SOIL ORGANIC CARBON STOCK IN PAMPEAN SOILS: CHANGES
ASSOCIATED TO ROTATION AND TILLAGE
Roberto Alvarez
Facultad de Agronomía, Universidad de Buenos Aires and CONICET. Av. San Martín
4453 (1417), Buenos Aires, Argentina. E-mail: ralvarez@agro.uba.ar
Abstract
During the last four decades the cropped area in the Pampas doubled, soybean was
introduced in rotations, replacing mainly corn, becoming nowadays the most
important grain crop, and no-till adoption by farmers was massively with 80% of the
area under this tillage system nowadays. Technological improvements, as the
generalized use of fertilizers, lead to yield increases of 100%. Using data from soil
surveys performed between 1960 and 1980, a soil organic carbon stock of 4.12 Gt to
1 m depth was estimated. In 2007-2008, 400 sites distributed along the region were
sampled for organic carbon evaluation and an artificial neural network model was
developed for carbon prediction as a function of vegetation cover and soil use. The
model, joined to satellite image classification of the area occupied by common
vegetation types and land uses, lead to an estimation of 4.22 Gt of organic carbon.
Despite the overall carbon stock did not change during the last decades, decreases
were observed in high carbon areas and increases in low carbon ones. Results from
7 long term field experiments were used for fitting a neural network model for
estimating organic carbon mineralization. Another neural network model was
adjusted to data generated in 116 experiments, which allowed relating crops yield
with carbon inputs from residues to the soil. The combination of these two models
was used to calculate the soil carbon balance for different rotations and soils. Carbon
balances are less negative than 40 years ago or turned out positive in low carbon
soils, a consequence of yield increases and higher carbon inputs from straw and
roots. The meta-analysis of data from 17 tillage experiments showed that under no-till
soil carbon increases by 5 % in rich organic matter soils and 15 % in low organic
matter soils at surface. No-till counteracted the impact of soybean on carbon inputs
helping carbon level maintenance. The models developed predict that in the following
years soil organic carbon of many pampeam subregions will decrease if the
proportion of soybean in rotation is not reduced.
Keywords: soil carbon, crop rotation, tillage system
The Pampas and its cropping history
The Pampas in Argentina (30-40ºS, 58-68ºW) is a vast plain of ca. 50 Mha (1), with a
flat or slightly rolling relief, and graminaceus grasslands as natural vegetation.
Forests occupy around 7 % of the area (2) without changes during the last decades
(3). Temperature ranges from 14ºC in the south to 18ºC in the north and rainfall
varies from 500 mm in the west to 1100 mm in the east (Figure 1). Predominant soils
are Mollisols, which evolved on loess like materials of eolian origin and present a
broad range of depth, texture and organic matter, being illite the main clay mineral
(4). The eolian origin of sediments (southwest to northeast) and the rainfall trend
1
(west-east) determined a soil gradient from sandy textured, shallow and with low
organic matter in the west to fine textured, deep and with high organic matter in the
east, (1, 4). Along the west and the south of the region a petrocalcic horizon is found
within the upper 1 m of many soils (5, 6, 7).
Figure 1. Map of the Pampean Region showing isohyets and isotherms.
At the end of the 19th Century (ca. 1870) agriculture was introduced in the central
humid portion of the region, on fertile soils, and expanded in the 20th Century to the
north, west and south (1, 8). Low external input farming systems predominate, based
on harvest crop and cattle grazing rotations (9). Because of economic reasons,
joined to the increase of rainfall during the last few decades, the cropped area
increased exponentially (10). Especially in humid environments the traditional
pasture-crop rotation was replaced by continuous cultivation (Figure 2). Well drained
soils are used for cropping and areas with hydromorphic soils destined to grazing (1).
Nowadays, more than 50% of the Pampean area is cropped with soybean (Glycine
max), corn (Zea mays), wheat (Triticum aestivum) and sunflower (Helianthus annuus)
(Figure 3A). Cultivation intensification since 1970 was accompanied with the
widespread adoption of soybean in rotations (9). This crop occupies around 60 % of
the agricultural area (11), replacing corn in rotations.
