A 33P tracing model for quantifying
gross P transformation rates in soil
Else K. Bünemann1 and Christoph Müller2,3
1
Institute of Agricultural Sciences, ETH Zurich, Eschikon 33,
CH-8315 Lindau, Switzerland
2 School of Biology and Environmental Science, Earth
Institute, University College Dublin, Dublin, Ireland
3 Department of Plant Ecology (IFZ), Justus-Liebig University
Giessen, Germany
The challenge
SOIL SOLUTION P
(Pi + Po)
MICROBIAL P
(Pi + Po)
INORGANIC P (Pi)
ORGANIC P (Po)
• Soil P dynamics: dominance of physicochemical processes
• Main microbial processes: mineralization and immobilization
=> How can we quantify them?
N mineralization and immobilization
IMM.
Mineral N
MIN.
t1
t2
MICROBIAL
and
ORGANIC N
For N:
• incubation and extraction (t1,t2);
dNmin(t1,t2) = net N mineralization
•
15N
isotopic dilution methods:
gross mineralization
- gross immobilization
= net N mineralization
P mineralization and immobilization
SOIL SOLUTION P
IMM.
MICROBIAL
MIN.
and
ORGANIC P
INORGANIC P (Pi)
For P:
• incubation and extraction (t1,t2);
no accumulation of soil solution P due to buffering
•
33P
isotopic dilution technique:
- assess gross rates (physicochemical and biological)
- derive net organic P mineralization
33P
isotopic dilution technique to measure
gross and net organic P mineralization
soil +
H2O
(1:10)
• 100 min exp, extrapolate specific activity (33P/31P)
in the soil solution
=> extrapolated SA (physicochemical processes)
• incubate 33P- labeled soils
=> measured SA (physicochemical+biological processes)
100
90
80
E-value (mg P kg-1) = 1/SA
33P
net
70
60
50
extrapolated E-value
extrapolated E-value
measured E-value
Microbial
P immobilization
measured
E-value
+ extrapolated E-value
gross
40
30
20
10
0
0
10
20
time after labelling (d)
30
Oehl et al.
SSSAJ 2001;
Bünemann et al.
SBB5 2007
Case study on organic P mineralization
derived from:
-isotopic dilution method -C mineralization
Cambisol, pHH2O 5.5,
44% sand, 22% clay
Mineral
P input
Treat-
Annual
plant P
uptake
kg P ha-1 yr-1
Microbial P
immobilization
Net P
mineralization
Net P
mineralization
--------- mg P kg-1 d-1 ----------------------
ment
NK
0
6.2 b
5.5 a
2.7 a
0.36 ns
NPK
17
16.6 a
2.2 b
0.9 b
0.39 ns
45 kg N ha-1 yr-1
83 kg K ha-1 yr-1
Uncertainty due to incomplete
extraction of microbial P
Bünemann
Bünemannetetal.
al.SBB
SBB2012
2012
A numerical 33P tracing model
Pof
Pos
Mf
• Conceptual model with 5 P pools
and 9 P transformations
Ms
Is
If
Pw
Sf
• Transformation rates: zero, first or
second order (Michaelis Menten)
kinetics
Ss
Df
Ds
Pif
Pis
• Initial pool sizes and tracer
distribution based on measured
values
Ti
Müller & Bünemann SBB 2014
• Markov Chain Monte Carlo method:
«random walk in the parameter space»
Entrapment in local minima avoided,
e.g. two parameter space:
400
2000
0
D
S
4000
Model parameter optimization
4
6
8
400
Ti
4000
Ds
200
2000
200
Müller et al. SBB 2007
Is
Mf
0
0 density
1
2
=> Probability
function
for
-3
each parameter (mean and
stdev)
x 10
4000
400
2000
0
0
0.2
0.4
200
Observed vs. modelled values
• Simulation of experimental
data (31P and 33P in Pw and
Pof): good agreement
between measured and
modeled values
• Each pool defined by
differential equations for
change in 31P and in 33P/31P
=> transformation rates
Müller & Bünemann SBB 2014
Conclusions from the modelling approach
=0.77 mg P kg-1 d-1
Pof
0.19
0.27
Mf
Pos
 lower than estimate based on C
mineralization
Is 0.04
Ms
0.31 If
0.84
Pw
0.0004
Sf
Ss
0.44
0.29
Df
Ds
Pif
• Net Po mineralization rate (NK)
= 0.46 – 0.35 mg P kg-1 day-1
Pis
Ti
0.31
=1.88 mg P kg-1 d-1
• Dominance of microbial
immobilization and
remineralization over slow
mineralization/immobilization
• Ratio of microbial to
physicochemical processes in
this soil (NK): about 1:2.5
Müller & Bünemann SBB 2014
Outlook
Modelling approach allows application of isotopic
dilution principles to non-steady-state conditions
(baseline of extrapolated E-values not needed)