Modeling Growth, Carbon Allocation and Nutrient Budgets of Phragmites australis in Lake Burullus, Egypt

Abstract

Phragmites australis is the major component of reed stands covering some 8200 ha along the shores of Lake Burullus (Egypt). We applied a published temperate zone reed model to assess growth and cycling of C and nutrients among the various organs of P. australis in this sub-tropical lake. We aim to quantify the role of reed stands for the C balance and nutrients cycling in the south Mediterranean wetland. Above-ground biomass was 3.5 times higher than the below-ground biomass. Root biomass represented 13% of the total below-ground, while leaves and panicles represented 16 and 3% of the above-ground biomass, respectively. Remobilization from rhizomes (15%) and reallocation from leaves (1%) were of little importance as assimilated sources. Nutrients accumulation by total above-ground biomass ranged between 2.7 to 46.8 g m⁻² yr⁻¹ for P and K, respectively. We calculated a C sequestration rate of 38.4 g C m⁻² yr⁻¹ for the dead rhizomes in the sediments. This value stresses the importance of P. australis stands for C sequestration in Lake Burullus. Further, as much as 254 t P and 5527 t N could potentially be removed annually from Lake Burullus by harvesting P. australis at maximum total above-ground biomass.

Appendix 1. List of A) Equations and B) Parameters used in the Reed Model to Simulate Growth and Carbon Distribution in a Phragmites australis Population in Lake Burullus, Egypt

A)

Rates of change (mol C m−2 per day) of the state variables (mol C m−2)

$\left( {{{\left( {\partial \,Rhizomes} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Rhizomes} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Growth–Rhizome Basal Respiration–Rhizome Mortality–Rhizome Mobilization

$\left( {{{\left( {\partial \,Roots} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Roots} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Root Growth–Root Basal Respiration–Root Mortality

$\left( {{{\left( {\partial \,Dead\,below - ground} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Dead\,below - ground} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Root Mortality+Rhizome Mortality–Below-ground Decay

$\left( {{{\left( {\partial \,Leaves} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Leaves} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Leaf Growth–Leaf Basal Respiration–Leaf Mortality–Leaf Reallocation

$\left( {{{\left( {\partial \,Stems} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Stems} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Stem Growth–Stem Basal Respiration–Stem Mortality–Stem Reallocation

$\left( {{{\left( {\partial \,Panicles} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Panicles} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Panicle Growth–Panicle Basal Respiration–Panicle Mortality–Panicle Reallocation

$\left( {{{\left( {\partial \,Dead\,leaves} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Dead\,leaves} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Leaf Mortality–Dead leaf Decay–Dead leaf Abscission

$\left( {{{\left( {\partial \,Dead\,stems} \right)} \mathord{\left/{\vphantom {{\left( {\partial \,Dead\,stems} \right)} {\left( {\partial \,t} \right)}}} \right.} {\left( {\partial \,t} \right)}}} \right)$=Stem Mortality+Panicle Mortality–Dead stem Decay

Main processes (mol C m−2 per day)a

Rhizome Mobilization=Remobilization rate20 . Rhizomes . f (T)

Organ Reallocation=Reallocation rate20 . Organ . f (T)

Glucose Production=Photosynthesis+Rhizome Mobilization+(Leaf, Panicle and Stem) Reallocation

Organ Growth=pOrgan . Glucose Production . (1–Respiration Fraction)

Organ Basal Respiration=Respiration rate20 . Organ . f (T)

Organ Mortality=Mortality rate20 . Organ . f (T)

Dead Organ Decay=Decay rate20 . Dead Organ . f (T)

Dead Leaves Abscission=Abscission rate20 . Dead Leaves . f (T)

Functionsb

$f(T) = {\theta^{\left( {T - 20} \right)}}$

Photosynthesis (mol C m−2 per day)=integral $\left( {Px,t.LAI.\,\partial \,x,x = 0,x = \infty } \right)$

${P_{x,t}}\left( {mol\,C\,{m^{ - 2}}\,{\hbox{leaf}}\,{\hbox{per}}\,{\hbox{day}}} \right) = {\hbox{P}}{\max_{20}}.\,f(T).\,\left( {1 - {{\exp }^{ - {1_{x,t.}}{{\rm{LUE}} \mathord{\left/{\vphantom {{\rm{LUE}} {\left( {{\rm{P}}{{\max }_{20}}.\,f\,(T)} \right)}}} \right.} {\left( {{\rm{P}}{{\max }_{20}}.\,f\,(T)} \right)}}}}} \right)$