Year
1
2
3
4
5
6
7
8
Figure 2. Common rotations used in the central portion of the Pampas.
2
Seeded surface (Mha)
30
Fertilizer consumption (Mt)
Soil organic carbon depletion (8, 12, 13), losses of nutrients due to negative balances
(14), and soil erosion (15), are common degradation processes in the Pampas.
Fertilizers use (Figure 3B) and no-till adoption (Figure 3C) widespread exponentially
for counteracting these negative effects of agriculture and, also, for economic
reasons. At the same time, genetic improvement determined potential yield increases
(16, 17). The combination of all these effects lead to yield increased at field
conditions that are, despite soil degradation, 2-3 fold greater than four decades ago
(Figure 3D). The Pampas had been considered as one of the most suitable areas for
grain crop production in the World because of its extension and yield potential (18),
but concern has increased in the region of the possible effects of the agricultural
expansion and soybean adoption on soil organic carbon due to its low residue input
to the soil.
A
25
20
Soybean
15
10
5
Wheat + corn + sunflower
0
1970
1980
1990
2000
4.0
B
3.0
2.0
1.0
0.0
1970
2010
1980
2000
2010
D
Average yield (kg ha-1)
No-till surface (Mha)
2010
4000
C
80
60
40
20
3000
2000
1000
0
1970
2000
Year
Year
100
1990
0
1980
1990
2000
2010
Year
1970
1980
1990
Year
Figure 3. A. Seeded surface of grain crops in the Pampas. B. Overall fertilizer
consumption. C. Cropped surface under no-till. D. Yield of the average rotation.
Elaborated with data form (11).
Cropping effects on soil carbon stocks
Research on soil organic carbon focus on productivity and on carbon sequestration
for mitigating the greenhouse effect (11, 20). Climate impacts net primary productivity
and mineralization regulating carbon inputs and outputs from the soil (21, 22). Soil
3
organic carbon is greater under humid or cold climates than under arid or warm
scenarios (22, 23). Texture, by affecting productivity (24) and mineralization (25),
impacts organic carbon, leading to greater carbon content in fine particle soils (26,
27). Net primary productivity is greater in forests than in grasslands and these
ecosystems have greater productivity than crops (28). Additionally, as in cultivated
areas part of the biomass is harvested, carbon input to the soil is greater in natural
than in cultivates systems (28, 29). The allocation of carbon into above- belowground
biomass, and roots distribution in depth depend on vegetation type (30, 31). The
combination of these factors determines that organic carbon is less stratified in
grasslands than in forest (31) and in cropped soils than in natural ecosystems (22).
Under cultivation a depletion of organic carbon content of surface soil usually occurs
(32, 28), but recent studies also showed deep soil organic carbon reductions (33,
34). In arid regions inorganic carbonate accounts for more than 50% of total soil
carbon (35, 36) and may be affected by management. Irrigation with carbonate-rich
water can increase soil inorganic carbon (37) but tillage exposition of buried soil (38)
or acidification due to fertilization (37) lead to inorganic carbon depletion.
Between 1960 and 1980 an area of ca. 74 Mha, which included the Pampas, was
surveyed (39, 40, 41, 42, 43, 44). Around 2000 soil profiles with their corresponding
influence area were described. Using a methodology previously used (4) soil
information was integrated at county level. Profile data were modelled in depth and
organic-inorganic carbon and textural properties estimated for fixed soil layers (45).
The surveyed area had and organic carbon stock to 1 m depth of 5.73 Gt and an
inorganic carbon stock of 3.65 Gt. Organic carbon represented in average 60 % of
soil carbon with higher proportions (up to 100%) in the humid portion of the region
and lower proportions (20%) in arid-semiarid areas. In the 0-25 cm soil layer around
50% of the organic carbon was sequestered.
