1. Long-term fertilization
and soil warming
effects on recalcitrant
litter decomposition
Bas Dingemans
[Faculty of Science
Biology]

2. Contents
Introduction
/Recalcitrance/Warming/Nitrogen/Research question
Methods
/Site description/Experimental setup
Results
/Incubation/Litter quality
Discussion
/Quality paradox/Interaction effect/More analyses
Conclusion
/Neutralizing effect/Caution with quality
[Faculty of Science
Biology]

3. Introduction
/Recalcitrance
Accumulation of SOM is
determined by the rates
of primary production
and decomposition.
PP CO2
Focus on the quality of
produced material and
the decomposition of
this material.
[Faculty of Science
Biology]

4. Introduction
Phase 1 Phase 2 Phase 3
/Recalcitrance
solubles
non-lignified carbohydrates
Most decomposition ex- lignified carbohydrates
periments have been lignin + lignin-like compounds
Remaining mass
done with “fresh litter”.
The recalcitrance of soil
organic matter is a lot
higher.
By extending my litter
decomposition experi-
ments in time, I’m follow-
ing the later stages in de-
composition.
Time
Berg & Laskowski 2006
[Faculty of Science
Biology]

5. a long-lasting stimulating effect on CO2-emission in subarctic short-ter
peatlands. tion disp
To partition the sources of increased ecosystem respiration rates, fractiona
we compared the effects of OTCs during the snow-free season peat resp
between intact and trenched-plus-clipped parts of plots in a com- respirati
Introduction panion experiment. Heterotrophic respiration of peat (trenched-
plus-clipped treatment) accounted for 70% of the total ecosystem
Informa
/Warming respiration rate (intact treatment) and both heterotrophic and plant-
related (aboveground, roots, rhizosphere) respiration rates
Dorrepaal et al. 2009:
Soil respiration increas- 0.6
Respiration (g CO2 m–2 h–1)
es with higher tempera-
tures. 0.4
Kirschbaum 1995: SOM
0.2
decomposition increas-
es more with increasing
0.0
temperature than net pri- Ecosystem Rh Ra
mary production. respiration
Figure 3
Dorrepaal et al. 2009
Figure 2 | Ecosystem respiration rates and their heterotrophic and plant- subjected
related components in a subarctic bog subjected to experimental warming and summ
Davidson & Janssens or ambient conditions. Spring and summer warming (black bars) stimulated winter sn
2006: Recalcitrant car- total ecosystem respiration (P 5 0.001), and stimulated heterotrophic (Rh)
and plant-related (Ra) respiration components equally (P , 0.001;
(P 5 0.03
duration
bon is more sensitive to warming 3 flux-component: P 5 0.65) compared with ambient conditions effects on
temperature than labile (white bars). Response patterns remained unchanged over the first two
experimental years (period 3 warming: P 5 0.41 for ecosystem respiration,
ecosystem
P 5 0.38)
carbon. P 5 0.18 for Rh and Ra), which were averaged. Error bars represent s.e.m. of (period 3
treatments (n 5 5 plots). s.e.m. of
Davidson & Janssens 2006
©2009 Macmillan[Faculty of Science ri
Publishers Limited. All
Biology]

