Modelling soil C sequestration in spruce forest ecosystems along a Swedish transect based on current conditions

Abstract

The change of current pools of soil C in Norway spruce ecosystems in Sweden were studied using a process-based model (CoupModel). Simulations were conducted for four sites representing different regions covering most of the forested area in Sweden and representing annual mean temperatures from 0.7°C to 7.1°C. The development of both tree layer and field layer (understory) was simulated during a 100-year period using data on standing stock volumes from the Swedish Forest Inventory to calibrate tree growth using different assumptions regarding N supply to the plants. The model successfully described the general patterns of forest stand dynamics along the Swedish climatic transect, with decreasing tree growth rates and increasing field layer biomass from south to north. However, the current tree growth pattern for the northern parts of Sweden could not be explained without organic N uptake and/or enhanced mineralisation rates compared to the southern parts. Depending on the assumption made regarding N supply to the tree, different soil C sequestration rates were obtained. The approach to supply trees with both mineralised N and organic N, keeping the soil C:N ratio constant during the simulation period was found to be the most realistic alternative. With this approach the soils in the northern region of Sweden lost 5 g C m⁻² year⁻¹, the soils in the central region lost 2 g C m⁻² year⁻¹, and the soils in the two southern regions sequestered 9 and 23 g C m⁻² year⁻¹, respectively. In addition to climatic effects, the feedback between C and N turnover plays an important role that needs to be more clearly understood to improve estimates of C sequestration in boreal forest ecosystems.

Appendix 1

List of equations

Equation	Definition	No.
Plant biotic processes
$C_{{Atm \to a}} = \varepsilon _{L} f(T_{l} )f(CN_{l} )f(E_{{ta}} /E_{{tp}} )R_{{s,pl}}$where ε L is a parameter representing the radiation use efficiency.	Rate of photosynthesis (g C m−2 day−1)	(1)
$f(T_{l} ) = \begin{array}{*{20}c} {0} \\ {{{{\left( {T_{l} - p_{{mn}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {T_{l} - p_{{mn}} } \right)}} {{\left( {p_{{o1}} - p_{{mn}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {p_{{o1}} - p_{{mn}} } \right)}}}} \\ {1} \\ {{1 - {{\left( {T_{l} - p_{{o2}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {T_{l} - p_{{o2}} } \right)}} {{\left( {p_{{mx}} - p_{{o2}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {p_{{mx}} - p_{{o2}} } \right)}}}} \\ {0} \\ \end{array} \quad \begin{array}{*{20}c} {{T_{l} < p_{{mn}} }} \\ {{p_{{mn}} \le T_{l} \le p_{{o1}} }} \\ {{p_{{o1}} < T_{l} < p_{{o2}} }} \\ {{p_{{o2}} \le T_{l} \le p_{{mx}} }} \\ {{T_{l} > p_{{mx}} }} \\ \end{array}$where p mn , p o1 , p o2 and p mx are parameters.	Response function for leaf temperature (−)	(2)
$f(CN_{l} ) = \begin{array}{*{20}c} {1} & {{CN_{{leaf}} < p_{{CN,Opt}} }} \\ {{1 + \frac{{CN_{{leaf}} - p_{{CN,Opt}} }} {{p_{{CN,Opt - }} p_{{CN,Th}} }}}} & {{p_{{CN,Opt}} \le CN_{{leaf}} \le p_{{CN,Th}} }} \\ {0} & {{CN_{{leaf}} > p_{{CN,Th}} }} \\ \end{array}$where p CN,Opt and p CN,Th are parameters and CN leaf is the C:N ratio in the leaf.	Response function for leaf C:N ratio (−)	(3)
