Published December 14, 2012
Journal of Environmental Quality
TECHNICAL
REPORTS
TECHNICAL
REPORTS
Landscape and Watershed Processes
Carbon Export from the Raccoon River, Iowa: Patterns, Processes,
and Opportunities
Christopher S. Jones* and Keith E. Schilling
C
arbon (C) is exchanged between the atmosphere,
Farmed landscapes are engineered for productivity, and research
suggests they contribute a disproportionate share of inorganic C
to the Mississippi River and Gulf of Mexico. Here we use alkalinity
and total organic C (TOC) measurements collected from the
Raccoon River of Iowa to (i) evaluate inorganic and organic C
concentrations and export patterns, (ii) compare current trends
to historical conditions, and (iii) link C transport processes to
current land use management. Export of inorganic C averaged
106,000 Mg per year and contributes 90% of the C flux from the
basin. Alkalinity concentrations are unchanged from 1931 to
1944 levels (~53 mg L-1 C), but inorganic C loads have doubled
due to increasing discharge. Carbonate-rich glacial deposits and
agricultural lime provide a large source of inorganic C, and results
confirm that alkalinity export in the Raccoon Basin is transport
limited. Although fertilization and tillage practices have possibly
helped increase C fluxes over the last 70+ yr, the overriding
factor on inorganic C export is discharge. Discharge control over
C export provides an opportunity for agriculture in terms of
quantifying C sequestration for potential C trading. Controlling
water flux through soils can limit inorganic C export similar to
practices such as reduced tillage and managed rotations.
biosphere, lithosphere, pedosphere, and hydrosphere
in the global C cycle. In one biogeochemical exchange,
atmospheric CO2 is sequestered through chemical weathering of
carbonate rock according to the reaction
CaCO3 + CO2 + H2O ® Ca2+ + 2HCO3−[1]
where the resultant bicarbonate (2HCO3-) consists of C
liberated from the rock and C contributed by CO2. Production
of organic matter (OM) by photosynthesis is another sink for
atmospheric CO2 (Stryer, 1981),
CO2 + 2H2O + light ® CH2O + O2 + H2O[2]
Subsequent oxidation of OM (Amiotte-Suchet et al., 2003) via
the reaction
CH2O + O2 ® CO2 + H2O[3]
liberates the CO2, which can then return to the atmosphere
or react with rocks during chemical weathering (Eq. [1]).
Photosynthesis links to rock weathering because the flux of CO2
from the oxidation of soil organic matter (SOM) is a major source
of CO2 for carbonate rock dissolution (Ludwig et al., 1998;
Amiotte-Suchet et al., 2003). Belowground plant respiration
also contributes CO2 to the soil (Edwards et al., 1970).
Once sequestered, atmospheric C remains in soil or is
transported to river systems and eventually delivered to the oceans
(Ludwig et al., 1999). An estimated 1 Gt of C is transported
annually to the oceans, with about 55% in the inorganic form
and 45% in the organic form (Amiotte-Suchet et al., 2003),
both in particulate and dissolved states. Most of the inorganic
C transported by rivers is in the form of bicarbonate alkalinity
(Amiotte-Suchet et al., 2003; Raymond et al., 2008). Estimates
of global atmospheric C consumed by continental erosion
and transported as alkalinity range from 0.23 to 0.38 Gt yr−1
(Holland, 1978; Berner et al., 1983; Meybeck, 1987; Probst,
1992; Amiotte-Suchet and Probst, 1995; Ludwig et al., 1998;
Gaillardet et al., 1999; Amiotte-Suchet et al., 2003). River
transport of bicarbonate is the primary source of ocean alkalinity
Copyright © American Society of Agronomy, Crop Science Society of America,
and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.
All rights reserved. No part of this periodical may be reproduced or transmitted
in any form or by any means, electronic or mechanical, including photocopying,
recording, or any information storage and retrieval system, without permission in
writing from the publisher.
C.S. Jones, Iowa Soybean Association, SW 1255 Prairie Trail Parkway, Ankeny, IA
50023; K.E. Schilling, Iowa Geological and Water Survey, 109 Trowbridge Hall, Iowa
City, IA 52242-1319. Assigned to Associate Editor J.M. Arocena.
J. Environ. Qual. 42:155–163 (2013)
doi:10.2134/jeq2012.0159
Freely available online through the author-supported open-access option
Received 16 Apr. 2012.
*Corresponding author ([email protected]).
Abbreviations: DML, Des Moines Lobe; DMRWQN, Des Moines River Water Quality
Network; DMWW, Des Moines Water Works; DOC, dissolved organic carbon; POC,
particulate organic carbon; SOC, soil organic carbon; TOC, Total organic carbon;
UMB, Upper Mississippi River Basin.
155
and helps regulate the calcium carbonate (CaCO3) saturation
alkalinity, the dominant form of C leaving the basin via the river.
point and pCO2 in the oceans (Zeebe and Westbroek, 2003).
