The effects of system management on soil carbon dynamics
A. Di Tizio1, A. Lagomarsino1,
M.C. Moscatelli2, S. Marinari2 and S. Grego R. Mancinelli2
1
Department of Agrobiology and Agrochemistry
2
Department of Crop Production
University of Tuscia, Viterbo, Italy
Keywords: soil respiration, carbon pools, management
Abstract
Soil organic carbon plays an essential role in determining the soil quality, which is a central
aspect to maintain in the long term sustainability and productivity in the agricultural ecosystems.
Agriculture can be considered as a sink for atmospheric CO 2 through carbon sequestration into biomass
product and soil organic matter, but it can also be a source for greenhouse gases, including CO 2. The
impact of different agricultural managements (conventional and organic) and tillage level (deep and
minimum tillage, DT and MT, respectively) on soil organic carbon pools content and CO 2 emission were
investigated in this work. Both systems have a three-year crop rotation including pea – durum wheat –
tomato; the organic system is implemented with the introduction of common vetch (Vicia sativa L.) and
sorghum (Sorghum vulgare bicolor) as cover crops. The results, reported as average of three crops, showed
highest values of total organic carbon (TOC) and total nitrogen (TN) in organic soil MT. Similar results
were found for microbial biomass carbon (MBC) and nitrogen (MBN). The most labile carbon pools
(water soluble carbon and labile carbon) showed higher values in the organic system than in the
conventional one, with the lowest value in conventional system DT. Soil CO 2 emissions were also greater in
the organic system than in the conventional one, with higher values in MT with respect to DT.
INTRODUCTION
Agriculture can be considered as a sink for CO2 through carbon sequestration into
biomass product and soil organic matter, but it can also be a source for greenhouse gases,
including CO2. Soil carbon sequestration benefits can be summarized on improved soil
quality, increased soil productivity, reduced risk of soil erosion and sedimentation (Lal 2007).
Soil organic carbon in permanently cropped fields can be increased trough a number of
management practices including reduced tillage, cover crops and green manure (Paustian and
Cole, 1998). These management options might enhance microbial function and promote soil
organic carbon sequestration (Jareki and Lal, 2003). In particular, tillage determines the
maximum depth to which plant residues are incorporated in soils and therefore it affects the
vertical distribution of soil organic matter (Etana et al., 1999). In addition, conventional
tillage induces rapid mineralization on SOM and potential loss of C and N from soil. On the
contrary, the green manure of cover crops and incorporation of crop residues, related to
organic management practices, are most effective to enhance soil structure by increasing its
organic carbon content, size and stability of aggregates, water retention and infiltration (Singh
et al., 2007).
The present work investigated the effects of different agricultural managements
(conventional and organic) and tillage level (deep and minimum tillage) on soil organic
carbon content and CO2 emission.
MATERIALS AND METHODS
A long-term field study, established in 2001, is conducted at University of Tuscia
experimental farm (Viterbo) on a volcanic soil (Typic Xerofluvent). In the experimental site
organic and conventional system management are compared in a randomized block design
with three replications. Both systems have a three-years crop rotation :pea (pisum sativum L.)
– winter durum wheat (Triticum durum Desf.) – tomato (Licopersicum esculentum Mill.). In
the organic management, the rotation is implemented with common vetch (Vicia sativa L.)
and sorghum (Sorgum vulgare bicolor) cover crops. For each management deep and
minimum tillage levels (DT and MT, respectively) are adopted. The three crops are
simultaneously present in the experimental field that includes 36 plots: 2 systems x 2 tillage
levels x 3 crops x 3 field replicates.
Soil samples were collected in winter and in summer 2006. Two soil cores were taken
in each plot and then pooled together, for a total of 36 soil samples at each sampling date. Soil
samples were sieved (<2mm) and for biochemical analyses the moisture content adjusted to
60% of their water holding capacity (WHC). Total organic carbon (TOC) and total nitrogen
(TN) were determined using an elemental analyzer (Thermo Soil NC - Flash EA1112). Total
extractable carbon (TEC) was determined by by dichromate oxidation and titration
techniques.
Microbial biomass carbon (MBC) was estimated following the Fumigation Extraction
(FE) method (Vance et al., 1987). C extracted from non fumigated samples represents the
labile pool of K2SO4 extractable C (ExC).
Microbial biomass nitrogen (MBN) was estimated on fumigated and non fumigated
samples following Joergensen and Brookes (1990). N extracted from non fumigated samples
represents the labile pool of K2SO4 extractable N (ExN). Water Soluble Carbon (WSC) was
determined following the method as described for Burford and Bremner, 1975.
Soil CO2 emission was measured in three randomized areas for each plot every 10
days from March to October 2006. The gas flux was measured using the non-steady-state
through-flow chamber: EGM-4 instrument (PP Systems, Stotfold, UK), a portable infrared
gas analyzer (IRGA), described in details by Pumpanen et al. (2004).
Data are presented as average of the two sampling dates and the three crops.
Analysis of variance (ANOVA) and LSD Post Hoc test were performed to evaluate the
main effects of system (organic/conventional), tillage (conventional/minimum) and their
interactions on the parameters analysed using Systat 11.0 statistical software package (SPSS
Inc.).
RESULTS AND DISCUSSION
The physical and chemical characteristics of organic and conventional soil are
reported in table 1. Total organic carbon (TOC) and total nitrogen (TN) content were
positively affected by minimum tillage. The highest values for the two parameters were found
in soil under organic MT management (15% and 25% increase with respect to organic DT
respectively for TOC and TN). These findings are, in agreement with several reviews
assessing the positive impact of conservation tillage on soil C sequestration (Franzluebbers
and Follett, 2005; Lal, 2007; Six et al., 2004).
