# Presentation: 526716

## Increased nitrogen uptake supports higher NPP but not carbon storage in CO2-enriched sweetgum trees

**Richard J. Norby, Sara S. Jawdy, Joanne Ledford, Johnna D. Sholtis, and Dale W. Johnson**

**Environmental Sciences Division **
**Oak Ridge National Laboratory**
**Oak Ridge, Tennessee**

**Research supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research Program**

## Progressive Nitrogen Limitation: 

**a tree-based analysis**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

## Progressive Nitrogen Limitation: 

**a tree-based analysis**

**Progressive N limitation may occur through impacts on relative N availability in soil, but the feedbacks start and end with the physiological responses of plants**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

## Progressive Nitrogen Limitation: 

**a tree-based analysis**

**Progressive N limitation may occur through impacts on relative N availability in soil, but the feedbacks start and end with the physiological responses of plants**
**The vast majority of data on C-N interactions in CO****2****-enriched trees comes from individual plants or expanding systems that are acquiring increasing amounts of N**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

## Progressive Nitrogen Limitation: 

**a tree-based analysis**

**Progressive N limitation may occur through impacts on relative N availability in soil, but the feedbacks start and end with the physiological responses of plants**
**The vast majority of data on C-N interactions in CO****2****-enriched trees comes from individual plants or expanding systems that are acquiring increasing amounts of N**
**Non-expanding forests, however, recycle most of their N, and this feature cannot be simulated in short-term studies with seedlings and saplings**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

## Progressive Nitrogen Limitation: 

**a tree-based analysis**

**Progressive N limitation may occur through impacts on relative N availability in soil, but the feedbacks start and end with the physiological responses of plants**
**The vast majority of data on C-N interactions in CO****2****-enriched trees comes from individual plants or expanding systems that are acquiring increasing amounts of N**
**Non-expanding forests, however, recycle most of their N, and this feature cannot be simulated in short-term studies with seedlings and saplings**
**FACE experiments in forests provide an opportunity to test the concepts of progressive N limitation in a realistic system**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

## Oak Ridge Experiment on CO2 Enrichment of Sweetgum

**Oak Ridge Experiment on**** CO****2**** Enrichment of Sweetgum**

***Liquidambar styraciflua***** monoculture plantation started in 1988**
**the closed canopy constrains growth responses**
**full occupancy of the soil by the root system constrains the nutrient cycle**
**tree growth is in a linear phase**

## Experimental Design

**Experimental Design**

***Liquidambar styraciflua***** plantation started in 1988**
**2 elevated, 3 control plots (2 with blowers)**

**Each plot is 25 m diameter (20 m diameter inside buffer) **
**Full year of pre-treatment measurement in 1997**
**CO****2**** exposure (542 ppm) started spring, 1998**
**Exposure is from sunrise to sunset, April through October**
**Brookhaven exposure system**

## Calculation of NPP

- <date/time>

**Calculation of NPP**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

**Stem**
**Allometry : DM = f(BA, H, taper, density)**

**Fine root**
**Minirhizotrons and in-growth cores**

**Coarse root**
**Allometry: DM = f(BA)**

**Leaf mass and LAI**
**Litter traps and APAR**

## Nitrogen Budget Methods

- <date/time>

**Nitrogen Budget Methods**

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

**leaf samples from  hydraulic lift & litter baskets, **
**soil cores for soil N and fine root N**
**increment cores for wood N**
**throughfall collectors**

## Net Primary Productivity

**Oak Ridge Experiment on CO****2**** Enrichment of Sweetgum**

**CO****2**** has consistently stimulated NPP **
**Average increase is 23% (16-28%)**
**LAI (~5.5) has not been increasing with time of CO****2**

**Net Primary Productivity**

## Aboveground Woody Increment

**No difference in growth prior to treatment (1997)**
**CO****2**** significantly increased growth in 1st year of treatment (33%), but not in subsequent years (5-15%)**