Our first study objective was to compare inorganic and organic
Alkalinity exported to the oceans from weathering processes can
C concentrations and loads to identify relative contributions
be considered a negative feedback to global warming because
of C sources. A second objective was to use pre-1945 Raccoon
atmospheric CO2 is consumed and the C is stored at the ocean
River alkalinity data to compare current patterns with historical
bottom (Berner and Berner, 1996).
conditions. Our third objective involved linking C transport
Export of alkalinity from the Mississippi River has been
processes in the Raccoon River to land management, climate,
examined (Gaillardet et al., 1999; Raymond and Cole, 2003;
and sustainability of current land use practices in the UMB.
Raymond et al., 2008). Raymond and Cole (2003) showed that
We conclude by discussing how riverine C export may provide
alkalinity export from the Mississippi River increased 59% in the
opportunities for C sequestration and trading.
period 1954 to 2001 due to the conversion of noncropped lands
to croplands and increased discharge. They further suggested
Methods and Materials
that increased alkalinity loading may be linked to increased N
Raccoon River Watershed
loading (Turner and Rabalais, 1994; Goolsby and Battaglin,
The Raccoon River drains a watershed of 9389 km2 in west2001), with enhanced soil respiration (Schlesinger and Andrews,
central Iowa and empties into the Des Moines River at Des
2000) and nitrification (Skiba et al., 1992) producing acids that
Moines, Iowa (Fig. 1). The Des Moines River flows southeast
increase chemical weathering rates.
and enters the Upper Mississippi River near Keokuk, Iowa. The
Spatial variations in alkalinity loading within the Mississippi
North, Middle, and South Raccoon rivers form major tributary
basin were delineated by Raymond and Cole (2003) into three
branches of the Raccoon. The North Raccoon River flows through
subbasins: Missouri, Ohio, and Upper Mississippi. The Upper
the recently glaciated Des Moines Lobe (DML) landform region,
Mississippi Basin (UMB) had the highest portion of land in
which is dominated by low relief and poor surface drainage
crops (55%) and contributed 49% of alkalinity to the total
(Prior, 1991). The geology of the area consists largely of thick
flux, even though its portion of total discharge was only 34%.
pebbly glacial till in flat till plains, clay and peat in depressions,
Furthermore, 56% of the alkalinity increase reported for the
and sand and gravel deposits in major floodplains and moraines.
1954–2001 period originated from the Upper Mississippi basin.
Over half the agricultural land area in the DML has subsurface
Increased discharge was considered the primary
cause of the increased alkalinity flux (Raymond
and Cole, 2003).
Farmed landscapes in the UMB are
engineered for high levels of productivity
(Schilling et al., 2012), and as a result, crops
both sequester atmospheric CO2 and contribute
alkalinity to rivers and streams. Reductions
in evapotranspiration due to crop production
increase discharge and the discharge to
precipitation ratio (Gedney et al., 2006; Betts et
al., 2007) and increase acceleration of the water
budget in the UMB (Schilling, 2005; Zhang
and Schilling, 2006). Raymond et al. (2008)
estimated that about 52% of the alkalinity flux
increase was due to increased discharge not linked
to precipitation (i.e., land use factors), ~20% was
due to increased precipitation, and ~30% was
due to other factors. They point out that lime
application to agricultural fields has probably
played a role in the increased flux, but examination
of the lime application rates and the total U.S.
contribution of alkalinity export to the oceans
resulting from dissolution of this lime (West and
McBride, 2005) implies that this practice is not a
major contributor to the increased alkalinity flux
from the Mississippi River.
In light of the conclusions reached by Raymond
and Cole (2003) and Raymond et al. (2008),
subwatersheds within the high-alkalinity export
area of the UMB merit examination. In this study,
we evaluate C export from the Raccoon River of
west-central Iowa, an intensely cultivated area of
the American Midwest, and focus attention on Fig. 1. Location of the Raccoon River watershed.
156
Journal of Environmental Quality
drainage (Schilling et al., 2008). The South Raccoon River drains
an older pre-Illinoian glacial landscape with higher relief and well
developed drainage and is characterized by approximately 1 to 6 m
of wind-blown silt (loess) overlying a clay-rich paleosol developed
in a dense, fine-grained, weathered glacial till. The Middle Raccoon
River drains both of these contrasting landforms. Overall, glacial
drift deposits can exceed 100 m thick in the watershed. Low
permeable Pennsylvanian (interbedded shale, limestone, and coal)
and isolated Cretaceous (sandstone and shale) rocks that underlie
the glacial deposits are not a significant water source to streams.