${I_{x,t}}\left( {W\,{m^{ - 2}}{\hbox{leaf}}} \right) = f\left( {{I_{0,\,\rho }}\lambda } \right)$

LAI (m2 leaf m−2 soil)=SLAc . Leaves

Remobilization rate20 (mol C m−2 per day)=aRh . RhizomesbRh

1. ^a Organ is living leaves, stems, panicles, rhizomes or roots (mol C m⁻²); Dead Organ is dead leaves, stems or belowground matter (mol C m⁻²). pOrgan is the fraction of assimilates allocated to the respective organ, explained in Fig. 1. Rate₂₀ is the rate (per day) at 20°C, f (T) is the temperature function correcting the rates for the prevailing temperature (T, °C)
2. ^bθ is the Arrhenius constant. P _{x, t} is the instantaneous gross CO₂ assimilation rate at depth x in the canopy per unit leaf surface; LAI is leaf area index. I _{x, t} is the photosynthetically active radiation at depth x. It is calculated as a function of incident light (I₀, W m⁻²), reflection of individual leaves (ρ) and extinction coefficient for diffuse radiation (λ) as in the model SUCROS. Pmax₂₀ is the maximal assimilation rate per unit of leaf surface at 20°C, LUE is the initial light use efficiency. SLA_c is the specific leaf area (m² mol C⁻¹). aRh and bRh are factors for calculating rhizome mobilization rate

B)

Parameter	Value
Initial rhizomes biomass (g DW m−2)	2024 a
Initial roots biomass (g DW m−2)	316 a
θ	1.07 b
Tcrit (oC_days)	488 d
T2 (stop rhizome remobilization) (days)	202 d
T3 (start flowering) (days)	247 d
T4 (start senescence) (days)	350 d
Max. assimilation rate (mol C (m2 leaf)−1 per day)	7.7 d
LUE (Light use efficiency) (mol C W−1 per day)	0.012 c
Specific leaf area (m2 leaf mol C−1)	0.60 a
λ (m2 leaf m−2 soil)−1	0.40 b
Ρ	0.20 b
Leaves reallocation rate (senescence) (per day)	0.001 d
Stems reallocation rate (senescence) (per day)	0.001 d
aRh	0.58 c
bRh	−0.50 c
Roots respiration rate20 (per day)	0.0018 d
Rhizomes respiration rate20 (per day)	0.002 c
Above-ground respiration rate20 (per day)	0.001 d
Roots respiration fraction (mol C mol C−1)	0.15 c
Rhizomes respiration fraction (mol C mol C−1)	0.15 c
Leaves respiration fraction (mol C mol C−1)	0.15 c
Stems respiration fraction (mol C mol C−1)	0.11 c
Panicles respiration fraction (mol C mol C−1)	0.18 c
Leaves mortality rate20 (growth phase) (per day)	0.0069 d
Stems mortality rate20 (growth phase) (per day)	0.0011 d
Leaves mortality rate20 (senescence) (per day)	0.20 c
Stems mortality rate20 (senescence) (per day)	0.20 c
Below-ground mortality rate20 (per day)	0.00015c
Abscission rate20 (per day)	0.05 c
Above-ground decay rate20 (per day)	0.0077 c
Below-ground decay rate20 (per day)	0.0020 c
ShapeBelow	0.15 c
cRoots	0.063 d
cPanicles	0.70 c
cLeaves	0.504 d
Leaf coefficient (per day)	−0.035 c
Leaves N contents (mg N g DW−1)	14.9 a
Stems N contents (mg N g DW−1)	5.8 a
Below-ground N contents (mg N g DW−1)	7.8 a
Leaves C contents (g C g DW−1)	0.43 b
Stems C contents (g C g DW−1)	0.41 b
Panicles C contents (g C g DW−1)	0.45 b
Below-ground C contents (g C g DW−1)	0.43 b

1. T1, T2, T3, and T4 are time of phenological events; Tcrit is the critical accumulated temperature at which growth starts. ShapeBelow, cRoot, cPanicles, cLeaf and leaf coefficient relate to the partitioning functions for assimilate, as explained in Fig. 1. Other parameters are explained in Appendix 1a
2. ^a Measured
3. ^b Values as in Soetaert et al. (2004)
4. ^c Parameters manually calibrated
5. ^d Derived by automatic calibration