During 2007-2008 another soil survey, covering and area of 48 Mha along the
Pampas, was performed sampling 386 soils under contrasting vegetation types and
land uses (Figure 4) (46). Organic and inorganic carbon stocks were reported (Figure
5) and significant differences (P= 0.05) were detected between scenarios in organic
carbon (tree soil > uncropped soil > cropped soil under the pasture phase of a mixed
rotation = cropped soil under the agriculture phase of rotation or under continuous
cultivation > flooded soil destined to grazing), but not in carbonate carbon.
Approximately, 50% of organic carbon was sequestered in the upper 25 cm of soil
profile and 90% of carbonate carbon below 50 cm depth under all vegetation typesland uses.
As reported in other regions of the World soil organic carbon was higher in the
Pampas under trees than in the other soils (33), possible because of greater carbon
inputs of forests compared to other ecosystems (47). Dryland cultivation usually
reduces ca. 30-50% organic carbon at surface (20-30 cm) (33, 48) by decreasing
carbon inputs in comparison to natural ecosystems (49). In the Pampas crops carbon
inputs are 30-70 % of grasslands inputs (50), leading to the organic carbon losses
reported here between cropped and uncropped sites. Soil erosion is another factor
that causes carbon losses in the Pampas (15). Rotation phase had only minimal
impact on carbon stocks, as showed previously in local long-term field experiments
(51, 52). In the Pampas net primary productivity is 35 % lower in flooded lands than
in well drained soils (53), leading to low organic carbon stocks.
4
Figure 4. Map of the Pampean Region showing sites sampled in 2007-2008.
SOC (t ha-1)
0
20
40
60 0
20
40
60 0
20
40
60 0
20
40
60 0
20
40
60
0
Depth (cm)
a
b
c
c
d
25
a
b
c
c
d
50
a
b
b
b
c
75
a
b
b
b
b
100
SIC (t ha-1)
0
20
40
60 0
20
40
60 0
20
40
60 0
20
40
60 0
20
40
60
Depth (cm)
0
a
ab
a
a
a
b
a
25
a
a
a
50
a
a
a
a
a
75
a
a
a
a
a
Pasture
Cropped
Flooded
100
Trees
Uncropped
Figure 5. Soil organic (SOC) and inorganic (SIC) carbon stocks to 1 m depth of
pampean soils surveyed in 2007-2008 under different vegetation types and land
uses (Trees= natural or planted trees, Uncropped= sites never cultivated under
graminaceus vegetation, Pasture= cropped soils sampled during the pasture phase
of a mixed rotation, Agriculture= cropped soil sampled during the agriculture phase of
a mixed rotation or under continuous cultivation, Flooded= hidromorphic soils at foot
slope positions devoted to grazing). Different letters for a soil layer indicate significant
differences (P= 0.05). Elaborated with data from (46).
5
120
-1
Observed organic carbon (t ha )
An artificial neural network was fitted with good performance for soil organic carbon
prediction by soil layer using as inputs vegetation type-land use, soil depth and
texture, temperature and rainfall of the site (Figure 6) (54). The model showed that
organic carbon increased with annual rainfall of the site and clay content of the soil
layer considered and decreased with soil depth, mean temperature of the site and
sand content of the soil layer. The network, joined to satellite image classification for
assessing vegetation type-land use area at county scale, allowed the estimation of
present soil organic carbon of the Pampas (55). For the area surveyed in 2007-2008
(48 Mha) organic carbon stock to 1 m depth was estimated as 4.22 Gt, similar to the
1960-1980 survey for the same area (Figure 7). Consequently, no depletion of the
organic carbon stock was detected between both surveyed times. Comparing results
for the 0-25 cm stratum, carbon stocks were 2.12 Gt in 1960-1980 and 1.93 Gt in
2007-2008, indicating a 9% decrease. Making comparison at county scale showed
that in average areas with carbon stocks greater than 95 t ha-1 to 1 m depth loss
carbon but below this threshold gains were more common (Figure 8).
100
80
60
y=x
R2 = 0.642
40
20
0
0
20
40
60
80
100
120
Estimated organic carbon (t ha-1)
Figure 6. Artificial neural network model fitted to soil organic carbon contents in
pampean soils. Full drops: training data set, empty drops: validation data set. Redraw
from (54).