Agriculture is the predominant land use throughout the
watershed. Row crops of corn (Zea mays L.) and soybeans [Glycine
max (L.) Merr.] comprise about 80% of the basin area (Schilling
et al., 2008). In the early 20th century, considerably more land
was planted to small grains (primarily oats [Avena sativa L.])
and alfalfa (Medicago sativa L.) compared with the present-day
(Hatfield et al., 2009). Oat and alfalfa crops were steadily replaced
by soybeans over the past 75 yr (USDA NASS, 2009).
Since 1916, annual discharge varies from 0.15 to 5.11 km3
(USGS, 2011). Annual average temperature of seven locations
(Guthrie Center, Storm Lake, Carroll, Perry, Jefferson, Des
Moines, and Rockwell City) has ranged from 7.9 to 12.4°C, with
an average of 9.8°C during the life of the flow gauge. Little or
no trend in temperature is apparent since 1951 (0.1°C increase).
Annual average precipitation at four locations (Guthrie Center,
Storm Lake, Rockwell City, and Des Moines) during the same
period ranged from 493 to 1207 mm, with an average of 796 mm
(Iowa Environmental Mesonet, 2012).
Water withdrawn from the river for irrigation is negligible
because precipitation is adequate for crop production.
Withdrawals for industrial or municipal purposes are also
insignificant upstream of the Van Meter gauge. A few km
downstream of the gauge, the Des Moines Water Works
(DMWW) uses the stream as its principal source of supply.
Because its watershed is intensely agricultural and its water is a
major source of municipal supply, the Raccoon River has been
extensively characterized for nitrate N (Lucey and Goolsby,
1993; Schilling and Lutz, 2004; Schilling and Zhang, 2004;
Hatfield et al., 2009; Jha et al., 2010), sediment ( Jones and
Schilling, 2011), and bacteria (Schilling et al., 2009) loading.
We obtained near-daily Raccoon River alkalinity data
since 1994 and approximately biweekly data for the 1931–
1944 period from the DMWW laboratory archives. We also
examined organic C data generated by the Des Moines River
Water Quality Network (DMRWQN). This data set includes
approximately biweekly data for the Raccoon River at Van Meter
beginning in 2000. The DMWW sample location (41.57827
N, −93.64971W) is 5 stream km upstream from the Raccoon
confluence with the Des Moines River. The DMRWQN sample
location (41.534034N, −93.95061W) is 42 stream km upstream
of the Raccoon–Des Moines confluence.
calculated from alkalinity assuming a factor of 2 correction for
charge difference between carbonate and bicarbonate (Table 1).
With a few exceptions, DMWW grab samples from the river
were collected daily at approximately 07:00 h Central Time,
Monday through Friday, for the 1994–2011 period and biweekly
for the 1931–1944 period. Exceptions include some holidays
and days when river ice, floods, or extreme weather prevented
collection. Water samples were collected from near the shoreline
or from a river intake structure at DMWW’s Fleur Drive
Treatment Plant by lowering a sample catcher apparatus into
moving river water. After collection, the sample was immediately
delivered to the utility’s testing laboratory, where alkalinity and
other analyses were conducted that morning.
Total alkalinity was determined using Standard Methods
procedure 2320B (APHA, 2005). The frequently turbid river
samples were filtered to facilitate visual determination of endpoint.
Before 1994, when electronic record-keeping began, alkalinity
and other data were carefully recorded by hand in bound books
stored in a library at the DMWW treatment plant and imply
that staff were using the best available laboratory practices of
records management. Review of early Standard Methods (APHA,
1920) editions, which accompany the data books, indicates that
alkalinity was quantified by titration since at least 1920. For these
reasons, we believe the early DMWW data can be compared
favorably with the modern data generated by essentially the same
procedure. In the modern era, DMWW laboratory quality control
procedures demonstrate a long-term accuracy of 99.2% recovery
for known concentration samples and a precision of 0.2% (relative
percent difference) for duplicated samples. Duplicate and fortified
(samples spiked with a known concentration of the analyte)
samples, along with background checks of laboratory-grade water
(blanks), are evaluated daily in the DMWW laboratory.
Data for nonanalysis days (weekends and holidays) were
extrapolated using values from the measurement days immediately
before and after. Daily loads were quantified by multiplying the
daily discharge by the concentration. Loads for nonanalysis days
were calculated by multiplying the estimated concentration by
the actual daily discharge. For early DMWW data (1931–1944),
alkalinity loads were calculated by multiplying annual average
alkalinity by annual average discharge.
The DMRWQN samples were collected similarly but less
frequently. The DMRWQN organic C analysis was conducted
using Standard Method procedure 5310B (APHA, 2005), hightemperature combustion, followed by infrared detection of
CO2. Loading of total organic C (TOC) was estimated using the
USGS program LOADEST (Runkel et al., 2004). Laboratory
quality control procedures for DMRWQN data demonstrate a
long-term accuracy of 100.8% recovery for known concentration
samples and a precision of 2.1% (relative percent difference) for
duplicated samples. Quality control samples are analyzed at a
frequency of 10% for the DMRWQN monitoring program.
