15
δ N
Values in Leaves and
Soils of Boreal and Tropical
Fertilization Experiments
Jordan R Mayor1,2*, BL Turner1, EAG Schuur2, SJ Wright1
1 Smithsonian
Tropical Research Institute, Soils Laboratory, Balboa, Ancón, República de Panamá
2 Department of Biology, Ecosystem Ecology Lab, University of Florida, Gainesville, Florida 32611
* corresponding author’s email: [email protected], and phone: (352) 283-1731
Main Questions
Introduction
Ecosystem ecologists and biogeochemists have sought to use nitrogen isotope ratios
(15N:14N represented as standardized δ15N) as integrative metrics of ecosystem N
cycling yet have been hampered by the possibility of multiple simultaneous
explanations for plant patterns. For instance, it remains uncertain whether
environmental or genetic and physiological factors interact to control variation in
foliar δ15N values, the most common measurement.
Can plant δ15N values be reliably used as a N-cycle integrator, like
C:N:P ratios, to predict changes in soil fertility or plant
productivity?
How do ecosystem δ15N values from fertilization experiments
compare to patterns from natural boreal and tropical forests?
Furthermore, it remains to be seen if fertilization alters plant δ15N values in predictable
ways that may be realistic proxies for future growth conditions under increased
deposition, mineralization, or disturbance events.
How do we separate sources from pathways of N-cycling in order to
understand plant δ15N values ?
Panamá Results
KCl [NH4] ug/g & KCl [NO3] ug/g vs. Rx
A)
12
10
+]
[NH4 = blue
[NO3-] = red
15NO3 vs. Rx
B)
& Org_KCl_NO3
2MOrg_KCl_NH4
KCl extractable
(µg g-1)
5
10
δ15
NO315NO3
8
6
0
-5
4
2
-10
15
[NO3-] = red
B)
10
600
400
5
0
200
N
P
NP
Rx
N
N
C
P
N
C
N
N+P
NP
Rx
PP
-1
★
-2
HEISCO OENOMA TET2PA ALSEBL
HEISCO OENOMA TET2PA ALSEBL
Species
2
1
A. blackiana, R2=0.86
1.8
5
15
15NO3
15NO3 -
δ NO3
6
7
15N
1.6
G)
1
0
1
2
3
4
δ1515NO3
NO3-
5
6
15N
8
TD15N
δ15
NTDN
9
10
1.6
1.4
1.5
1.1
1
2
15NO δ15NO3
3
3
4
-10
1
0.8
H)
-5
0
5
0.4
-2 -1 0 1 2 3 4 5 6 7 8
Methods
2
1.5
All species, R2=0.78
1
0.5
0
-0.5
C
N
15NO δ15NO3
3
-2
-1
N
NP
NP
Rx
foliar15N vs. Rx & foliarN
NP
NP
P
P
A) Salt extractable pools of NH4+ and NO3- were significantly higher in both N
and NP treatments. B) δ15NO3- values from ion exchange resins were significantly
enriched in the P treatment only and less variable in the N and NP treatments.
C) Foliar δ15N values were significantly enriched in both the N and NP treatments
as was foliar N concentrations. D) δ15N values of ectomycorrhiza (ECM) fungi
were significantly depleted in the N treatment only. E) Synthesis of findings and
isotope mixing model solutions (see below) suggest the patterns are caused be a
declining dependence on ECM fungi for soil N forms and a switch to direct root
uptake of the mineral forms of N resulting from fertilization.
-3
-4
-5
-6
N
NP
P
Rx
15
N
Rx
ALASKA RESULTS
C)
C
C
N
Legend
-2
-7
P
C
P
0.4
0.8 1 1.2 1.4 1.6 1.8
foliarN
fungal15N_2 vs. Rx
Legend
D)
12.5
3 pool N source:
δ15Navailable = ƒDON • δ15NDON + ƒNH4 • δ15NNH4 + ƒNO3 • δ15NNO3
δ15Navailable =(1 – Tr) • δ15Nfungi + Tr • δ15Ntransfer
2 pool plant mixture:
δ15Nroot = δ15Navailable – ∆ƒ • (1 – Tr) • ƒ
2 pool fungal mixture:
δ15Nfungi = δ15Navailable + ∆ƒ • Tr
E)
10
7.5
5
2.5
-1
-1.5
I)
0.6
15NO3 δ15
NO3
5
T. panamensis, R2=0.98
1.2
1.7
1.3
7
7
T. panamensis, R2=0.94
1.9
2
1.2
6
2.1
T. panamensis, R2=0.92
1.4
5
-0.5
-4 -3 -2 -1 0
15N
foliar
15N
4
F)
A) Salt extractable NO3- was significantly higher
in both N and NP treatments. B) δ15NO3- values
from ion exchange resins were significantly
enriched in the N and NP treatments. C) Foliar
δ15N values were significantly enriched in Alseis
blackiana, Heisteria concinna in the N treatments
and Tetragastris panamensis in the P treatment,
whereas Oenocarpus mapora was depleted in the
NP treatment. D) Significant relationships
between the δ15N values of these plants in and
specific N forms taken from soil cores directly
beneath the species indicate species and
treatment specific tracing of N forms that
deviates from all species in the control plot with
the exception of H. concinna.