Assuming that during the last century forest and flooded land surface remained
unchanged in the Pampas and that cultivation expanded on natural grassland, a net
flux of 230 Mt of carbon to the atmosphere was estimated using the neural network
developed. This is equivalent to the fossil fuel consumption of 6 years in the region
(56). Main carbon looses to 1 m depth were produced, apparently, before the 19601980 survey and the expansion of agriculture and soybean adoption. Cultivation had
no impact on inorganic carbon, possibly as the consequence of the short cropping
history of the region and its low fertilizer use.
6
SOC (t ha-1)
1960-1980
SOC (t ha-1)
2007-2008
4.12 Gt
SIC (t ha-1)
1960-1980
4.22 Gt
SOC (t ha-1)
No data
SIC (t ha-1)
No data
1.87 Gt
Figure 7. Soil organic (SOC) and inorganic (SIC) carbon stocks at county level to 1 m
depth in the Pampas at two sampling times. Numbers under the maps indicated total
carbon stock of the shaded are. Data from (55).
7
Organic carbon change (t ha-1 m-1)
100
10.0
10.0
8.8
12.4
75-95
95-115
7.0
50
0
-50
-100
35-55
55-75
115-180
Organic carbon 1960-1980 survey (t ha-1 m-1)
Figure 8. Box plot of the soil organic carbon change to 1 m depth between 20072008 and 1960-1980, as a function of organic carbon in 1960-1980. Boxes were
calculated with county carbon stocks showing 5, 25, 50, 75 and 95 percentiles. The
numbers over the boxes are the area (Mha) summed of the counties aggregated in
each box. Redrawn from (55).
Carbon balance in cropped soils
Long-term experiments are used for evaluating management effects on soil organic
carbon (58, 59, 60) because its changes are slow and difficult to detect (57). The
impact on soil carbon of practices not tested in these experiments can not be
assessed and results can not be extrapolated to other regions or to the future (57).
Process-based models attain these goals but their use is constrained when
information for the parameterization-validation process is not available (61). Simple
models, like Roth-C (62), need information on crop productivity and carbon input to
the soil, meanwhile the use of sophisticated models like Centtury (63) that simulate
these processes, is restricted by available information for parametrization-validation
in developing countries. The carbon budget approach is another option for studying
soil carbon dynamics in many ecosystems (64). Because carbon fluxes to or from the
soil are much greater than soil carbon changes, increases or decreases can be
determined by this approach in short time periods (65, 66). Variations from kilograms
to tons per hectare can be detected in yearly periods (67, 68). Inputs-outputs of
carbon to the soil are usually experimentally assessed under field conditions, but
when carbon changes under scenarios from which there are no available data are of
interested, empirical modelling of ecosystem carbon fluxes must be performed.
Carbon budget has been estimated in some pampean experiments in which
contrasting tillage systems and rotations were tested. Integration of data from these
experiments indicated that temperature is a main controlling factor of soil
heterotrophic respiration, which shows a strong seasonal variation (Figure 9). Tillage
8
C-CO2 emission (kg ha d )
-1
80
40
Semiarid Pampa
-1
Humid Pampa
-1
-1
C-CO2 emission (kg ha d )
system has no deep effect on annual respiration under both humid and semiarid
climates, but carbon input to the soil is affected by tillage in some environments.
Meanwhile under humid scenarios tillage management has no impact on soil water
content and crops productivity (71, 73), under semiarid climate greater water content
and productivity had been reported under no-till when compared to tillage
management (74). Consequently, under no-till soil carbon budget turned to neutral in
the Semiarid Pampas in comparison to tilled systems that loose carbon (Figure 10).
Experiments conducted in the Humid Pampa showed that tillage systems do not
affect soil carbon budget (50).
60
40
20
0
30
20
10
0
0
60
120
180
240
Julian day
300
360
0
60
120
180
240
300
360
Julian day
Figure 9. A. Seasonal evolution of soil heterotrophic respiration in the humid and the
semiarid portions of the Pampas under contrasting tillage treatments. Elaborated with
data from (69, 15, 70, 74, 72) for Humid Pampa and (74) for Semiarid Pampa. Full
drops: tilled treatments, empty drops: no-till.
