Running title: Climate change dominates future carbon export
Increase in dissolved inorganic carbon flux from the Mississippi River to Gulf of
Mexico due to climatic and anthropogenic changes over the 21st century
Wei Ren1, Hanqin Tian1*, Bo Tao1, Jia Yang1, Shufen Pan1, Wei-Jun Cai2, Steven E.
Lohren3, Ruoying He4, and Charles S. Hopkinson5
1
International Center for Climate and Global Change Research, Auburn University,
Auburn, AL 36849, USA;
2
School of Marine Science and Policy, The University of
Delaware, Newark, DE 19716, USA; 3School for Marine Science and Technology,
University of Massachusetts-Dartmouth, New Bedford, MA 02744, USA; 4Department of
Marine, Earth & Atmospheric Sciences, North Carolina State University, Raleigh, NC
27695, USA; 5Department of Marine Sciences, University of Georgia, Athens, GA 30602,
USA
*Corresponding author:
Dr. Hanqin Tian
E-Mail: tianhan@auburn.edu
Tel: 1-334-844-1059
Fax: 1-334-844-1084
Submitted to Journal of Geophysical Research - Biogeosciences
Feb 16, 2015
1
Supplementary Text
The Dynamic Land Ecosystem Model:
In the Dynamic Land Ecosystem Model (DLEM2.0), three major processes are involved
to simulate the export of water, carbon and nutrients from land surface to coastal areas
include 1) the generation of runoff and leachates, 2) the leaching of water, carbon and
nutrients from land to river networks in the form of overland flow and base flow, and 3)
transportation of riverine materials along river channels from upstream areas to coastal
regions. Details regarding hydrological processes in DLEM 2.0 can be found in our
recent work[Liu et al., 2013a]. Here we provide a brief description of DIC export
simulation in leachate production and in-stream transformations. Carbon transport along
river channels are controlled by water movement from upstream grid cells to downstream
grid cells [Liu et al., 2013; Yang et al., 2014].
Soil carbon decomposition:
The size of soil carbon pools and the carbon fluxes transferred between pools determine
the source and loss of soil organic and inorganic carbon, which ultimately produce soil
DIC, DOC and POC leachates to riverine systems. In DLEM 2.0, there are eight soil
carbon pools: woody detritus; two litter pools; three microbial pools; and two slow soil
organic matter pools, native organic matter and passive soil organic matter. All organic
carbon input, received from tissue turnover, manure, crop residue, and branch
fragmentation, are totally allocated to litter pools according to a carbon to nitrogen ratio.
Then the carbon fluxes are transferred between pools through biological decomposition,
physical adsorption and desorption, and leaching. The equations to estimate soil and litter
decomposition use first-order decay rate constants (𝑘𝐶_𝑃𝑜𝑜𝑙 ), which are adopted from the
models of CENTURY [Parton et al., 1993], CN-SIM [Petersen et al., 2005], and IBIS
2
[Liu et al., 2005]. Generally, heterotrophic respiration is the critical process that largely
determines the generation of soil DOC and DIC. In DLEM 2.0, decomposition rate of
each soil organic carbon pool 𝑘𝑐_𝑝𝑜𝑜𝑙 is influenced by soil temperature, soil water content,
nutrient availability and soil texture:
𝑘𝐶_𝑃𝑜𝑜𝑙 = 𝐾𝑚𝑎𝑥/365 × 𝑓(𝑇) × 𝑓(𝑊) × 𝑓(𝑁) × 𝑓(𝑐𝑙𝑎𝑦)
(1)
where kmaxi is the maximum decay rate (year-1); f(T) is the average soil temperature
scalar; f(W) is the soil moisture scalar; f(clay) is the soil texture scalar; f(N) is the
nitrogen scalar. T is air temperature (Celsius degree).
Leaching of DIC from land to rivers:
R lchDICc = 𝐻2 𝐶𝑂3 + 𝐶𝑠𝑐𝑜2 + 𝑊𝑅𝑐𝑜3
(2)
Production of dissolved inorganic C in DLEM2.0 includes three processes:
dissolution of atmospheric CO2, dissolution of soil CO2, and input from carbonate rock
weathering. Dissolution of atmospheric CO2 is assumed to be the primary source of DIC
in surface runoff. This process is simulated according to Henry’s Law. Carbon dioxide
dissolution includes two reactions. The first reaction is the process that free CO2 enters
water and becomes H2CO3. Dissolution of soil CO2 to soil water is an important
contributor of stream DIC export. To account for CO2 dissolution in soil, soil CO2
concentration (Csco2) is calculated with an empirical equation (at the depth of 1 m)
obtained from the transport equation by Jassal et al.[Jassal et al., 2005]. Another source
of DIC is soil carbonate weathering [Van Cappellen et al., 1993], where 𝑊𝑅𝑐𝑜3 is the
weathering rate of soil carbonate rock (g C/ s).