6. ng-term nutrient fertilization these inferences were based on abovegroundreduction in the thickness of the layer, because neither %C nor
and surface soil
measurements only. The lack of soil-profile measurements reflects affected by fertilization (Supplementary Infor-
bulk density was
helle C. Mack1*, Edward A. G. Schuur1*, M. Syndonia Bret-Harte2, the expectation that the large heterogeneous belowground C pool mineral soil, fertilization reduced %C by 50%
us R. Shaver3 & F. Stuart Chapin III2 mation). In the upper
(P ¼ 0.04), whereas the depth to the frozen soil surface and mineral
partment of Botany, University of Florida, Gainesville, Florida 32611, USA
titute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska
75, USA
Introduction
e Ecosystems Center, Marine Biological Laboratory, Woods Hole,
sachusetts 02543, USA
/Nitrogen
se authors contributed equally to this work
....................................................................................................................................................................
August 2007 NUTRIENT August 2007
LIMITATION AND DECOMPOSITION
NUTRIENT LIMITATION AND DECOMPOSITION
2109
obal warming is predicted to be most pronounced at high
tudes, and observational evidence over the past 25 years
Mack et al. 2005: Nitro-
gests that this warming is already under way1. One-third of
global soil carbon pool is stored in northern latitudes2, so
gen enrichment results
re is considerable interest in understanding how the carbon
ance of northern ecosystems will respond to climate warm-
3,4
in increased primary pro-
. Observations of controls over plant productivity in tundra
d boreal ecosystems5,6 have been used to build a conceptual
duction and a stronger
del of response to warming, where warmer soils and increased
increase in decomposi-
omposition of plant litter increase nutrient availability,
ich, in turn, stimulates plant production and increases eco-
tion.
tem carbon storage6,7. Here we present the results of a long-
m fertilization experiment in Alaskan tundra, in which
Figure 1 Effect of fertilization on vascular plant aboveground net primary production
(ANPP) in tundra. Fertilized plots in moist acidic tundra near Toolik Lake, Alaska, have
Mack et al. 2005
reased nutrient availability caused a net ecosystem loss of received 10 g N m22 yr21 and 5 g P m22 yr21 since 1981. Values are means (^1 fertilization on tundra carbon and nitrogen pools after 20 yr of
Figure 2 Effects of
most 2,000 grams of carbon per square meter over 20 years. We standard error, s.e.); means from 1982–95 are reported in ref. fertilization. a, c, Mean (^1 s.e.) above- and belowground carbon (a) and nitrogen (c)
19; the year-2000 data
Craine et al. 2006: Low
nd that annual aboveground plant production doubled
ing the experiment. Losses of carbon and nitrogen from
are from this study (n ¼ 4). Components of ANPP (new leaves pools in unmanipulated control and fertilized treatments of moist acidic tundra near Toolik
and reproductive parts,
new stems and secondary growth) are shown in SupplementaryLake, 1.Fig. Alaska. Aboveground pools include shoots, standing dead plant material, and
0 nitrogen availability©can Publishing Group
2004 Nature
rhizomes. Belowground pools include surface litter, roots, and organic and mineral soil.
NATURE | VOL 431 | 23 SEPTEMBER 2004 | www.nature.com/nature
increase litter decompo- NATURE | VOL 431 | 23 SEPTEMBER 2004 | www.nature.com/nature
©2004 Nature Pub
sition as microbes use
labile substrates to ac- Figure 2 Effects of fertilization on tundra carbon and nitrogen pools after 20 yr of
fertilization. a, c, Mean (^1 s.e.) above- and belowground carbon (a) and nitrogen (c)
quire nitrogen from re- pools in unmanipulated control and fertilized treatments of moist acidic tundra near Toolik
Lake, Alaska. Aboveground pools include shoots, standing dead plant material, and
calcitrant organic matter rhizomes. Belowground pools include surface litter, roots, and organic and mineral soil.
(microbial nitrogen min- NATURE | VOL 431 | 23 SEPTEMBER 2004 | www.nature.com/nature
©2004 Nature Pub
ing).
FIG. 2. Relationships between (a and b) labile-C FIG. 2. Relationships betweenandandandlabile-C pool sizeal. 2006 substrate [C] and (c and d) recalc
Craine et decay rate
pool size (CL) vs. substrate [C] (a (c b) d) recalcitrant-C (CL) vs. (kR) vs.
substrate [N] for leaves with no nutrients added (thin line) [N] for added with no nutrients added (thin line) and N added (thick line).
substrate and N leaves (thick line).
[Faculty of Science
followed N-mining theory with N fertilization decreas-
followed N-mining theory with N fertilization decreas-
ing kR (decay rate of the recalcitrant-C pool) bykR (decay rate of the recalcitrant-C pool) by 29% on
ing 29% on Biology]
average (Fig. 1b). Declines in kR with N average (Fig. 1b). Declines in kR with N fertilization
fertilization

7. Introduction
/Research question
Does long-term fertilization amplify or neutralize the positive effect of soil
warming on the decomposition of recalcitrant litter?
Is chemical composition a good predictor of decomposability of recalcitrant
litter?
[Faculty of Science
Biology]

8. Iceland
Methods
/Site description
Field site is located near
Hveragerði in Iceland.
Geothermally heated val- Reykjavik Hveragerdi
ley with patchwork of
heated and ambient wet
grassland soils.
Land age
< 0.8 M y
0.8 - 3.3 M y
3.3 - 15 M y
[Faculty of Science
Biology]

9. Water flow
Methods
/Experimental Bufferzone
setup Plot
C N
Plots consist of two adja-
cent subplots, a fertilized
and upstream its unferti-
lized control. 25
Air temperature: ~10°C Ambient
Warmed
20
Soil temperature (°C)
Plots were layed out in
2005 on warmed (~25°C) 15
and ambient grass patch-
es. 10
5
Dead standing litter
(grasses and sedges) 0
from every plot was har- Fertilized Unfertilized
vested in May 2009 and Plot treatment
pouled per treatment.
[Faculty of Science
Biology]

