EFFECTS OF CHANGES IN FALLOW LENGTH ON SOIL ORGANIC C DUE TO CLIMATE CHANGE AND
SOCIOECONOMIC FACTORS IN POTATO-BASED CROPPING SYSTEMS IN THE BOLIVIAN HIGHLANDS
P.P. Motavalli1, J. Aguilera2, B. Jintaridth1, C. Valdivia1, M. Gonzales2 and C. Chambilla2
1College of Agriculture, Food and Natural Resources, University of Missouri, Columbia, MO 65211 USA
2Fundación PROINPA, La Paz, Bolivia
Lake
Titicaca
La Paz
Umala
Altiplano
Kellhuiri
Vinto Coopani
San Juan Cerca
San José de Llanga
Figure 1 A-D. (A-D) location and landscape example of the
study communities and villages in the Altiplano of Bolivia.
A.
Objectives
1.
2.
To determine the effects of cropping, fallow
periods and landscape position on soil total
organic C and soil organic C fractions.
To compare different analytical techniques to
evaluate changes in soil organic C fractions.
B.
C.
• Four small communities in Umala in the central
Altiplano of Bolivia (Fig. 1 A-D) were selected as
study sites in 2006 to represent typical Aymara
farming communities at different relative
elevations (Table 1).
• Replicate soil samples were collected to a 20 cm
depth from agricultural fields of the same
cropping system and sandy loam soil type (locally
classified as “cha’lla” soil) to determine changes
in soil C due to cropping and fallow length.
Average
time in
fallow
- years -
3,757
3,781
3,769
96
31
64
1 .8 4
2 .5 5
2 .2 0
3 .9 6
7 .3 7
5 .6 6
3 .5
7 .4
5 .4
Upper elevation
Vinto Coopani
Kellihuiri
Average
3,986
4,071
4,028
29
25
27
0 .8 5
1 .1 7
2 .0 0
3 .9 6
4 .7 7
4 .3 6
5 .1
6 .3
5 .7
Community
Lower elevation
San José de Llanga
San Juan Cerca
*Fields areas in 2006/2007
**Number of cattle plus sheep
Upper elevation
Vinto Coopani
1.60
1.40
A.
Y = 0.91 + 0.0157X - 0.000226X2
R2 = 0.37***, n = 39
Kellihuiri
Fallow
length
Total
organic C
KMnO4
active C*
POM C**
- years -
--- % ---
- mg kg-1-
- % of TOC -
1
4
10
20
Uncropped
0.44 ± 0.01
0.46 ± 0.01
0.46 ± 0.02
0.50 ± 0.02
0.52 ± 0.08
162 ± 15
171 ± 6
170 ± 7
186 ± 14
209 ± 7
37.2 ± 3.9
37.6 ± 1.4
38.0 ± 0.2
38.0 ± 0.9
37.5 ± 0.4
1
10
20
30
40
Uncropped
0.56 ± 0.02
0.57 ± 0.04
0.57 ± 0.04
0.62 ± 0.02
0.77 ± 0.15
0.83 ± 0.01
163 ± 11
178 ± 4
182 ± 8
222 ± 30
307 ± 43
358 ± 11
28.0 ± 3.3
32.9 ± 1.6
33.1 ± 0.6
35.5 ± 2.4
47.7 ± 10.2
44.3 ± 12.5
1
17
25
Uncropped
0.75 ± 0.16
1.00 ± 0.17
1.04 ± 0.14
0.92 ± 0.04
352 ± 13
366 ± 6
374 ± 5
250 ± 12
33.9 ± 2.3
38.2 ± 0.9
38.3 ± 0.5
28.5 ± 0.0
1
6
8
1.10 ± 0.12
1.06 ± 0.19
1.22 ± 0.16
337 ± 50
361 ± 32
444 ± 4
32.6 ± 0.7
39.9 ± 7.6
41.6 ± 0.4
1.20
1.00
San Jose de Llanga
Juan
YSan
= 0.52
+ Cerca
0.00255X
Vinto
Coopani
2
R = 0.15 *, n = 42
Kellihuiri
0.80
Upper communities
*Average ± standard deviation
**Particulate organic matter C
A.
