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16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Can Organic Farming Contribute to Carbon Sequestration? A
Survey in a Pear Orchard in Emilia-Romagna Region, Italy
Ciavatta C.1, Gioacchini P.2, & Montecchio D.3
Key words: Organic Carbon Sequestration, Organic farming, Pear orchard
Abstract
The effect of organic fertilisation on the level of total organic carbon (TOC) in an 18years old pear orchard (cv. Abate Fetel) was evaluated vs. a conventional pear
orchard mineral fertilized (control). In both orchards soil samples (Typic Udochrept
loamy soil) were taken at two depths (0-15 and 30-50 cm) along the row (tilled and
mainly amended with compost) and in the inter-row space (grassed with different
Graminaceae species in the organic orchard, bare in the conventional orchard). The
area (elevation 20 m), located in Bologna province, Emilia-Romagna Region (Italy), is
characterised by mean annual temperature 13.1 °C and rainfall around 750 mm.
In the horizon 0-15 cm of the row an increase of about 14 tons ha-1 of TOC has been
calculated after 18-years of cultivation and amendment compared to the control soil,
which had received just mineral fertilisation. A significant increase of TOC (about 6.3
tons ha-1) was also measured in the top layer (0-15 cm) in the grassed inter-row,
where this C sink is exclusively due to the cover crop. A survey of the role of organic
vs. conventional farming on soil C sink/source is started in 2007 in 8 typical organic
orchard farms located in Emilia-Romagna Region and it is still running.
Introduction
Soil organic carbon (C) preservation in agro-ecosystems is a crucial point to maintain
soil fertility and productivity and to reduce losses of CO 2 in the atmosphere. The use
of different soil management can contribute to the soil carbon sequestration and its
distribution in the soil profile (Lal, 2002) to mitigate the greenhouse effect (Lal, 2003).
Organic farming has been reported to have a positive effect on C sequestration as a
result of increased root yields, higher humification rate constant and the direct
application of organic matter through organic amendments (Ciavatta et al., 1997;
Francioso et al., 2000; 2005; Kundu et al., 2007; Bhattacharyya et al., 2007). In order
to increase the level of organic C in the top layer of soil, perennial grass species are
often used in organic orchards.
Aim of this study was to determine the contribution of organic amendment to the soil
carbon sink in an 18-years old pear orchard (cv. Abate Fetel) vs. a conventional pear
orchard mineral fertilized (control).
1
Department of Agro-Environmental Science & Technology, viale Fanin n. 40, I-40127 Bologna,
Italy, E-Mail [email protected], Internet http://www.dista.agrsci.unibo.it
2
As Above
3
As Above
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
Materials and methods
Soil samples (Typic Udochrept loamy soil) were taken from an 18-years old,
organically fertilised pear orchard (cv. Abate Fetel) of a certified organic farm located
in San Matteo della Decima (Bologna), Emilia-Romagna Region (Italy) and from a
mineral fertilised pear orchard of a conventional orchard (control) located about 100 m
far away. The area (elevation 20 m) is characterised by the following mean climate
conditions (1921-2004): mean annual temperature 13.1 °C and rainfall around 750
mm. Along the 18-years the main organic fertilisation was done with 4-6 tons ha-1 of
compost (28% organic C, 2.5% total N, 2.3 organic N, P 2O5 1.4%, K2O 1.7%).
Compost was prepared by composting (130 days period) a blend of 70% (v/v) plant
trimming (mowing and pruning) and 30% (v/v) sewage sludge (50% waste waters and
50% food processing). The yearly nitrogen mineral fertilisation was around 90 kg N
ha-1 (ammonium sulphate and urea).
