to occur.

6. Trees grown in high–CO2 plots first began producing pollen while younger and at smaller sizes relative to ambient grown trees.  Pollen cone and airborne pollen grain abundances were significantly greater in the fumigated stands.  We conclude that the greater number of matured trees in high–CO2 plots resulted in greater pollen production at the stand level.  Precocious pollen production has important implications for fertilization and pollen dispersal from young, dense stands.  Increasing levels of airborne pollen raises concerns for escalating rates of human respiratory disease (LaDeau and Clark 2006a).  

7. Trees growing under elevated CO2 matured earlier and produced more seeds and cones per unit basal area than ambient grown trees.  By 2004, trees grown in high–CO2 had produced an average 300 more seeds per tree than ambient grown trees.  Although there was a trend toward decreasing mean CO2 effect (difference in fecundity between elevated and ambient treatments) over time, the hierarchical Bayes analysis indicates that this decrease comes from the emergence of a few highly fecund ambient grown trees by 2002, rather than acclimation or downregulation among the fumigated trees. Although biomass responses can sometimes be large, the increase in fecundity can have long-term impacts on forest dynamics that transcend the current generation (LaDeau and Clark 2006b).

8. CO2 enhancements of growth and survival in understory plants were not large in comparison to variation among individual plants.  There was no tendency for shade-intolerant species to preferentially benefit from elevated CO2.  Because small differences in annual growth rates accumulated over time, the modest CO2 responses could still have long term impacts (Mohan et al. 2007).

 9. Poison ivy grows 149% more, and chemically shifts to produce a more poisonous form of urushiol, under elevated atmospheric CO2.  (Mohan et al. 2006) This paper received over 130,000 listings on Google News.com; covered in media sources including the U.S., Canada, Europe, India, Japan, and Iran).

10. The increase in CO2 efflux from trees grown in the elevated treatment was explained by the higher growth rates and higher soil CO2 concentrations found at the site, and was not a direct effect of elevated CO2 per se.  Previous studies may have over estimated autotrophic respiration in forests exposed to elevated CO2 (Moore et al. 2007). 

11. Lower digestibility of foliage, greater protein precipitation capacity in frass, and lower nitrogen concentration of larvae indicate that growth under elevated CO2 reduced the food quality of oak leaves for caterpillars (Knepp et al. 2007). By subtly altering aspects of leaf chemistry, the ever-increasing concentration of CO2 in the atmosphere will change the trophic dynamics in forest ecosystems.

12. The depression of photosynthesis detected by fluorescence caused by fungal infections and galls extended >2.5 times further from the visible damage and was 40% more depressed than chewing damage caused by insects. Areas of depressed photosynthesis around fungal infections on oaks growing in elevated CO2 were more than 5 times larger than those grown in ambient conditions, suggesting that this element of global change may influence the indirect effects of biotic damage on photosynthesis (Aldea et al. 2006).

Belowground allocation:

13. We used first principles and published data from four FACE experiments on forest tree species to conceptualize the total allocation of C to belowground (TBCA) under current [CO2], and to predict the likely effect of elevated [CO2] (Palmroth et al. 2006). We showed that at a Duke FACE site where leaf area index (L) was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary production, were accompanied by opposite changes in TBCA. A similar pattern emerged when data was combined from the four FACE experiments, using leaf area duration (LD) to account for differences in growing-season length.

14. Elevated [CO2]-induced enhancement of TBCA in the combined data decreased from 50% (700 g C m-2 y-1) at the lowest LD to ~30% (200 g C m-2 y-1) at the highest LD. The consistency of the trend in TBCA with L and its response to [CO2] across the sites provides a norm for predictions of ecosystem carbon cycling, and is particularly useful for models that use L to estimate components of the terrestrial carbon balance.

15. This model provides a tool for generating caps for TBCA and its enhancement under elevated [CO2]. We showed that both vary with commonly available ecosystem quantities, leaf area index and net aboveground primary productivity, which themselves vary spatially with N availability and temporally with weather conditions.

16. Increases in root biomass and soil respiration averaged 30% at high CO2 during the past two years. Increases at 2-m depth were smaller, only 18% higher for elevated CO2 rings in the tenth year, but the CO2 effect is slowly but steadily increasing in this deepest layer. Overall, the effect of CO2 on root biomass and carbon cycling is the greatest in summer, remains consistently higher in elevated CO2 compared to ambient CO2, and shows no sign of disappearing or diminishing thus far in the experiment.  In fact the CO2 response for some variables is still increasing after the first decade of exposure.

17. Intact amino acid (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. Yet we found no evidence supporting increased recovery of free amino acids in fine roots under elevated CO2, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency (Hofmockel et al. 2007).

Web Site:  http://face.env.duke.edu

FACTS-I Publications