Carbon Measurements
Results
At the DMWW laboratory, total alkalinity is quantified by
titration with acid to pH 4.5 and reported as mg L−1 CaCO3.
Bicarbonate concentrations for the daily DMWW data were
calculated using sample pH and total alkalinity to ensure alkalinity
was indeed being exported in this form. For the 2000–2011 period,
bicarbonate was 0.982 of total alkalinity, and thus inorganic C was
www.agronomy.org • www.crops.org • www.soils.org
Inorganic Carbon Concentrations and Loads
From 1994 through 2011, daily discharge of the Raccoon
River ranged from 1.9 to 1120 m3 s−1 and averaged 64.3
m3 s−1 (Fig. 2a). Daily river discharge exceeded 600 m3 s−1 in
1998, 2004, 2007, 2008, and 2010, with values greater than
157
Historical DMWW data from 1931 to 1944 (n = ~24
per year) showed average annual Raccoon River alkalinity
concentrations to be 220 mg L−1 (52.9 mg C L−1) (Fig. 6). The
historical concentrations were similar to the 2000–2011 period,
when alkalinity averaged 224 mg L−1 (53.8 mg C L−1) (Table 1).
Average stream discharge was much smaller during the 1931–
1944 period (35 m3 s−1) compared with the 2000–2011 period
(65 m3 s−1). The average annual Raccoon River inorganic C
export during the 1931–1944 period (58,600 Mg C) was about
56% of that recorded from 2000 to 2011.
1000 m3 s-1 recorded in 1998 and 2008. In contrast to flow,
daily inorganic C concentrations were much less variable (Fig.
2b), with minimum (18.8 mg L−1) and maximum (93.6 mg L−1)
concentrations fluctuating around a mean of 53.8 ± 13.5 mg L−1
C. Daily inorganic C load exported by the Raccoon River
peaked on 15 June 1998, when 3588 Mg of inorganic C was
lost from the watershed (Fig. 2c). Daily average inorganic C
load was 290 Mg from 1994 to 2011 and totaled 1909,664 Mg
for the period. Annual Raccoon River inorganic C load ranged
from 13,902 Mg in 2000 to 267,908 Mg in 2010 (Fig. 3) and
averaged 106,092 Mg.
Median daily inorganic C loads were substantially less than
average values (132.9 Mg d−1). Of the total inorganic C mass
lost during the 18-yr monitoring period, 13% of the total C was
exported during 131 d (2% of the time), and 70% of the C load
was exported during the upper 25% of days (Fig. 4). Although
daily inorganic C concentrations were negatively related to
daily discharge (p < 0.05), inorganic C loads were significantly
positively related to discharge (p < 0.001) (Fig. 5).
Comparison of Inorganic and Organic Carbon
Concentrations and Loads
From 2000 to 2011, TOC concentrations were significantly
lower than inorganic C concentrations (Table 1). Total
organic C concentrations ranged from 3 to 28 mg L−1 and
averaged 5.2 mg L−1 (n = 258), whereas average inorganic C
concentrations were 10 times higher during the monitoring
period (53.8 mg L−1). Inorganic loads were considerably higher
than TOC loads, averaging 105,360 and 11,299 Mg, respectively
Table 1. Summary of annual inorganic carbon and total organic carbon concentrations and loads in the Raccoon River from 2000 to 2011.
Year
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
Avg.
Daily flow†
m3 s-1
10.0
(10.3)†††
55.9
(79.1)
31.6
(33.7)
47.0
(87.6)
59.5
(96.6)
39.2
(60.8)
28.6
(31.9)
104.5
(119.9)
111.4
(151.9)
73.9
(63.8)
158.1
(164.5)
56.3
(71.5)
64.7
(46.9)
Alkalinity (as
CaCO3)‡
192.6
(44.5)
188.7
(48.4)
223.1
(56.8)
214.6
(49.9)
204.9
(47.3)
206.3
(46.5)
218.2
(54.1)
245.2
(63.6)
245.1
(55.4)
247.0
(53.8)
265.4
(47.9)
240.2
(44.3)
224.3
(5.8)
Inorganic C§
Measured
TOC¶
———— mg L-1 ————
46.2
5.48
(10.7)
(0.93)
45.3
5.45
(11.6)
(1.87)
53.5
4.55
(13.6)
(1.60)
51.5
6.08
(12.0)
(2.06)
49.2
6.59
(11.4)
(2.61)
49.5
6.73
(11.2)
(3.07)
52.4
5.86
(13.0)
(1.93)
58.8
4.17
(15.3)
(1.31)
58.8
5.82
(13.3)
(5.47)
59.3
4.36
(12.9)
(2.97)
63.7
3.64
(11.5)
(0.73)
57.7
3.27
(10.6)
(0.46)
53.8
(1.4)
5.2
(1.35)
Total Inorganic Total Organic
C#
C††
Inorganic C
fraction of
total C‡‡
Organic C Ratio Inorganic
fraction
to
of total C§§
organic C¶¶
———— Mg ————
13,901.5
1575.7
0.815
0.185
8.82
70,166.7
10,608.8
0.768
0.232
6.61
52,418.4
5700.1
0.821
0.179
9.20
73,876.2
10,414.5
0.780
0.220
7.09
81,776.8
13,080.7
0.758
0.242
6.25
59,259.0
7706.1
0.794
0.206
7.69
47,630.5
4831.4
0.831
0.169
9.86
174,868.5
20,165.1
0.813
0.187
8.67
183,912.3
21,372.1
0.811
0.189
8.61
137,481.2
10,221.9
0.871
0.129
13.45
267,907.9
24,009.2
0.848
0.152
11.16
101,115.2
5899.9
0.895
0.105
17.14
105,359.5
11,298.8
0.823
0.177
9.32
† Average daily flow at the Van Meter USGS gauge.