0.60
O/R ratio = 0.69
0.40
Y = 0.52 + 0.00255X
2
R = 0.15*, n = 42
Figure 4.
MIR-DRIFT
scans and O/R
ratios of soils
with A) 1 year
of fallow and
B) 40 years of
fallow from
Umala.
Lower communities
0.20
2000
B.
Results and Discussion
Materials and Methods
Plowed
Fallow
fields per
fields per
household* household
--- ha ----- ha ---
Lower elevation
San José de Llanga
San Juan Cerca
Average
D.
Figure 2 A-D. Fallow and native vegetation growing during
the fallow period has an important role in (A) grazing for
sheep, (B & C)) a source of fuel for cooking and (D) soil
fertility restoration.
Altitude
- ma s l -
• Based on community surveys, fallow land
represents between 68 to 82% of cultivated lands
among both lower and upper elevation
communities (Table 1). Average time of fallow
varied between 3.5 to 7.4 years among the
communities and community members
perceived that the duration of fallow has
generally decreased over the last 10 years due to
increased human population and increased
forage production.
• Soil total organic C was significantly lower in
soils collected from farm fields in the lower
elevation communities compared to the upper
elevation communities (Table 2). This difference
was probably due to the generally higher sand
content of the soils in the lower communities
and possibly due to the more intensive
mechanized tillage used in those communities
compared to animal-based tillage in the upper
communities.
B.
1600
released (mg kg-1)
Increasing human population in the Altiplano and
competing land uses have caused a growing
reduction in the use and length of fallow (Coûteaux et
al., 2008). Moreover, practices, such as use of
mechanized disc plowing and cutting native
vegetation for fuel, have reduced the regrowth of
natural vegetation during the fallow period, possibly
diminishing the amount of organic inputs and the rate
of soil fertility restoration. The impact of fallowing in
this environment on soil properties is unclear. For
example, Hervé (1994) in a comparison of soils with
up to 20 years of fallow from the Bolivian Altiplano
did not observe any clear improvement in soil
physical (e.g., bulk density) and chemical properties
(e.g., soil total organic C) with increasing fallow.
Cabaneiro et al. (2008) in the Andes in Venezuela
determined that fallowing only increased labile
organic C in soils collected from slopes with a
northeast compared to a southwest aspect, possibly
due to differences in microclimate.
D. Umala landscape
C. Umala
Community
Cumulative CO2-C
The Bolivian highland plateau region (Altiplano) is
a semi-arid region in the Andes Mountains that
occupies approximately 27% of the area of Bolivia
and has a range in elevation of between 3600 and
4300 m above sea level. The region’s climate is
characterized by high diurnal temperature variations,
frost risks, low and irregular precipitation and high
risks of drought during the growing season (Garcia et
al., 2007). Potato-based cropping systems and
livestock rearing of cows, sheep and camelids (e.g.,
alpaca and lama) are the primary agricultural
activities (Valdivia et al., 2001). Crop rotations are
initiated with potato followed by two to three years of
cereal crops (e.g., quinoa, barley) and then an
extended uncultivated fallow period which can last
from 1 to over 40 years. This long fallow period
allows for restoration of soil fertility, control of crop
disease and pests, grazing of natural vegetation (e.g.,
“thola” (Parastrephia lepidophylla)) and fuel for
cooking (Bottner et al., 2006; see Fig. 2 A-D)).
No. of
Households
Y = 1184 + 7.67X
R2 = 0.24***, n = 39
• Increases in soil total organic C due to fallowing
were more rapid in soils collected from
communities at higher elevations compared to the
lower elevation soils and generally a maximum
accumulation was reached at higher elevations after
approximately 20 to 30 years of fallow (Fig. 3A).