10. Methods
/Experimental
setup
Litter was incubated at
two different tempera-
tures (15°C and 25°C)
with and without extra ni-
trogen (urea)
for 365 days with three
harvests (0, 175 and 365 0 days 175 days 365 days
days).
Harvested material was
used for determination of
mass loss, C:N ratio and
lignin.
[Faculty of Science
Biology]

11. Lignin determination
Methods
/Experimental dry litter
setup water,
methanol, lipids, sugars,
chloroform soluble phenolics
Lignin content was
measured by sequential
extraction of lipids, water
hydrochlo-
solubles and hydrolys- ric acid starch, fructans,
able carbon. pectins, hemicel-
lulose
C and N
By analysing the carbon analysis,
and nitrogen content of calculation
the residue the lignin
content was calculated. cellulose lignin
Poorter & Villar 1997
[Faculty of Science
Biology]

12. Results
Mass remaining after 365 days (%)
/Incubation Ambient
Warmed
When litter was incu- 55
bated at plot-own situa-
tion an accelerated de-
50
composition was found
in litter from unfertilized,
warmed plots incubated 45
at 25°C without addition-
al nitrogen.
40
No significant tempera-
ture effect was measured
Fertilized Unfertilized
within the fertilized treat-
ment. Treatment
No significant fertiliza-
tion effect was measured
within the ambient treat-
ment. [Faculty of Science
Biology]

13. 43 Ambient
Litter carbon content (%)
Warmed
42
Results 41
/Litter quality
40
39
38
Initial litter C concentra- 37
tion is higher and N con-
centration is lower in lit-
Litter nitrogen content (%)
1.6
ter from warmed plots. 1.5
1.4
Fertilization of the plots 1.3
1.2
leads to a lower carbon 1.1
and nitrogen concentra- 1.0
tion in the litter.
36
Initial carbon to nitrogen
Litter C:N ratio (g g-1)
34
ratio is higher in litter 32
from warmed plots. 30
28
Fertilization of the 26
warmed plots leads to a Fertilized Unfertilized
higher C:N ratio in the lit- Plot treatment
ter. [Faculty of Science
Biology]

14. Ambient
Litter lignin content (%)
Warmed
Results 15
/Litter quality
10
Initial litter lignin concen-
tration is higher in litter 5
from warmed plots.
Initial litter lignin concen-
tration is lower in litter 12
Litter lignin:N ratio (g g-1)
from fertilized plots
10
8
6
4
2
Fertilized Unfertilized
Plot treatment
[Faculty of Science
Biology]

15. Discussion
/Quality paradox
Warming of plots causes a ‘time shift’, i.e. the litter from warmed plots is in a
further stage in the decomposition process. Due to the loss of easily decom-
posable carbon the concentration of recalcitrant carbon is higher.
Spring Summer Autumn Winter
Ambient Snow
Warmed
Harvest End growing
season
Fertilizing of plots causes a higher biomass production. Fertilized plants grow
faster due to the production of easily composable (i.e. decomposable) plant
material. The production of recalcitrant plant material takes more time and the
overall recalcitrant compound concentration will be lower.
[Faculty of Science
Biology]

16. Discussion
Mass remaining after 365 days (%)
/Interaction ef- Ambient
fect 55
Warmed
Nitrogen fertilization
may neutralize the posi- 50
tive effect of increased
temperature on the de-
composition of recalci- 45
trant litter in grasslands.
40
Fertilized Unfertilized
Treatment
[Faculty of Science
Biology]

17. Incubator treatment
Discussion 70
Control 15°C Control 25°C
/More analyses
60 Ambient
Warmed
Incubating without ad- 50
Mass remaining after 365 days (%)
ditional nitrogen shows
a clear positive effect of 40
plot temperature on the
decomposition rate. 30
This effect is gone when
incubating with addition- 70
Nitrogen 15°C Nitrogen 25°C
al nitrogen.
60
Further discussion
50
about this research at
dECOlab meeting. 40
30
Fertilized Unfertilized Fertilized Unfertilized
Plot treatment
[Faculty of Science
Biology]

18. Conclusion
/Neutralizing effect
Nitrogen fertilization may neutralize the positive effect of increased temperature
on the decomposition of recalcitrant litter in grasslands.
/Caution with quality
Linking litter decomposability to the chemical composition of litter is “tricky busi-
ness”. A high C:N ratio or a high lignin concentration does not necessarily mean
low decomposability.
[Faculty of Science
Biology]

19. Acknowledge-
ments
/Thanks to
Mariet/UU
Anne/NIOO
Bjorn/UU
Paul/NIOO
Riks/NIOO/UU
Gerrit/UU
Jos/UU
Tryggvi/UI
Gisli Pall/Grund
Rannveig/AUI [Faculty of Science
Biology]