‡ Average daily total alkalinity as mg L-1 CaCO3 generated by the Des Moines Water Works Laboratory.
§ Inorganic C concentration calculated from the total alkalinity (total alkalinity × 0.12 × 2).
¶ Average of biweekly total organic C measurements generated by Des Moines River Water Quality Network.
# Summed total of daily loads obtained by multiplying concentration by discharge, with concentrations extrapolated for nonanalysis days.
†† Loads estimated by LOADEST.
‡‡ Inorganic C load divided by sum of Total Inorganic and Total Organic load.
§§ Organic C load divided by sum of Total Inorganic and Total Organic load.
¶¶ Inorganic C load divided by Organic C load.
††† Standard deviations of measured values in parentheses.
158
Journal of Environmental Quality
(Table 1). Of the total C load exported in the Raccoon River
from 2000 to 2011, 90.3% was inorganic C, and 9.7% was
TOC (Table 1). The fraction of C export for inorganic C
varied from 86.2 to 94.5% and was higher during the latter
3 yr of the study period. The average ratio of inorganic to
organic C loads was 9.3 for the study period, but this ratio
was substantially higher in 2011 (17.1) (Table 1).
Discussion
Sources of Carbon
Glacially derived deposits in Iowa contain a large proportion
of CaCO3 and CaMg(CO3)2 minerals (Ruhe, 1969). In the
DML landscape region, through which much of the Raccoon
River flows (Fig. 1), calcareous soils formed from the weathering
of calcareous parent materials, such as limestone and dolomite,
common to the Midwest (Rabenhorst et al., 1991; Prior,
1991; Rogovska et al., 2007). Ruhe (1969) reported the soil
CaCO3 distribution in a typical DML hillslope ranges from
1 to 20% CaCO3. Similarly, Rogovska et al. (2007) measured
soil CaCO3 as high as 30% with most values <10 to 15%. High
values of CaCO3 are associated with soils having pH >7.5
(Rogovska et al., 2007). Upper soil profiles and well drained
areas are often leached of carbonates, which can accumulate
in unweathered zones and in low-lying areas. In the Southern
Iowa landscape drained by the South Raccoon River, soils are
formed in loess and older pre-Illinoian till that were originally
calcareous (Kay and Apfel, 1929; Hutton, 1948). The upper Fig. 2. Daily Raccoon River discharge (a), inorganic C concentration (b), and
horizons of both units (~3–4 m) are typically oxidized and inorganic C loads (c), 1994–2011.
leached of carbonates, but deeper zones are unleached and
Nonetheless, lime application in the Raccoon River should be
contain abundant inherited carbonates (Ruhe, 1969; Hallberg,
considered a source of inorganic C in the basin that is available
1978; Kemmis et al., 1981). Loess deposits occasionally contain
for leaching and transport.
secondary carbonates as nodules (“loess kindchen”). With a soil
Much of the TOC is derived from allochthonous (SOM,
matrix dominated by carbonate, alkalinity export is generated
terrestrial plants, litter decomposition) and autochthonous
when CO2 encounters the unleached soil profile and carbonate
(phytoplankton, benthic algae, macrophytes) sources ubiquitous
weathering occurs. Simple water flux through calcareous soils
to the Upper Mississippi River basin (Bianchi et al., 2007;
leaches inorganic C (White and Blum, 1995; Stumm and
Warrner et al., 2009). Particulate organic carbon (POC),
Morgan, 1996), so landscapes that produce greater water yield
consisting mainly of eroded soil and vegetation debris, has been
export more alkalinity.
highly correlated with total sediment flux (Ludwig et al., 1996).