• Cumulative CO2-C mineralized during incubation
provides a relative measure of active organic C and
results from this analysis showed similar
differences between the lower and upper elevation
communities as was observed for total organic C
except this form of organic C continued to
accumulate in the upper communities over 40 years
of fallow (Fig 3B). Others have observed in the
Andes in Venezuala that the maximum level of
active C occurred after 8 years of fallow (Cabaneiro
et al., 2008).
• Additional measurements of soil C fractions
showed a general increase with fallowing in KMnO4
active C and POM C in all communities with
fallowing except for San José de Llanga (Table 2).
• Ongoing analysis using MIR-DRIFT has shown
some sensitivity in detecting changes in the ratio of
labile to recalcitrant C (O/R ratio) with fallowing
(Fig. 4).
• Portable NIR units may allow for rapid
determination of soil C fractions due to
management practices, such as fallowing. Fig. 5
shows the results of developing a predictive model
for KMnO4 active C using NIR spectra and how it
significantly compared to measured KMnO4 active
C. Development of a low-cost NIR field unit may
facilitate these type of measurements in locations,
such as the Altiplano, where laboratory facilities are
difficult and costly to access.
Upper communities
1200
Conclusions
O/R ratio = 0.79
800
400
Y = 626 + 1.93X
R = 0.08 NS, n = 42
2
Lower communities
500
0
0
10
20
30
40
50
Time of fallow (years)
Figure 3. Changes in A) soil total organic C and B) cumulative
CO2-C after 84 days of incubation of soils with different times
of cropping and fallow. Regression lines show relationship for
lower and upper elevation communities.
Y = 0.820X + 48.8
2
R = 0.82**, n = 49
400
(mg kg-1)
Introduction
• Soils were air-dried and passed through a 2-mm
sieve and analyzed for soil total organic C using
a LECO C/N analyzer.
• Active C was determined using a dilute 0.02 M
KMnO4 oxidation procedure described by Weil
et al. (2003).
• Particulate organic matter C (POM-C) was
determined using the wet sieving procedure
described by Cambardella and Elliot (1994).
• Near infrared (NIR) spectra of the soils were
determined using a portable field spectrometer
Fieldspec Pro FR (Analytical Spectral Devices,
Inc, Boulder, CO) based on methods detailed in
Sudduth and Hummel (1993).
• Ratios of labile (O containing) and recalcitrant
(R) C functional groups were determined by
MIR-DRIFT analysis of HF-treated humic acid
fractions separated from the samples (Ding et
al., 2002).
• Soils were also incubated for 84 days in 150 mL
Falcon filter units at -47 kPa soil moisture
tension and a constant temperature of 25 ºC.
Samples were periodically placed in sealed
mason jars and the headspace swept with CO2free air. Changes in head space CO2
concentration in the head space due to soil CO2
evolution was then determined using a gas
chromatograph (GC) (Buck Scientific Inc., East
Norwalk, CT, USA) fitted with a thermal
conductivity detector (TCD).
Table 2. Effects of changes in length of fallow period
on soil total organic C and soil C fractions in Umala.
Predicted KMnO4 active C
B.
Soil total organic C (%)
A.
Table 1. Selected characteristics of the lower and upper
communities in Umala in 2006/2007.
300
200
100
100
200
300
400
500
Measured KMnO4 active C
(mg kg-1)
Figure 5. Predicted KMnO4 active C of soils from
Umala using NIR spectra versus measured values.
• The decreasing length of the fallow period and
reduction in native vegetation caused by competing
uses and mechanized tillage, may be removing an
important mechanism by which total and active soil
organic C is restored in potato-based cropping
systems in this region.
• The impact of fallowing appears to be greater in
soils collected at higher elevations which may reflect
differences in climate and management practices
with elevation in this region.
• Several analytical methods for detecting changes in
soil active organic C fractions were used in this
study and they were sensitive to the effects of
fallowing. Additional development of low-cost and
rapid field methods for detecting changes in soil C
fractions would be useful for developing and
monitoring sustainable agricultural practices.