Abstract
Atmospheric nitrogen (N) deposition can increase forest growth. Because N deposition commonly increases foliar N concentrations, it is thought that this increase in forest growth is a consequence of enhanced leaf-level photosynthesis. However, tests of this mechanism have been infrequent, and increases in photosynthesis have not been consistently observed in mature forests subject to chronic N deposition. In four mature northern hardwood forests in the north-central United States, chronic N additions (30 kg N ha(-1) yr(-1) as NaNO3 for 14 years) have increased aboveground growth but have not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increases in growth, we hypothesized that the NO3(-) additions increased foliar N concentrations and leaf-level photosynthesis in the dominant species in these forests (sugar maple, Acer saccharum). The NO3(-) additions significantly increased foliar N. However, there was no significant difference between the ambient and +NO3(-) treatments in two seasons (2006-2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (micromol CO2 s(-1) g(-1) N) was significantly decreased (-13%) by NO3(-) additions. Furthermore, we found no consistent NO3(-) effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for delta 13C and delta 18O, isotopes that can be used together to integrate changes in photosynthesis over time. We observed a small but significant NO3(-) effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number. These photosynthesis and leaf area data together suggest that NO3(-) additions have not stimulated photosynthesis. There is no evidence that nutrient deficiencies have developed at these sites, so unlike other studies of photosynthesis in N-saturated forests, we cannot attribute the lack of a stimulation of photosynthesis to nutrient limitations. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation.
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Claims 7
Claim · model-written summary
N additions increased aboveground growth.
- Taxon
- sugar maple
Acer saccharume1 - Treatment
- chronic N additions (30 kg N ha(-1) yr(-1) as NaNO3 for 14 years)e0
- Outcome
- aboveground growthe0
- Direction
- increasede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“chronic N additions (30 kg N ha(-1) yr(-1) as NaNO3 for 14 years) have increased aboveground growth”
e1“sugar maple, Acer saccharum”
Claim · model-written summary
N additions did not affect canopy leaf biomass or leaf area index.
- Taxon
- sugar mapleAcer saccharumt0
- Treatment
- NO3(-) additionse1
- Outcome
- canopy leaf biomass or leaf area indexe0
- Direction
- have not affectede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“have not affected canopy leaf biomass or leaf area index.”
t0“sugar maple”
e1“The NO3(-) additions significantly increased foliar N. However, there was no significant difference between the ambient and +NO3(-) treatments in two seasons (2006-2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (micromol CO2 s(-1) g(-1) N) was significantly decreased (-13%) by NO3(-) additions. Furthermore, we found no consistent NO3(-) effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for delta 13C and delta 18O, isotopes that can be used together to integrate changes in photosynthesis over time.”
Claim · model-written summary
Nitrate additions significantly increased foliar N.
- Taxon
- sugar mapleAcer saccharume1
- Treatment
- NO3(-) additionse0
- Outcome
- foliar Ne0
- Direction
- increasede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The NO3(-) additions significantly increased foliar N.”
e1“sugar maple, Acer saccharum”
Claim · model-written summary
Photosynthesis did not differ significantly between ambient and nitrate treatments.
- Taxon
- sugar mapleAcer saccharume1
- Treatment
- +NO3(-) treatmentse0
- Outcome
- photosynthesise0
- Direction
- no significant differencee0
- Comparator
- ambiente0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“there was no significant difference between the ambient and +NO3(-) treatments in two seasons (2006-2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches.”
e1“sugar maple, Acer saccharum”
Claim · model-written summary
Nitrate reduced photosynthetic nitrogen use efficiency by 13%.
- Taxon
- sugar mapleAcer saccharume1
- Treatment
- NO3(-) additionse0
- Outcome
- photosynthetic nitrogen use efficiencye0
- Direction
- decreasede0
- Value
- -13%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“photosynthetic nitrogen use efficiency (micromol CO2 s(-1) g(-1) N) was significantly decreased (-13%) by NO3(-) additions.”
e1“sugar maple, Acer saccharum”
Claim · model-written summary
No consistent nitrate effect was detected in foliage or litter isotope measures.
- Taxon
- sugar mapleAcer saccharume1
- Treatment
- NO3(-)e0
- Outcome
- delta 13C and delta 18Oe0
- Direction
- no consistent NO3(-) effecte0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“no consistent NO3(-) effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for delta 13C and delta 18O”
e1“sugar maple, Acer saccharum”
Claim · model-written summary
Nitrate affected individual leaf area and mass, varying by site.
- Taxon
- sugar mapleAcer saccharume1
- Treatment
- NO3(-)e0
- Outcome
- average area and mass of individual leavese0
- Qualifier
- varied by sitee0
Not reportedComparatorDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a small but significant NO3(-) effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number.”
e1“sugar maple, Acer saccharum”
The same questions in other papers
nitrogen · growth · Acer saccharum (sugar maple)Positive and negative results
2 in the opposite direction
- no effect
- no effect
Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.