Differential effects of reduced
versus oxidized N in speciesrich grasslands and heathlands
Roland Bobbink
Landscape Ecology, Utrecht University &
B-WARE Research Centre, Nijmegen
OUTLINE OF PRESENTATION
• Introduction
- impacts of N deposition
- reduced versus oxidized N
- species-rich grassland & heaths
• Effect studies
- correlative field studies
- waterculture & pot experiments
- mesocosm experiments
- field addition studies (very rare)
• Concluding remarks
Overview impacts of N deposition
• direct toxicity of gases & aerosols;
• accumulation of nitrogen, resulting in
changes in productivity and species
composition in the long term;
• soil-mediated effects of acidification
• increased sensitivity to stresses and
disturbances (drought, frost, pathogens,
herbivores)
• THUS: Very complex, many interactions,
different timescales
Plant species richness and N
All vegetation
deposition (Sn/Sc)
types
Species richness ratio
1.00
0.80
0.60
0.40
0.20
0.00
0
5
10
15
Exceedance (g N m -2)
(n=44; mostly grasslands & heaths; P<0.01) (Bobbink 2004)
Overview impacts of reduced N
• membrane dysfunction & cell solute leakage;
• dysfunction of cell pH regulation (internal cell
acidification);
• nutritional imbalance (increased N%; lower cation
% by reduced uptake and increased shoot leaching;
accumulation N-rich amino-acids);
▬►reduction in plant growth
(roots!!) & survival
Species-rich grassland (pH 4.5
– 6.5)
Species-rich heaths
(pH 4.5 - 6.0)
Correlative field studies on plant
species and soil conditions
• 300 vegetation samples with full soil
chemistry
• acidic grasslands and heaths in the
Netherlands (sandy parts);
• Almost 90 areas (nature reserves!!)
• To avoid pseudo-replication: never more
than 2 randomly selected samples per area;
• Kleijn et al. (submitted)
Ammonium/Nitrate ratio
Growth site characteristics of common (blue) and
rare (red) species of heaths and acidic grasslands
150
100
50
0
3.9
4.4
4.9
5.4
5.9
pH
Kleijn et al (submitted)
Water culture & pot experiments
pH =4
(De Graaf et al. 1998)
Interaction of pH
and ammonium
Green = shoot
Brown = root
Van den Berg et al.
2005
(Soil pH 4.3)
Dorland 2004
Survival of Succisa with increasing
ammonium (Dorland et al. 2003)
pH soil 4.3
Mesocosm experiments
• Plant – plant interactions included;
• Plant – (natural) soil interactions;
• Also more “long-term” processes in
plants and soil incorporated;
• Best experimental alternative for field
addition in high N regions.
Influence of ammonium:nitrate ratio
(2 experiments; 3 grasses; 6 endangered species)
Biomass of grasses (Deschampsia &
Danthonia) after 2 yrs
biomass (g/m2)
200
150
100
50
0
N load
10
10
(4:0)
(3:1)
Soil pH ca. 6.8
10
10
40
40
(1:1)
(1:3)
(4:0) (3:1)
Ammonium:Nitrate ratio
40
40
(1:1)
(1:3)
Tomassen et al 1999
Cover of endangered forbs (as % of total) after
2 yrs addition of ammonium or nitrate
% rare forbs
75
50
25
0
N load
10
10
10
10
40
40
40
40
(4:0)
(3:1)
(1:1)
(1:3)
(4:0)
(3:1)
(1:1)
(1:3)
Soil pH ca. 6.8
Ammonium:Nitrate ratio
Tomassen et al 1999
Mechanism: full nitrification in grassland or heath
soil; Æ hardly any ammonium in soil! (Dorland et al.
Potential net NO 3- production (μ mol g-1 dry soil)
2004)
70
a
60
pH 5.5
pH 4.3
50
40
b
c
cd
30
20
10
0
0-5 cm
Control
6-10 cm
Control
0-5 cm
Acidified
6-10 cm
Acidified
Influence of ammonium:nitrate ratio
(second experiment; 3 grasses; 6 endangered species)
pH 4.3;
Start 09-01 /
end 03-04
Mortality of typical species (Van den Berg et al submitted)
Low ammonium
/ nitrate ratio
High ammonium /
nitrate ratio
Van den
Berg et al
submitted
And now: field addition experiments
with both reduced and oxidized N
PROBLEM: no DATA at all in
grassland or heaths!!!!
Rich fen: Addition study of reduced or oxidized
N (small scale / large scale exp)
Species-rich vegetation,
especially brown
mosses!!
Small-scale addition experiment 4 yrs
-2
-1
Bryophyte biomass after 4 years of N addition (5 g N m yr )
Scragh Bog, central Ireland
-2
living standing crop (g m )
300
a
250
200
a
a*
a
ab*
150
b
100
50
* differ significantly at the 10 % level
(Paulissen et al in prep)
NH4-N
NO3-N
control
Sphagnum
NH4-N
NO3-N
control
Scorpidium
0
Small-scale addition experiment 4 yrs
Tissue asparagine concentration
350
B
µmol g -1 dwt
300
250
AB
200
Scorpidium
150
Sphagnum
100
50
A
0
control
nitrate
ammonium
(Paulissen et al in prep)
Cover of Phanerogams (afer 4 yrs)
(5 g N m-2 jr-1)
100
b
80
ab
a
60
40
20
0
Control
Alpha = 0.10
NO3-N
NH4-N
N content rich fen Ireland (3 yrs)
New 5-10 yr experiment
dominant sedge
brown mosses
25
mg N / g DW
20
15
10
5
0
control
35NH4
70NH4
35NO3
70NO3
Conclusions:
• Toxic impacts of reduced N much larger than
those (if any) of oxidized N;
• Most sensitive systems (and their endangered
plants) have intermediate soil pH (4.5 – 6.5);
• Most sensitive plants for reduced N: bryophytes
& lichens (full shoot uptake!);
• Effects of reduced N mediated by (potential)
nitrification rate.
• (Long-term) field additions with both reduced
and oxidized N highly needed !! (ESF-BEGIN)
• Development of critical loads for reduced N and
oxidized N separately.