In both orchards soil samples (4 samples per plot) were collected at the end of April
2006 from four plots at two depths (0-15 and 30-50 cm) along the row (tilled and
mainly amended with compost) and in the inter-row space (grassed with different
perennial Graminaceae species in the organic orchard, bare in the conventional
orchard). The choice of the two sampling layers was related to the tillage depths
(around 20 cm): the top layer (0-15 cm) was annually tilled, while the deep layer (3050 cm) did not undergo any tillage. After sampling soil samples were air dried,
crushed to pass a 2 mm sieve and stored in sealed bags, according to Italian Official
Methods of Soil Analysis (2000).
The main physical-chemical characteristics of the organic orchard soil were: pH (in
water) 7.98; Texture: sand 30%, silt 48%, clay 22%; Total carbonates (CaCO 3) 14%;
Bicarbonates (HCO3-) 5.1%; Cation exchange capacity 23 cmolc kg-1; Total organic
carbon (TOC) 9.5 g kg-1; Total Kjeldahl nitrogen (TKN) 1.3 g kg-1; those of the control
soil were: pH (in water) 8.00; Texture: sand 32%, silt 49%, clay 19%; Total carbonates
(CaCO3) 16% g kg-1; Bicarbonates (HCO3-) 4.8%; Cation exchange capacity 21 cmolc
kg-1; TOC 8.4 g kg-1; TKN 0.9 g kg-1.
Results and discussion
Total organic carbon (TOC) content of soil samples of the organic and conventional
pear orchard taken at two depths (0-15 and 30-50 cm) along the row is shown in Fig.
1. The top layer of the organic orchard was significantly richer in TOC compared to the
conventional one. These differences disappeared in the deep layer, indicating that the
effect of the amendment was not distributed along the soil profile. The inter-row zone
showed a similar trend with a significantly higher content of TOC in the upper layer of
the organic orchard compared to the conventional one (Fig. 2). In this case the greater
amount of TOC was due to the C released from grasses as rhyzodepositions.
However, even in the inter-row, the deep layer of the two orchards had similar TOC
content suggesting that any C contribution to soil only affects the upper part of the soil
profile.
From a quantitative point of view, it can be calculated that a concentration of 1 g kg -1
of soil TOC corresponds to 1.875 tons ha-1, assuming a soil depth of 15 cm and a
density of 1.25 kg dm -3. Applying this assumption, it can be estimated that in the
horizon 0-15 cm of the row there was an increase of about 14 tons ha-1 of TOC after
18-years of cultivation and amendment vs. the control that has received just mineral
fertilisation (Fig. 1). On the same basis, the presence of grasses in the top layer of the
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
inter-row caused a significant increase in TOC that could be quantify in about 6.3 tons
ha-1 and that was due to the accumulation of their rhizodeposits.
Figure 1: Total organic carbon (TOC)
content in soil samples taken at two
depths (0-15 and 30-50 cm) in the row.
Tukey-HSD test: similar letters are not
significantly different at p ≤0.05.
Figure 2: Total organic carbon (TOC)
content in soil samples taken at two
depths (0-15 and 30-50 cm) in the
inter-row. Tukey-HSD test: similar
letters are not significantly different at
p ≤0.05.
Conclusions
The management of the organic orchard with the annual addition of compost over 18years and the presence of grasses was able to significantly increase the amount of
TOC in the 0-15 cm layer compared to the conventional orchard used as control.
These increases were equal to 14 and 6.3 tons ha-1 in the row and in the inter-row
respectively.
In order to confirm these preliminary results, a survey of the role of organic vs.
conventional farming on soil carbon sink/source is started in 2007 in Emilia-Romagna
Region. Eight typical organic orchard farms and eight conventional orchard farms
(control), located in different provinces of the Region, have been sampled and results
will be available in 2008.
Acknowledgments
Activity carried out within the project co-ordinated by CRPV (Vegetal Production
Research Center, Italy) in collaboration with ProBER (Organic Producers of EmiliaRomagna Region, Italy), funded by Emilia-Romagna Region (L.R. 28/98). Authors
gratefully thank Dr. P. Schiatti (ProBER) and S. Scagliarini (Agriculture and
Environment Centre) for technical support and soil sampling.
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16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/view/projects/conference.html
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