J)
3
4
5
6
7
8
9 10 11 12
δ15N
TD15N
TDN
15N
δ15N
2.4
3
0
E)
-0.2
-0.4
0.5
15N
2
0.2
0
D)
1.2
1.8
0.4
15N
1.4
2.2
H. concinna, R2=0.99
1
0.6
1.6
1
HEISCO OENOMA TET2PA
★ P ≤ 0.15, Dunnett’s
H. concinna, R2=0.79
0.8
0
PANAMA RESULTS
0
HEISCO OENOMA TET2PA ALSEBL
P
0
★
ALSEBL
NP
Rx
Legend
★
1
P
C
15N vs. Species by Rx
★
C)
NP
foliar
δ15N
foliar15N
NP
fungal
fungal15N_2
N
C
C
foliar
δ15N
15N
[NH4+] = blue
Legend
800
δ15N
C
C
15N
foliar15Nδ15N
A)
NO3_15N vs. Rx
Legend
-5
0
2
Org_KCl_NH4 & Org_KCl_NO3 vs. Rx
1000
δ15NO3NO3_15N
14
-1
2MKClKCl
(µg
[NH4]extractable
ug/g & KCl [NO3]
ug/gg )
Alaska Results
Alaska: Fertilization of 16 plots began in 2002, sampled in 2007. Single doses of
pelletized NH4+NO3- (N), ortho-PO4- (P), both (N+P), or none (C), annually
at a
TD15N
level of 200, in year 1, or 100 kg ha-1 yr-1, in subsequent years, per nutrient. Picea
mariana was sampled in triplicate from each plot at the height of needle expansion.
Panamá: Fertilization of 16 40×40m plots began in 1998, sampled in 2011. Four doses
of coated urea as ((NH2)2CO), 125 kg N ha-1 yr-1, phosphorus as triple
superphosphate (Ca(H2PO4)2.H2O) 50 kg P ha-1 yr-1, both, or none. Canopy heights
are to 40 and the forest is possibly >200 years old. Alseis blackiana, Heisteria concinna,
Oenocarpus mapora, and Tetragastris panamensis canopy leaves were shot in triplicate in
all plots where posible.
Sampling: δ15N values were measured using persulfate oxidation coupled to the
denitrifier method. δ15N values of total dissolved N were measured from five 15×4.2
cm cores extracted with 2M KCl the day of sampling at the height of the growing
season (Alaska) or beginning of the rainy season (Panamá). NH4+ and NO3- δ15N
values measured from ion exchange resin bags incubated for 4 (Panamá) to 8 (Alaska)
weeks.
0
C
N
Rx
NP
P
Conclusions
Fertilization experiments exhibit biome-specific effects on plant δ15N values. In the
tropics plants simply trace sources and in boreal forest they reflect the waning
influence of ectomycorrhizal fungi. Fertilization produces an excess of labile NO3-,
regardless of soil type or climate, and this causes significant enrichment of the
remaining δ15NO3-. If this metric is to be regarded as a useful integrator of
increasing N availability these artifacts must be seen to occur in natural ecosystems
that have undergone environmental changes or increased N deposition as well.
What is surprising is that it was P in boreal and N in tropical forests that triggered
these effects. This runs counter to established theory. Echoing other recent
assessments, the idea that N is in excess of plant demands in the tropics and boreal
forests are strictly N limited is not supported.
Acknowledgments
Support for JR Mayor in Panama was provided by an NSF International Research Fellowship
Program (OISE-1012703) and postdoctoral fellowship status from the Smithsonian Tropical
Research Institute. The Alaska work was supported by an NSF Doctoral Dissertation award
(DGE-0221599), a Forest Fungal Ecology Research award of the Mycological Society of America,
an International Association of GeoChemistry Student Research Grant and NSF and DOE
funding to MC Mack and EAG Schuur.