Despite the presence of carbonate-rich soils in Iowa, we
In the Raccoon River, recent work indicates that sediment loads
cannot ignore the addition of agricultural lime as a potentially
have decreased in the river since peaking in the early 1970s
important source of inorganic C. Our estimates using available
( Jones and Schilling, 2011), implying that POC loads may
data (R. White, personal communication, 2012) of the average
have similarly decreased over the last few decades. Since 1985,
annual inorganic C exported as a result of lime applications in
annual sediment loads are highly correlated with precipitation
the Raccoon River watershed could be as high as 32,775 Mg,
which would represent approximately 31% of the inorganic C
export if all of the applied lime-C was lost as riverine bicarbonate
export. Lime applications vary considerably across the state and
are more important in regions where surface soils have been
leached of carbonate minerals. In the Raccoon River basin, lime
applications are likely to be more prevalent in the smaller South
Raccoon River basin where older loess soils have been leached,
compared with the larger North Raccoon where recently glaciated
deposits are enriched with carbonates. Furthermore, West and
McBride (2005) suggest that as much as 25% of the applied
lime-C may be lost back to the atmosphere as CO2 gas. Finally,
reporting of agricultural lime application and consumption is
not well monitored, prompting West and McBride (2005) to
characterize US agricultural lime consumption data as “suspect.”
Fig. 3. Annual Raccoon River inorganic C loads.
www.agronomy.org • www.crops.org • www.soils.org
159
et al., 2008), DOC may contribute greatly to TOC
loads. Contributions of DOC from groundwater
(Vidon et al., 2008), tile drains, in-stream sources,
and possibly benthic sediments and eroded soils are
difficult to differentiate (Warrner et al., 2009).
Comparison to Regional Studies
Raymond and Cole (2003) estimated inorganic
C export for the entire Mississippi River basin above
St. Francisville, Louisiana to be 1.75 × 1013 g yr−1
for the end of their period of record (2001). They
also state that the UMB contributed 0.49 of the
total alkalinity flux (i.e., 8.58 × 1012 g yr−1). This
translates to an inorganic C yield for the UMB
(above the Missouri River confluence) of 1.93 × 105
g ha−1 yr−1. Raymond and Cole (2003) calculated
loads by multiplying annual average alkalinity
concentrations by annual average discharge from
an alkalinity data set that averaged 17 samples per
Fig. 4. Percentile rank of daily Raccoon River inorganic C loads for the 1994–2011 period.
year at St. Francisville. Using the DMWW data
( Jones and Schilling, 2011), suggesting that POC export may
set, which averages about 250 samples per year,
be largely driven by rainfall runoff patterns. This may explain
and calculating annual loads by summing daily loading data,
why the ratio of inorganic:organic C was unusually high in 2011
we estimate the average annual inorganic C yield for Raccoon
(17.1) (Table 1). The second half the of year was characterized
River Basin to be 1.24 × 105 g ha−1 yr−1 since 2000. This loading
by dry conditions, which surely reduced surface runoff and
rate is substantially less (0.64) than the UMB inorganic C
the transport of POC relative to inorganic C present in the
yield deduced by Raymond and Cole (2003), even though the
groundwater and tile water feeding the river. On the other hand,
fraction of cropland in the Raccoon is far higher (0.77 vs. 0.55).
dissolved organic C (DOC) losses can be significant in tileThe difference could be associated with a lower water yield in
drained Midwestern watersheds (Royer et al., 2005; Warrner
the Raccoon River Basin (2.10 × 105 m−3 yr−1) compared with
et al., 2009). In the North Raccoon River watershed, where
the UMB (2.67 × 105 m−3 yr−1) since 2000. On the other hand,
more than half of the cropped fields are tile drained (Schilling
inorganic C yields calculated using the DMWW data are more
consistent with Gaillardet et al. (1999) if we apply the 0.49
loading contribution for the UMB used by Raymond and Cole
(2003). Gaillardet et al. (1999) estimated inorganic C loading
Fig. 5. Daily Raccoon River inorganic C concentration and load
versus discharge.
160
Fig. 6. Comparison of Raccoon River inorganic C concentrations and
loads, 1931 to 1944 versus 2000 to 2011.
Journal of Environmental Quality
for the Mississippi to be 1.2 × 1013 g yr−1, which translates to
1.33 × 105 g ha−1 yr−1 for the UMB, only slightly larger than the
yield of inorganic C we calculate for the Raccoon River Basin.
Considering the intensity of crop production in the Raccoon
watershed, we expected the yield of inorganic C to be higher
in the Raccoon River Basin compared with the UMB. Further
work is needed to determine whether differences in inorganic C
loading rates among watershed areas is real or an artifact of using
difference data sets or methods.