$f{\left( {E_{{ta}} /E_{{tp}} } \right)} = \frac{{E_{{ta}} }} {{E_{{tp}} }}$	Response function for soil moisture (−)	(4)
$\begin{aligned}{} & C_{{a \to Root}} = f_{{root}} \cdot C_{{Atm \to a}} \\ & C_{{a \to Leaf}} = f_{{leaf}} \cdot C_{{Atm \to a}} \\ & C_{{a \to Stem}} = {\left( {{\left( {1 - f_{{root}} - f_{{leaf}} } \right)} \cdot C_{{Atm \to a}} } \right)} \cdot {\left( {1 - f_{{cr}} } \right)} \\ & C_{{a \to CRoot}} = {\left( {{\left( {1 - f_{{root}} - f_{{leaf}} } \right)} \cdot C_{{Atm \to a}} } \right)} \cdot f_{{cr}} \\ \end{aligned}$where f root , f leaf and f cr are parameters.	Carbon allocation to root, leaf, stem and coarse root respectively (g C m−2 day−1)	(5)
$C_{{Leaf \to Resp}} = k_{{mrespleaf}} \cdot f{\left( {T_{a} } \right)} \cdot C_{{leaf}} + k_{{gresp}} \cdot C_{{a \to Leaf}}$ where k mrespleaf is the maintenance respiration coefficient for leaves, k gresp is the growth respiration coefficient, and f(T a ) is the temperature response. The equation calculates respiration from stem, roots and coarse roots by exchanging k mrespleaf to k mrespstem , k mresproot , k mrespcroot , and using the corresponding storage pools.	Plant growth and maintenance respiration from leaves (g C m−2 day−1)	(6)
$f(T_{a} ) = t_{{Q10}} ^{{{(T_{a} - t_{{Q10bas}} )} \mathord{\left/ {\vphantom {{(T_{a} - t_{{Q10bas}} )} {10}}} \right. \kern-\nulldelimiterspace} {10}}}$ where t Q10 and t Q10bas are parameters.	Air temperature response function for respiration (−)	(7)
$C_{{Leaf \to Litter}} = l_{{Lc}} \cdot C_{{Leaf}} \cdot e^{{(A_{l} \cdot k_{{LAI}} )}}$ where l Lc is a rate parameter, A l the LAI and k LAI the LAI Enhanced Coefficient parameter. Stem, coarse roots and fine roots are calculated analogously using l Sc , l CRc and l Rc , except for the exponential function for enhanced leaf litterfall.	Leaf litter rate carbon (day−1)	(8)
$N_{{Demand}} = {\left( {\frac{{C_{{a \to Root}} }} {{cn_{{MinRoot}} }} + \frac{{C_{{a \to CRoot}} }} {{cn_{{MinCRoot}} }} + \frac{{C_{{a \to Stem}} }} {{cn_{{MinStem}} }} + \frac{{C_{{a \to Leaf}} }} {{cn_{{MinLeaf}} }}} \right)} - N_{{MobileReallo \to Leaf}}$ where cn MinRoot , cn MinCRoot , cn MinStem and cn MinLeaf are parameters and N MobileReallo is equal to the nitrogen content in the mobile nitrogen pool at that time-step.	Plant nitrogen demand (g N m−2 day−1)	(9)
$N_{{Mineral \to Plant}} = f_{{NUpt}} \cdot {\left( {N_{{NO_{3} }} + N_{{NH_{4} }} } \right)}$where f NUpt is a parameter.	Plant uptake of mineral nitrogen (g N m−2 day−1)	(10)
$\begin{aligned}{} & N_{{Organic \to Plant}} = N_{{Litter}} o_{L} \cdot \frac{{N_{{Litter}} o_{L} }} {{{\left( {N_{{Litter}} o_{L} + N_{{Humus}} o_{H} } \right)}}} \\ & + N_{{Humus}} o_{H} \cdot \frac{{N_{{Humus}} o_{H} }} {{{\left( {N_{{Litter}} o_{L} + N_{{Humus}} o_{H} } \right)}}} \\ \end{aligned}$ where o L and o H are parameters for maximum uptake rates.	Plant uptake of organic nitrogen (g N m−2 day−1)	(11)
$\begin{aligned}{} & N_{{a \to Root}} = \min (N_{a} ,\frac{{C_{{a \to Root}} }} {{cn_{{MinRoot}} }}) \\ & N_{{a \to CRoot}} = \min (N_{a} - N_{{a \to Root}} ,\frac{{C_{{a \to CRoot}} }} {{cn_{{MinCRoot}} }}) \\ & N_{{a \to Stem}} = \min (N_{a} - N_{{a \to Root}} - N_{{a \to CRoot}} ,\frac{{C_{{a \to Stem}} }} {{cn_{{MinStem}} }}) \\ & N_{{a \to Leaf}} = \min (N_{a} - N_{{a \to Root}} - N_{{a \to CRoot}} - N_{{a \to Stem}} ,\frac{{C_{{a \to Leaf}} }} {{cn_{{MinLeaf}} }}) \\ \end{aligned}$ where cn MinRoot ; cn MinCRoot , cn MinStem and cn MinLeaf are parameters.	Nitrogen allocation to root, coarse root, stem and leaf respectively (g N m−2 day−1)	(12)