3
In-stream process:
A considerable amount of DIC leaves water in the form of CO2. In DLEM2.0,
degasing is mainly driven by temperature, air CO2 concentration and degasing coefficient:
DICdeg = K deg × (𝑊𝑝𝑐𝑜2 − SaWpco2 ) × 12
𝑆𝑐ℎ −0.5
𝐾𝑑𝑒𝑔 = 𝐾600 × 600
K 600 = Sl × V × 2841.6 + 2.03
𝑆𝑐ℎ = 1911.1 − 118.11 × 𝑇𝑤𝑎𝑡𝑒𝑟 + 3.4527 × 𝑇𝑤𝑎𝑡𝑒𝑟 2 − 0.04132 × 𝑇𝑤𝑎𝑡𝑒𝑟 3
(3)
(3.1)
(3.2)
(3.3)
where DICdeg is the degassing rate in surface water bodies (g C/m2s); Kdeg is CO2 transfer
velocity; Wpco2 is the water CO2 concentration (M); SaWpco2 is the saturated water CO2
calculated at current atmospheric CO2 level (M). Calculation of SaWpco2 follows
Equation 4; Twater is the temperature of rivers or lakes in each grid cell (ºC). K600 is the
normalized gas transfer velocity for CO2 (m day-1); Sl is the slope of river channels or
lakes (dimensionless); V is river discharge velocity (m s-1); Sch is the Schmidt number
for CO2 in water (dimensionless).
Transportation from upstream region to coastal areas:
The time step for the streamflow simulation in DLEM 2.0 is 30 minutes. Hydrological
connections between different grid cells are simulated with a linear reservoir model in
which flow direction in each grid cell are obtained from Graham et al. (1999). Allocation
of water flow to lakes and streams is weighted by their areas. The residence time of water
in rivers is calculated according to Coe [2000]:
TR,riv =
D
(4)
u
u = min(umax , umin (ic /i0 )0.5 )
4
(5)
where D is the distance between centers of the local and downstream grid cell (m); umax
is equal to 5 ms-1, which is the maximum river flow velocity [Miller et al., n.d.]; umin is
the minimum river flow velocity, we set umin as 0.8 ms-1; ic is the downstream gradient
(mm-1), and i0 is a reference gradient (0.5×10-4 mm-1). For the residence time of water in
the lake, we assume it is 10 times longer than that of river in the same grid cell. Different
carbon species are assumed to distribute evenly in each river body and move downstream
with water. More detailed descriptions of the hydrological component in DLEM 2.0 were
published in recent work [Liu et al., 2013; Yang et al., 2014].
5
Figure S1. Key processes of dissolved inorganic carbon (DIC) related to loss and sources
in the Dynamic Land Ecosystem Model (DLEM) and the potential effects of future
environmental changes in climate, atmospheric CO2, and land use.
Figure S2. Spatial changes in precipitation (mmyear-1) in the 2000s and the 2090s under
the A2 and B1 scenarios, projected by three global climate models (CCCMA, CCM3,
ECHAM).
Figure S3. Spatial changes in average temperature (°C) in the 2000s and the 2090s under
the A2 and B1 scenarios, projected by three global climate models (CCCMA, CCM3,
ECHAM).
Figure S4. Spatial changes in cropland area (1000km2) in 2010 and 2099 under the A2
and B1 scenarios.
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Climate, atmospheric CO2, land use and management etc.
Soil
Source
Soil and root respiration
Soil CO2 dissolution
Soil rock weathering
Soil DIC
DIC leaching
CO2 degassing
Loss
River DIC
Atmospheric CO2 dissolution
River
DIC export
Ocean
DIC related loss
DIC related source
Figure S1
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Figure S2
8
B1scenario
A2 scenario
2000s
2000s
2090s
CCCMA
CCSM3
ECHAM
Figure S3
9
2090s
Figure S4
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