**Aboveground Woody Increment**

## Fine Root Productivity

**The increase in NPP is recovered primarily in fine root production**
**Annual fine root production has more than doubled since the 3****rd**** year of treatment**
**Mean residence time is 0.6 yr, and is not affected by CO****2**
**The allocation of the additional C to fast-turnover pool instead of woody biomass has important implications for C and N cycling **

**Fine Root Productivity**

## Photosynthesis is higher in elevated CO2....

**Photosynthesis is higher in elevated CO****2****....**

**elevated**

**ambient**

## ...even though foliar N concentration is lower

**elevated**

**ambient**

## Does the increase in PNUE imply a negative feedback on organic matter production so that C and N requirements stay within relatively constrained proportions?

**elevated**

**ambient**

**Hence, photosynthetic N-use efficiency is increased**

## The effect of CO2 on foliar [N] is primarily in the upper canopy. The variation in [N] is largely due to LMA

**N distribution in canopy**

## Whole-canopy N concentration has averaged 16.6 mg/g in ambient CO2, and is 8% less (15.3 mg/g) in elevated CO2

**Whole-canopy N concentration has averaged 16.6 mg/g in ambient CO****2****, and is 8% less (15.3 mg/g) in elevated CO****2**

**Canopy mass has been 11% higher in elevated CO****2****, so there is no difference in amount of N in the canopy**

**Whole-canopy [N]**

**Whole-canopy N mass**

## Leaf litter mass production was increased by CO2, so there was no difference in total N return

**Litter N mass**

**Litter [N] was 10% less in elevated CO****2**

**Resorption varied from 50 – 56% and was not affected by CO****2**

**N resorption**

**Litter [N] **

**Litter N mass**

## Wood increment assumes constant [N] in wood

**Assume no resorption from fine roots prior to abscission**
**Foliar leaching = throughfall – deposition**
**Assume constant value for leaching**

**N uptake = wood increment **
**	****+ leaf litter **
**	****+ gross fine root production      **
**	****+ foliar leaching**** **

## Nitrogen uptake

**1998         1999         2000         2001         2002**

**N uptake was 13 to 46% higher (average 28%) in elevated CO****2****, primarily because of the fine root component**

**Nitrogen uptake**

## N uptake and NPP are closely coupled

## Fine root production is significantly higher in elevated CO2. Fine root [N] was not significantly affected, so total annual uptake into fine roots was much higher

**Fine root production is significantly higher in elevated CO****2****. Fine root [N] was not significantly affected, so total annual uptake into fine roots was much higher**

**Fine root length production**

**N content of fine root production**

## Fine root standing crop

**Fine root standing crop**

**Root length duration is calculated as the area under the curve of the seasonal progression of root standing crop. **

## N uptake increased linearly with root length duration

## At the beginning of the experiment, >80% of the fine roots were in the upper 30 cm of soil, and <5% were at 45-60 cm depth 

**The stimulation of root production by elevated CO****2**** occurred primarily in deeper soil**

**1998**

**2002**

## The N content of deeper soil, where roots are proliferating in CO2 enriched plots, is much less than that of surface soil

**The N content of deeper soil, where roots are proliferating in CO****2**** enriched plots, is much less than that of surface soil**

## N

**N**

**CO****2**

**C assimilated by the canopy and N assimilated by the root system are distributed to leaves, fine roots, and wood**** **

## N

**N**

**CO****2**

**In elevated CO****2**** more C is assimilated by the canopy and most of the extra C is allocated to root production, especially in deeper soil. **
**Some of the extra C also stays in leaves, but little is allocated to wood**

## N

**N**

**CO****2**

**N**

**The additional root deployment supports (and requires) uptake of more N **
**The additional N stays in the root system; it is not allocated to increased leaf or wood production**
**The additional N is returned to the soil as the fine root crop turns over**

## N

**N**

**CO****2**

**N**

**After the deeper and more extensive root system of CO****2****-enriched trees is fully established, the additional N uptake is more than sufficient for the increased fine root production**
**More N is available for other processes, and C and N are allocated to increased wood production**