Land Use, Climate, and Alkalinity Export
Although much of the focus on alkalinity fluxes has been on
discharge, other factors may be contributing. Fertilization is one
such factor. An estimated 128,000 Mg yr−1 of N inputs enters the
Raccoon watershed in the form of chemical fertilizers, animal
manures, and legume fixation (Schilling et al., 2008). Nitrogen
fertilizer can increase bicarbonate formation and transport
through two mechanisms. First, the formation of nitric acid in
the soil can result in dissolution of carbonate minerals without
consuming atmospheric CO2 (Skiba et al., 1992; AmiotteSuchet, 1995). This mechanism has been shown to generate
6% of the bicarbonate load in a carbonate-abundant farmed
watershed (Semhi et al., 2000). Second, fertilization stimulates
soil respiration and thus production of the source of CO2 for
chemical weathering (Schlesinger and Andrews, 2000). On the
other hand, research has shown that N fertilization can enhance
sequestration of soil organic C (Halvorson et al., 2000), primarily
through return of crop residues to the soil. Overall, the role of N
fertilizer on bicarbonate formation and transport is complex and
worthy of future research.
Tillage practices have been implicated in the loss of soil
organic C (Doran, 1980; Allmaras et al., 2000; Baker and
Griffis, 2005), primarily through soil exposure to oxygen and loss
of organic C as CO2 to the atmosphere after oxidation. Losses
have also been linked to tile drainage systems (Royer et al., 2005;
Warrner et al., 2009). Some estimate that as much as 30 to 50%
of the SOM present before cultivation has been lost from soils
of the Midwestern United States, including Iowa (Allmaras et
al., 2000).
Increased temperatures can also be a factor in C transport
through enhanced ecosystem respiration (Griffis et al., 2003;
Suyker et al., 2004), and this is true for soils (Bond-Lamberty
and Thomson, 2010). However, an examination of climate data
(Iowa Environmental Mesonet, 2012) shows only about a 0.1°C
increase in annual average watershed temperature since 1951.
Increased temperature does not seem to be a major factor for
increase C transport in the Raccoon watershed.
Concentrations of alkalinity have changed little, if at all, in
the Raccoon River since the 1940s (Fig. 6). Raymond and Cole
(2003) deduced a slight increase in concentrations in the UMB,
whereas Raymond et al. (2008) indicated that dataset may have
been biased by a decade of low flows at the beginning of the
record. They found alkalinity flux at average flows increasing
substantially, especially in watersheds with annual precipitation
>0.6 m yr−1. Raymond et al. (2008) demonstrated that the
alkalinity flux from portions of the UMB is transport limited and
is mildly transport limited in regions where annual precipitation
is between 0.5 and 1.0 m yr−1, which includes the Raccoon
River watershed (0.80 m yr−1 since 1900) (Iowa Environmental
www.agronomy.org • www.crops.org • www.soils.org
Mesonet, 2012). In transport-limited systems, soil water is near
saturation with resident ions, and increasing inputs of weathering
agents does not dramatically affect bicarbonate concentrations.
As water throughput increases, however, the added water
encounters a reservoir rich in bicarbonate, ready for transport.
It does not appear that alkalinity concentrations have changed
appreciably in the Raccoon River in the last 80 yr (52.9 mg L−1
C, 1931–1944 vs. 53.8 mg L−1 C, 2000–2011). On the other
hand, streamflow and baseflow in the Raccoon River increased
through the 20th century (Schilling and Libra, 2003; Jones and
Schilling, 2011) due to increasing precipitation and land use
change from perennial to annual crops. Increasing water flux
through carbonate-rich soils has approximately doubled C export
from the Raccoon River across the latter half of the 20th century.
Alkalinity transport in the Raccoon basin does indeed seem to be
transport limited. It is interesting to note that given the increasing
intensity of agricultural production, large N inputs, and calcareous
soils, alkalinity concentration increases were not observed in the
Raccoon River, and the yield of alkalinity from the Raccoon River
watershed may be less than the rest of the UMB, according to the
UMB estimates of Raymond and Cole (2003).
Riverine Carbon Flux Compared with Grain
Carbon Export
Baker and Griffis (2005) used eddy covariance and mass
balance techniques to determine SOC loss from fields in Southern
Minnesota to be about 90 g m−2 over a 2-yr period in corn–
soybean rotation. Hernández-Ramírez et al. (2011) similarly
examined CO2 fluxes in corn–soybean rotations in fields <50 km
from the Raccoon River near Ames, Iowa, sharing the same DML
landform as the North Raccoon River. Soils are similar to those
found in the North Raccoon River watershed (typic Hapludolls)
(USDA–NRCS, 2012) (i.e., of the same age and type and formed
in the same glacial parent material under prairie vegetation).