Crops carbon inputs to the soil in straw and roots had been evaluated in numerous
experiments (n= 113) in the central humid portion of the Pampas (75). The results,
linked to data from experiments in which heterotrophic respiration was assessed,
allowed the development of methods for soil carbon budget estimation (75). Artificial
neural networks were generated for predicting soil microbial CO2-C emission and
carbon inputs from crops (Figure 11). Daily C-CO2 flux could be estimated using as
inputs the organic carbon mass in the 0-50 cm soil layer, temperature at –10 cm and
water content in the upper 30 cm of the profile. Microbial respiration increased with
soil carbon content and was positively affected by temperature, water content, and
their interaction. Crop carbon inputs could be predicted using as inputs crop species,
yield and rainfall during the growing cycle. Carbon inputs were linearly related with
yield but with great scatter of data, especially in wheat and soybean, because
harvest index variations. These variations were controlled by the yield x rainfall
interaction.
9
Humid Pampa
CO2
No-till
-1.9 Mg C ha-1 yr-1
Crop C
7.1
CO2
Tilled
- 0.78 Mg C ha-1 yr-1
9.0
Soil C
Crop C
7.1
7.9
Soil C
Semiarid Pampa
CO2
No-till
0.0 Mg C ha-1 yr-1
Crop C
CO2
Tilled
- 1.3 Mg C ha-1 yr-1
4.0
4.0
Soil C
Crop C
3.0
4.3
Soil C
Figure 10. Carbon flows in the Humid and Semiarid Pampas under different tillage
systems. Numbers under tillage system indicate annual soil carbon balance.
Numbers on the arrows indicate annual carbon fluxes (t ha -1 yr-1). Elaborated with
data from (71) for the Humid Pampa and (74) for the Semiarid Pampa.
12
y =x
2
R = 0.83
C input observed (t ha-1)
CO2-C observed (kg ha-1 d-1)
80
60
40
20
A
20
40
60
CO2-C estimated (kg ha-1 d-1)
9
6
4
3
B
0
0
y= x
2
R = 0.84
80
0
0
3
6
9
12
-1
C input estimated (t ha )
Figure 11. A. Observed vs. estimated soil heterotrophic respiration in the Pampas
using an artificial neural network. B. Observed vs. estimated carbon inputs from main
pampean crops to the soil using an artificial neural network. Full drops: training data
set, empty drops: validation data set. Redrawn from (75).
10
Soil carbon budget was calculated for a typical soil of the central portion of the Humid
Pampa and two different time periods as a function of soil organic carbon content
and the most common rotations implemented (Figure 12). Carbon budget was
negative in medium to high carbon soils but positive in low carbon soils;
consequence of greater mineralization in reach carbon profiles. In the period 20032006 carbon budget was less negative than in 1973-1976, despite the adoption of
soybean as the principal component of rotations and its low carbon inputs (Figure
13). This result was the consequence of yield increases (Figure 14A) and higher
biomass production in the Pampas (Figure 14B). Soil carbon trend in the future was
estimated assuming that yield gain rate and plant crop structure will remain constant
(Figure 152). A decrease of soil carbon was estimated for the next decades if
soybean proportion in rotations is maintained (more than 50%), but a recuperation of
carbon levels due to increased inputs from residues was estimated by the middle of
the century.
Soil carbon balance (t C ha-1 yr-1)
6
1973-1976
4
2003-2006
2
0
-2
-4
-6
50
55
60
65
70
-1
Organic carbon 0-50 cm (t ha )
Figure 12. Estimated soil carbon balance as a function of the initial soil organic
carbon level during two different time periods in the Humid Pampa . Elaborated with
data from (75).
11
8
B
A
Crop C input (t ha-1)
Soybean proportion in rotation
100
80
60
40
20
0
6
4
2
0
1973-76
2003-06
Corn
Time period
Wheat
Soybean
Crop
Figure 13. A. Evolution of soybean proportion in rotations used in the Humid Pampa.