**Future scenario 1**

## CO2

**N**

**CO****2**

**N**

**Future scenario 2**

**Soil N availability declines as more N is sequestered into resistant SOM pools, especially in deeper soil**
**Less N is available to support additional C fixation**

## Is there evidence that soil N supply will limit the response to CO2?

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**Except for the first year of the experiment, CO****2**** enrichment has not significantly increased woody dry matter increment**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**Except for the first year of the experiment, CO****2**** enrichment has not significantly increased woody dry matter increment**
**Unless [N] in wood of CO****2**** -enriched trees is much higher, total N content of perennial tissue should be increasing at similar rates in ambient and elevated CO****2**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**Except for the first year of the experiment, CO****2**** enrichment has not significantly increased woody dry matter increment**
**Unless [N] in wood of CO****2**** -enriched trees is much higher, total N content of perennial tissue should be increasing at similar rates in ambient and elevated CO****2**
**If the small (6%), non-significant increase in annual dry matter increment is sustained, these conclusions could change**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**What is the fate of the N in leaves and fine roots?**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**What is the fate of the N in leaves and fine roots?**
**Similar amounts of N returned to the soil in leaf litter, and litter decomposition is rapid**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**What is the fate of the N in leaves and fine roots?**
**Similar amounts of N returned to the soil in leaf litter, and litter decomposition is rapid**
**The fine root population turns over rapidly; mean residence time < 1 year**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**What is the fate of the N in leaves and fine roots?**
**Similar amounts of N returned to the soil in leaf litter, and litter decomposition is rapid**
**The fine root population turns over rapidly; mean residence time < 1 year**
**Fine root decomposition rate is apparently not affected by CO****2**

**Is there evidence that soil N supply will limit the response to CO****2****?**

## Is there evidence that soil N supply will limit the response to CO2?

**Soil C and N sequestration (Jastrow)**
**Soil C is increasing, particularly in microaggregates that facilitates movement of C into pools with long residence time**
**N in protected pools is increasing as well, but at a lesser rate**

## Is there evidence that soil N supply will limit the response to CO2?

**Soil C and N sequestration (Jastrow)**
**Soil C is increasing, particularly in microaggregates that facilitates movement of C into pools with long residence time**
**N in protected pools is increasing as well, but at a lesser rate**
**N availability (Zak)**
**No effect of CO****2**** on microbial N cycling**

## Is there evidence that soil N supply will limit the response to CO2?

**Soil C and N sequestration (Jastrow)**
**Soil C is increasing, particularly in microaggregates that facilitates movement of C into pools with long residence time**
**N in protected pools is increasing as well, but at a lesser rate**
**N availability (Zak)**
**No effect of CO****2**** on microbial N cycling**
**We have not made enough measurements in deeper soil where fine roots are proliferating**

## Conclusions

**The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported**

## Conclusions

**The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported**
**N uptake in elevated CO****2**** is keeping pace with NPP and is not providing a negative feedback on physiological processes**

## Conclusions

**The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported**
**N uptake in elevated CO****2**** is keeping pace with NPP and is not providing a negative feedback on physiological processes**
**C and N are cycling through the trees faster in elevated CO****2**** but are not being sequestered**

## Conclusions

**The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported**
**N uptake in elevated CO****2**** is keeping pace with NPP and is not providing a negative feedback on physiological processes**
**C and N are cycling through the trees faster in elevated CO****2**** but are not being sequestered**
**There is not currently evidence for progressive N limitation in this forest stand after 5 years of CO****2**** enrichment**

## Conclusions

**The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported**
**N uptake in elevated CO****2**** is keeping pace with NPP and is not providing a negative feedback on physiological processes**
**C and N are cycling through the trees faster in elevated CO****2**** but are not being sequestered**
**There is not currently evidence for progressive N limitation in this forest stand after 5 years of CO****2**** enrichment**
**Longer term responses will depend on how leaf and fine root N is cycled through the soil**