Increased Nitrogen Uptake Supports Higher NPP but Not Carbon Storage in CO2-Enriched Sweetgum Trees

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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Progressive Nitrogen Limitation: a tree-based analysis

Oak Ridge Experiment on CO2 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 CO2 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 CO2-enriched trees comes from individual plants or expanding systems that are acquiring increasing amounts of N

Oak Ridge Experiment on CO2 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 CO2-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 CO2 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 CO2-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 CO2 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 • CO2 exposure (542 ppm) started spring, 1998 • Exposure is from sunrise to sunset, April through October • Brookhaven exposure system

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Calculation of NPP Stem Allometry : DM = f(BA, H, taper, density) Coarse root Allometry: DM = f(BA) Fine root Minirhizotrons and in-growth cores Leaf mass and LAI Litter traps and APAR

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Nitrogen Budget Methods • leaf samples from hydraulic lift & litter baskets, • soil cores for soil N and fine root N • increment cores for wood N • throughfall collectors

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Net Primary Productivity • CO2 has consistently stimulated NPP • Average increase is 23% (16-28%) • LAI (~5.5) has not been increasing with time of CO2

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Aboveground Woody Increment • No difference in growth prior to treatment (1997) • CO2 significantly increased growth in 1st year of treatment (33%), but not in subsequent years (5-15%)

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Fine Root Productivity • The increase in NPP is recovered primarily in fine root production • Annual fine root production has more than doubled since the 3rd year of treatment • Mean residence time is 0.6 yr, and is not affected by CO2 • The allocation of the additional C to fast-turnover pool instead of woody biomass has important implications for C and N cycling

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Photosynthesis is higher in elevated CO2…. • ambient • elevated

Oak Ridge Experiment on CO2 Enrichment of Sweetgum …even though foliar N concentration is lower • ambient • elevated

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Hence, photosynthetic N-use efficiency is increased Does the increase in PNUE imply a negative feedback on organic matter production so that C and N requirements stay within relatively constrained proportions? • ambient • elevated

Oak Ridge Experiment on CO2 Enrichment of Sweetgum N distribution in canopy The effect of CO2 on foliar [N] is primarily in the upper canopy. The variation in [N] is largely due to LMA

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Whole-canopy [N] 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 mass Canopy mass has been 11% higher in elevated CO2, so there is no difference in amount of N in the canopy

Oak Ridge Experiment on CO2 Enrichment of Sweetgum N resorption Resorption varied from 50 – 56% and was not affected by CO2 Litter [N] Litter [N] was 10% less in elevated CO2 Litter N mass Litter N mass Leaf litter mass production was increased by CO2, so there was no difference in total N return

Oak Ridge Experiment on CO2 Enrichment of Sweetgum N uptake = wood increment + leaf litter + gross fine root production + foliar leaching • 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Nitrogen uptake 1998 1999 2000 2001 2002 N uptake was 13 to 46% higher (average 28%) in elevated CO2, primarily because of the fine root component

Oak Ridge Experiment on CO2 Enrichment of Sweetgum N uptake and NPP are closely coupled

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Fine root length production N content of fine root production 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Fine root standing crop Root length duration is calculated as the area under the curve of the seasonal progression of root standing crop.

Oak Ridge Experiment on CO2 Enrichment of Sweetgum N uptake increased linearly with root length duration

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 1998 2002 The stimulation of root production by elevated CO2 occurred primarily in deeper soil

Oak Ridge Experiment on CO2 Enrichment of Sweetgum The N content of deeper soil, where roots are proliferating in CO2 enriched plots, is much less than that of surface soil

Oak Ridge Experiment on CO2 Enrichment of Sweetgum CO2 C assimilated by the canopy and N assimilated by the root system are distributed to leaves, fine roots, and wood N

Oak Ridge Experiment on CO2 Enrichment of Sweetgum CO2 In elevated CO2 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum CO2 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum CO2 Future scenario 1 After the deeper and more extensive root system of CO2-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 N N

Oak Ridge Experiment on CO2 Enrichment of Sweetgum CO2 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 N N

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Is there evidence that soil N supply will limit the response to CO2? Is more N being sequestered in CO2-enriched trees?

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Is there evidence that soil N supply will limit the response to CO2? • Is more N being sequestered in CO2-enriched trees? • Except for the first year of the experiment, CO2 enrichment has not significantly increased woody dry matter increment

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Is there evidence that soil N supply will limit the response to CO2? • Is more N being sequestered in CO2-enriched trees? • Except for the first year of the experiment, CO2 enrichment has not significantly increased woody dry matter increment • Unless [N] in wood of CO2 -enriched trees is much higher, total N content of perennial tissue should be increasing at similar rates in ambient and elevated CO2

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Is there evidence that soil N supply will limit the response to CO2? • Is more N being sequestered in CO2-enriched trees? • Except for the first year of the experiment, CO2 enrichment has not significantly increased woody dry matter increment • Unless [N] in wood of CO2 -enriched trees is much higher, total N content of perennial tissue should be increasing at similar rates in ambient and elevated CO2 • If the small (6%), non-significant increase in annual dry matter increment is sustained, these conclusions could change

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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?

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 on microbial N cycling

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 on microbial N cycling • We have not made enough measurements in deeper soil where fine roots are proliferating

Oak Ridge Experiment on CO2 Enrichment of Sweetgum Conclusions • The hypothesis that full soil occupancy by roots in a non-expanding system would constrain nutrient uptake was not supported

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 is keeping pace with NPP and is not providing a negative feedback on physiological processes

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 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 CO2 but are not being sequestered

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 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 CO2 but are not being sequestered • There is not currently evidence for progressive N limitation in this forest stand after 5 years of CO2 enrichment

Oak Ridge Experiment on CO2 Enrichment of Sweetgum 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 CO2 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 CO2 but are not being sequestered • There is not currently evidence for progressive N limitation in this forest stand after 5 years of CO2 enrichment • Longer term responses will depend on how leaf and fine root N is cycled through the soil

Increased Nitrogen Uptake Supports Higher NPP but Not Carbon Storage in CO2-Enriched Sweetgum Trees