Redrawn from (76). B. Average carbon inputs (straw + roots) from crops to the soil
determined in 113 field experiments performed between 1997 and 2006. Data from
(75).
8
C-C-C-W
W/S-C-S-S-S
4000
2000
A
0
Rotation C input (t ha-1 a-1)
Rotation yield (kg ha-1 yr-1)
6000
Natural ecosystem C input
6
4
2
0
1973-76
2003-06
Time period
B
1973-76
2003-06
Time period
Figure 14. A. Yield evolution of rotation in the central portion of the Humid Pampa.
Elaborated with data from (11). B. Most common rotation carbon inputs in the central
portion of the Humid Pampa. Data from (75). C= corn, W= wheat and S= soybean,
W/S= one year double crop wheat/soybean. Natural ecosystem carbon input was
estimated using a climatic model for net primary productivity estimation and
assuming that 90 % of photosynthetically fixed carbon became as input to the soil
(77).
12
Soil organic C 0-30 cm (t ha-1)
100
Natural ecosystem
80
60
Cropped soil
40
20
0
0
2010
20
2020
40
60
2040
2060
Year
80
2080
100
2100
Figure 15. Future soil organic carbon evolution in the central portion of the Humid
Pampa estimated using the neural networks models from Figure 11 for a double crop
wheat/soybean-corn-soybean-soybean-soybean rotation. Natural ecosystem carbon
level was determined by sampling 10 uncropped sites. Future yield gains of crops
were estimated (averaged of the last four decades) as 35, 41 and 112 kg ha-1 yr-1 for
wheat, soybean and corn respectively.
The Pampas has great potential for carbon sequestration because of its high net
primary productivity (78). Using NDVI, it had been estimated that the region lost 24
Mt of the net primary productivity during a 23 years period due to soil degradation
(79). These losses were not produced by the introduction of soybean in rotations; the
substitution of pastures by crops (11) seems a more probable cause.
Potential soil organic carbon increase and global warming mitigation arises from
cropland conversion from tilled systems to no-till (80, 81, 82). A meta-analysis of
results from 17 pampeam experiments, in which different tillage systems were
contrasted (83) (Figure 16), showed that no-till adoption produced an increase of
organic carbon in the previously tilled soil layer (Figure 17A). The carbon increase
was not associated to time since no-till initiation, climate, soil texture or rotation. As a
mean, carbon increase was ca. 2.8 t ha-1, which accounted for 5% change in high
carbon soils and up to 15% in low carbon soils (Figures 17B).
13
Figure 16. Map of the Pampas showing tillage experiments location.
A
-1
C change under no-till(t ha )
Mean change: 2.8 t C ha
-1
15
10
5
0
0
5
10
-5
20
B
15
10
5
0
Years
-10
15
C change under no-till (%)
20
20
0
20
40
60
80
100
-1
Tillage C (t ha )
Figure 17. A. Change of organic carbon stock (no-till carbon – tilled soil carbon) in
the upper 20-25 cm of the soil in 17 pampean field experiments. B. Average carbon
change (0-20 cm) in pampean soils as a function of carbon level under tilled
management. Data are presented in an equivalent mass basis. Redrawn from (82).
Carbon sequestration rate under no-till is usually an S-shape process as a function of
time, reaching steady state after 15-25 years from initiation (84; 82). Average rates of
no-till gains ranged from ca. 300 to 500 kg C ha -1 yr-1 (85; 82), with an overall
sequestration potential from ca. 3 to 12 t C ha-1 (84, 86, 85). In the Pampas mean
carbon sequestration rate under no-till is lower than these averages without
association with time since its adoption. The higher relative effect of no-till on low
carbon soils, which are found mainly in the Semiarid Pampas, may be the result of
greater water content, yield and carbon inputs to the soil under this tillage system
(74). The average soil carbon sequestration potential of the region is equivalent to
6% of stocks in the 0-20 cm layer as a mean for the entire region. Massive adoption
of no-till in the Pampas seems to counterbalance partially cultivation expansion
impacts on soil carbon reserves.
14
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