Carbon dioxide fluxes were monitored using eddy-covariance
and soil chambers in adjacent production fields (HernándezRamírez et al., 2011). When export of grain C was considered,
their corn–soybean rotation showed an apparent decrease in soil
C of 104 ± 57 g C m−2 yr−1 over a period of 4 yr (2004–2007),
with the negative flux due almost entirely to the export of grain
C. Raccoon River C flux during the same 4-yr period (inorganic
and organic C) averaged 10.9 g C m−2 yr−1. Because only half of
the inorganic C load can be linked to SOC (Eq. [1], where one
CO2–C originates from the soil and one carbonate-C is liberated
from the rock), the riverine export of SOC (linked to both riverine
organic and inorganic carbon) is at most 6.1 g C m−2 yr−1. Hence,
loss of SOC via river flux is only about 1/20th the C loss from
grain harvest calculated by Hernández-Ramírez et al. (2011).
They measured total SOC of their field site as 23,900 g m−2 to a
depth of 1.2 m. The calculated maximum Raccoon River SOC
export of 6.1 g C m−2 yr−1 translates to only 0.03% of the SOC
measured by Hernández-Ramírez et al. (2011). Assuming the
soils in the Hernández-Ramírez et al. (2011) study are indeed
similar to those in the Raccoon Basin, which seems very likely, we
therefore conclude here that loss of SOC via chemical weathering
of carbonate minerals and river transport of alkalinity does not
seriously threaten the productive sustainability of soils in the
Raccoon River watershed.
161
Opportunity for Agriculture
Sequestration of soil C provides multiple agricultural and
environmental benefits. Consideration has been given to the
implementation of various C credit trading systems that could
provide economic incentives to farmers participating in C
sequestration initiatives (Marland et al., 2001a, 2001b). Various
strategies have been proposed to reduce loss of SOC, mainly
through tillage modifications, residue management, and use
of cover crops. Baker and Griffis (2005) examined the use of
strip tillage and cover cropping on SOC flux. The cumulative
difference between conventional tillage and strip tillage on C
sequestration was approximately 10 g m−2 yr−1. The addition
of a spring oats cover crop provided no benefit in the form of
C sequestration. They concluded that policies that monetarily
stimulate practices to increase SOC may not be especially
effective. Halvorson et al. (2002) experimented with tillage,
fertilization, and crop rotations in North Dakota and examined
their effects on SOC. Their best system improvement, no-till on
a wheat-sunflower rotation, improved sequestration of SOC by
23 g m−2 yr−1. West and Post (2002) found that changing from
conventional tillage to no-till sequesters an average of 57 g
C m−2 yr−1 and that enhancing rotation complexity can sequester
an average of 20 g C m−2 yr−1.
We have studied another tile-drained watershed on the DML
Landform, Lyons Creek (Schilling et al., 2012), which lies about
50 km east of the Raccoon River watershed. Preliminary data
from three tile mains in the Lyons Creek basin show total C
(bicarbonate-C plus TOC) yield via tile flow to range from 24
to 39 g m−2 yr−1. These values from intensively drained cropped
fields are similar to those found in the much larger Raccoon
River watershed during the same time period (2008–2010,
~23 g m−2 yr−1) and compare favorably with C sequestration
rates from land management practices. This may present an
interesting opportunity for agriculture. If farmers can reduce
water throughput through engineered and drained systems, most
of which are likely transport limited for C export, they may be
able to achieve C sequestration rates relatively similar to practices
such as reduced tillage and crop rotation complexity. For example,
the use of in-line water level control structures (Gilliam et al.,
1979; Wesstrom et al., 2001) to manage water flow out of field
tiles presents an easy and inexpensive opportunity for farmers
to control C loss without operational challenges presented by
tillage and crop rotation modification. Furthermore, monitoring
C loss through these engineered systems is a simple matter of
collecting flow and water quality data, which lends itself well to
any C credit trading systems. Managing water flux further offers
the benefit of reducing nitrogen and phosphorous export from
these hydrologically modified systems (Gilliam et al., 1979;
Wesstrom et al., 2001).
Conclusions
The Raccoon River in west-central Iowa has been extensively
studied because it drains an intensively cultivated watershed
representative of the Corn Belt region of the United States and
is frequently monitored as a drinking water supply source. Using
near-daily measurements of river alkalinity for an 18-yr period
(1994–2011) obtained from DMWW, we report that inorganic
C export from the Raccoon River averaged 106,000 Mg per year
162
and contributes 90% of the C flux from the basin. Inorganic C
concentrations were relatively unchanged from levels measured
more than a half century earlier (~53 mg L−1 C), but loads
approximately doubled since the 1931–1944 period due to the
effects of increasing discharge on stream loads. Carbonate-rich
glacial deposits and agricultural lime provide a large source
of inorganic C, and our results confirm that alkalinity export
in the Raccoon Basin is transport limited as suggested by
Raymond et al. (2008) for many areas in the UMR. In the end,
discharge control over C export may provide an opportunity
for agriculture. Controlling water flux through soils can limit C
export on par with land management practices such as reduced
tillage and managed rotations.
Acknowledgments
The authors acknowledge DMWW, Iowa Soybean Association, and
Iowa Department of Natural Resources for supporting this research.
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