Similar results are reported in table 2a and 2b for C and N labile pools (microbial
biomass carbon, nitrogen, and extractable nitrogen) ,that showed the highest values in soil
under organic MT management. No significant changes were detected on passive carbon pool
(TEC).
From these remarks we hypothesize that in the short period (5 years after beginning of
experimental field) the reduced tillage, relative to conservative management (including green
manure), improved labile carbon and nitrogen availability, which consequently affected total
carbon and nitrogen stock. The passive pool was less sensitive to soil management systems,
according with other authors (Carvalho Leite et al., 2004).
The CO2 production during the period of measurement (from March to October 2006)
showed higher rates in soil under organic management in comparison with conventional one
(figure 1). Consequently a higher cumulative amount of CO2 produced in the organic system
was observed (22.58 tons ha-1 and 21.93 tons ha-1, respectively). This trend was probably a
consequence of the green manure practice, which produced an increase of microbial biomass
and therefore in respiration activities, as also reported by Tejada et al. (2008). Tillage
practices induced as well differences in soil respiration rates, with higher values in organic
MT with respect to organic DT (+ 12 %). This result, in agreement with the higher content of
soil organic carbon, confirmed previous studies (Hendrix et al., 1988; Follet and Schimel,
1989) that related soil respiration to carbon availability for microbial biomass with a
consequent greater biological activity.
CONCLUSIONS
The organic management and the minimum tillage practices enhanced in the short
term the CO2 emission from soil. This trend was related to the greater values of labile carbon
pool in the organic system with minimum tillage, confirming that the management (mainly
the green manure practice) and the tillage level had a positive effect on carbon sequestration,
while the passive pool was less sensitive in short term.
AKNOWLEDGEMENT
The authors are grateful to Prof. Enio Campiglia and Prof. Fabio Caporali to made
available the experimental field, Prof. Paolo de Angelis for allowing the use of elemental
analyzer.
REFERENCES
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TABLES
Table 1
Physical and chemical properties of organic and conventional soils.
DT = deep tillage; MT = minimum tillage; Data reported in brackets are the standard error (n
= 9). Different letters indicate significant differences for each parameter.
* = p ≤ 0.05.
texture
TOC
TN
C:N
TEC
Management tillage
pH H2O pH KCl
(USDA)
%
%
ratio
%
a
a
a
a
a
6.91
5.52
1.10
0.12
9.55
0.70 a
DT
clay
(0.05) (0.06) (0.05) (0.01) (0.31) (0.03)
Organic
loam
6.81 a 5.49 a 1.27 b * 0.15 b * 9.10 a
0.74 a
MT
(0.04) (0.08) (0.03) (0.01) (0.44) (0.05)
6.84 a 5.47 a 1.15 a 0.13 ab 9.46 a
0.69 a
DT
sandy
(0.05) (0.05) (0.06) (0.01) (0.29) (0.03)
Conventional
clay
6.91 a 5.62 a 1.18 a 0.13 ab 9.28 a
0.70 a
MT
loam
(0.05) (0.06) (0.03) (0.01) (0.33) (0.08)
Table 2a
Biochemical properties of organic and conventional soils.
DT = deep tillage; MT = minimum tillage; Data reported in brackets are the standard error (n
= 9). Different letters indicate significant differences for each parameter.
** = p ≤ 0.01; * = p ≤ 0.05.
MBC
MBN
MBC MBN ratio
Management
tillage
g C g-1
g N g-1
g C g N-1
245.21 a
52.27 a
5.91 a
DT
(23.95)
(2.51)
(0.58)
Organic
b*
b*
314.78
74.19
5.13 a
MT
(22.92)
(10.31)
(0.64)
a
c**
210.15
44.52
4.40 a
DT
(16.15)
(2.52)
(0.48)
Conventional
a
a
217.78
57.41
4.80 a
MT
(26.83)
(3.50)
(0.77)
Table 2b
Biochemical properties of organic and conventional soils.
DT = deep tillage; MT = minimum tillage; Data reported in brackets are the standard error (n
= 9). Different letters indicate significant differences for each parameter.
** = p ≤ 0.01; * = p ≤ 0.05.
ExC
ExN
WSC
LC
Management
tillage
g C g-1
g N g-1
g C g-1
g C g-1
103.26 a
21.17 ab
44.55 a
331.94a
DT
(10.04)
(2.01)
(4.44)
(22.57)
Organic
a
b
a
92.08
30.38
42.07
370.49a
MT
(7.30)
(5.96)
(4.49)
(21.52)
a
a
b*
78.49
16.96
31.23
274.16b*
DT
(8.21)
(1.08)
(3.04)
(12.21)
Conventional
a
ab
ab
90.65
21.70
34.20
312.14ab
MT
(5.37)
(3.02)
(3.45)
(23.30)
FIGURES
1.00
0.90
O DT
O MT
C DT
C MT
0.80
g CO2 m-2h-1
0.70
0.60
0.50
0.40
0.30
0.20
0.10
29
/0
3/
20
06
12
/0
4/
20
06
26
/0
4/
20
06
10
/0
5/
20
06
24
/0
5/
20
06
07
/0
6/
20
06
21
/0
6/
20
06
05
/0
7/
20
06
19
/0
7/
20
06
02
/0
8/
20
06
16
/0
8/
20
06
30
/0
8/
20
06
13
/0
9/
20
06
27
/0
9/
20
06
11
/1
0/
20
06
25
/1
0/
20
06
0.00
Fig. 1. CO2 emission from soil measured with EGM-4 (IRGA) from March to October 2006.
O = organic; C = conventional; DT = deep tillage; MT = minimum tillage.
The bars represent the standard error (n = 9).