$N_{{Leaf \to Litter}} = l_{{Lc}} \cdot N_{{Leaf}} \cdot e^{{(A_{l} \cdot k_{{LAI}} )}}$	Leaf litter rate nitrogen (day−1)	(13)
$C_{{Leaf \to Mobile}} = C_{{Leaf \to LitterSurface}} \cdot m_{{retain}}$ where m retain is a parameter.	Carbon retention at leaf litterfall (g C m−2 day−1)	(14)
$N_{{Leaf \to Mobile}} = {C_{{Leaf \to Mobile}} } \mathord{\left/ {\vphantom {{C_{{Leaf \to Mobile}} } {cn_{{MinLeaf}} }}} \right. \kern-\nulldelimiterspace} {cn_{{MinLeaf}} }$ where cn MinLeaf is a parameter.	Nitrogen retention at leaf litterfall (g N m−2 day−1)	(15)
$C_{{Mobile \to Leaf}} = C_{{Mobile}} \cdot m_{{shoot}}$ where m shoot is a parameter.	Reallocation of carbon from mobile pool to leaves at leafing (g C m−2 day−1)	(16)
$N_{{Mobile \to Leaf}} = N_{{Mobile}} \cdot m_{{shoot}}$	Reallocation of nitrogen from mobile pool to leaves at leafing (g N m−2 day−1).	(17)
Soil nitrogen and carbon processes
$C_{{DecompL}} = k_{l} f(T)f(\theta )C_{{Litter}}$ where k l is a parameter. Decomposition of the humus pool is calculated analogously using the parameter k h .	Decomposition of litter, carbon (g C m−2 day−1)	(18)
$\begin{array}{l}C_{{Litter \to CO_{2} }} = (1 - f_{{e,l}} ) \cdot C_{{DecompL}}\\C_{{Litter \to Humus}} = f_{{e,l}} f_{{h,l}} C_{{DecompL}}\\C_{{Litter \to Litter}} = f_{{e,l}} (1 - f_{{h,l}} ) \cdot C_{{Decomp}}\end{array}$ where f e,l and f h,l are parameters.	Decomposition products from litter (g C m−2 day−1)	(19)
$C_{{Humus \to CO_{2} }} = (1 - f_{{eh}} ) \cdot C_{{DecompH}}$ where f e,h parameter is a parameter.	Decomposition products from humus (g C m−2 day−1)	(20)
$N_{{Litter \to Humus}} = C_{{Litter \to Humus}} /cn_{m}$ where cn m is a parameter representing the C:N ratio of the microbes.	Decomposition of litter, nitrogen (g N m−2 day−1)	(21)
$N_{{Litter \to NH_{4} }} = C_{{DecompL}} {\left( {\frac{1} {{CN_{{Litter}} }} - \frac{{f_{{e,l}} }} {{cn_{m} }}} \right)}$ Mineralisation / immobilisation of nitrogen from humus is calculated analogously. A negative value of the flux means that a net immobilisation takes place.	Mineralisation/immobilisation of nitrogen (g N m−2 day−1)	(22)
$C_{{Litter1 \to DO}} = d_{{DOL1}} f(T)f(\theta )C_{{Litter1}}$ where d DOL is a rate parameter and f(T) and f(θ) are the common response functions for temperature and soil moisture. The equation is used analogously to calculate the flux of nitrogen from litter to the dissolved organic nitrogen pool.	Flux from litter to dissolved organic carbon (g C m−2 day−1)	(23)
$C_{{Humus \to DO}} = f(T)f(\theta ) \cdot {\left( {d_{{DOH}} C_{{Humus}} - d_{{DOD}} (z)C_{{DO}} } \right)}$ where d DOH is the rate parameter for formation of dissolved organic C, d DOD is the rate parameter for the fixation of dissolved organic C, f(T) and f(θ) are the common response functions for temperature and soil moisture, θ(z) is the soil moisture content and Δz is the depth of the soil horizon. The equation is used analogously to calculate the flux of nitrogen from humus to the dissolved organic nitrogen pool.	Flux from humus to dissolved organic carbon (g C m−2 day−1)	(24)
$q_{{DOC}} = \frac{{C_{{DO}} (z)}} {{\theta (z)\Delta z}} \cdot q_{w}$ where q w is the vertical water flow. The equation is used analogously to calculate the flux of dissolved organic nitrogen.	Vertical redistribution of dissolved organic carbon (gC m−2 day−1)	(25)
$\begin{array}{ll}f(T)=1 & T > t_{max}\\f(T)=\left(\frac{T-t_{min}}{t_{max}-t_{min}}\right)^2 & t_{min}<T < t_{max}\\ f(T)=0 & T < t_{min}\end{array}$ where t min and t max are parameters.	Response function for soil temperature (Ratkowsky function) (-)	(26)
$\begin{array}{ll}f(\theta (z))=p_{\theta satact} & \theta (z)= \theta_ {s}\\ f(\theta (z))=min\left(\begin{array}{l}\left( \frac{\theta_s - \theta (z)}{p_{\theta Upp}}\right)^{p_{\theta p}} (1-p_{\theta satact})+p_{\theta satact},\\\left(\frac{\theta(z)-\theta_{wilt}}{p_{\theta Low}}\right)^{p_{\theta p}} \end{array} \right) & \theta_{wilt}<\theta(z)<\theta _s\\f(\theta)=0 & \theta(z)< \theta _{wilt}\\ \end{array}$ where pθsatact, pθUpp, pθLow and pθp are parameters and the variables, θs,θwilt and θ are the soil moisture content at saturation, at wilting point and at the actual soil moisture content respectively.	Response function for soil moisture (-)	(27)
Plant abiotic processes
$A_{l} = C_{{Leaf}} \cdot p_{{l,sp}}$ where p l,sp is a parameter for the specific leaf mass.	Leaf area index (m2 m−2)	(28)
$R_{{s,pl}} = {\left( {1 - e^{{ - k_{{rn}} \frac{{A_{l} }} {{f_{{cc}} }}}} } \right)} \cdot f_{{cc}} {\left( {1 - a_{{pl}} } \right)}R_{{is}}$ where k rn is the light use extinction coefficient given as a single parameter common for all plants, f cc the surface canopy cover and a pl the plant albedo.	Plant interception of global radiation (MJ m−2 day−1)	(29)
$r_{s} = \frac{1} {{\max (A_{l} {\kern 1pt} g_{l} ,0.001)}}$ where g l is the stomatal conductance.	Stomatal resistance (s m−1)	(30)
$g_{l} = \frac{{R_{{is}} }} {{R_{{is}} + g_{{ris}} }}\frac{{g_{{\max }} }} {{1 + \frac{{{\left( {e_{s} - e_{a} } \right)}}} {{g_{{vpd}} }}}}$ where g ris , g max and g vpd are parameter.	Stomatal conductance per leaf area (m s−1)	(31)
$f_{{cc}} = p_{{c\max }} (1 - e^{{ - p_{{ck}} A_{l} }} )$ where p cmax and p ck are parameters .	Surface canopy cover (m2 m−2)	(32)
$L_{\upsilon } E_{{tp}} = \frac{{\Delta R_{n} + \rho _{a} c_{p} \frac{{(e_{s} - e)}} {{r_{a} }}}} {{\Delta + \gamma {\left( {1 + \frac{{r_{s} }} {{r_{a} }}} \right)}}}$ where R n is net radiation available for transpiration, e s is the vapour pressure at saturation, e is the actual vapour pressure, ρ a is air density, c p is the specific heat of air at constant pressure, L ν is the latent heat of vaporisation, Δ is the slope of saturated vapour pressure versus temperature curve, γ is the psychrometric “constant”, r s is an “effective” surface resistance and r a is the aerodynamic resistance.	Potential transpiration (mm day−1)	(33)
$E_{{ta}} ^{*} = E_{{tp}} ^{*} {\int\limits_{z_{r} }^0 {f_{\psi } (z)f_{T} (z)r(z)} }$ where r(z) is the relative root density distribution, z r is root depth and f ψ , and f T are response functions for soil water potential and soil temperature.	Actual transpiration before compensatory uptake (mm day−1)	(34)
$E_{{ta}} = E_{{ta}} ^{*} + f_{{umov}} \cdot (E_{{tp}} ^{*} - E_{{ta}} ^{*} )$ where f umov is the degree of compensation, E * ta is the uptake without any account of compensatory uptake and E * tp is the potential transpiration with eventual reduction due to interception evaporation.	Actual transpiration (mm day−1)	(35)