16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008

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How Perennial Grass has Modified Distribution of Organic Carbon in a Peach Orchard in Emilia-Romagna Region (Italy)

Montecchio, D.[1], Francioso, O.[2], Gioacchini, P.[3]& Ciavatta, C.[4]

Key words: Organic Carbon, Humification Parameters, DRIFT, TG-DTA

Abstract

In this study, the distribution of the total and humified organic carbon in a peach orchard tilled-irrigated on the row and perennial grassed on the inter-row space after 16 years of cultivation were evaluated. The TOC has shown differences not statistically significant in the 0-20 cm horizon, whereas the difference in the row vs. inter-row 20-40 cm horizon was significant. The highest content of humic substances was found in the 0-20 cm of the inter-row with perennial grass vs. row tilled soil: the absence of tillage increases the accumulation of humified compounds. DRIFT and TG-DTA analysis pointed out only some small structural variation in the humic fraction of the samples taken from the layer at depth 20-40 cm.

Introduction

Cultivation practices in agricultural systems have remarkable influence on dynamic of soil organic carbon (Francioso et al., 2000; 2005a; Gioacchini et al., 2006). Sowings, perennial grass species and irrigation, among the others, are the major factors affecting the dynamic of the organic carbon in orchards. Perennial sods prevent soil erosion, improve traffic conditions, enhance water infiltration into the soil, suppress pests, interact with beneficial organisms, modify orchard temperatures and light conditions for improved fruit quality, reduce dust and mite infestations, and provide substrate or food and habitat for a multitude of soil-borne organisms. The adoption of different soil management can contribute to the soil carbon sequestration and distribution in soil profile (Lal, 2002) to mitigate the greenhouse effect (Lal, 2003). Perennial grass species can contributes to the formation of a soil horizon reach in organic carbon in the top layer (Wedin et al., 1995).

Aim of this study was to measure the distribution of the total and humified organic carbon in a peach orchard tilled-irrigated on the row and perennial grassed on the inter-row space after 16 years of cultivation using chemical analysis, infrared spectroscopy and thermogravimetric (TG) and differential thermal analysis (DTA).

Materials and methods

Soil samples (Typic Xerochrept) were taken in a 16-years peach orchard farm located in Roncadello (Forlì-Cesena province), Emilia-Romagna Region (Italy). Samples were collected in August 2005, after harvest, from plots at two depth (0-20 and 20-40 cm) along the row (tilled soil) and in the inter-row space (perennial grassed with different Graminaceae species). Soil samples were air dried, crushed to pass a 2 mm sieve and stored in sealed bags. In the last decade the mean annual fertilisation added was 40 kg N ha-1 of organic N fertiliser. The main physical-chemical characteristics of the soil were: pH (in water) 7.24; texture: sand 25%; silt 35%; clay 40%; total calcium carbonate (CaCO3) 110 g kg-1; cation exchange capacity 25 cmolc kg-1; TOC 9.5 g kg-1; TKN 1.3 g kg-1; Olsen-P 18 mg kg-1; Exchangeable-K 170 mg kg-1.

Total (TOC), extracted (TEC), humified (HA+FA) and non humified organic carbon (NH) were determined according to Ciavatta et al. (1997) method. DRIFT and TG-DTA analysis were performed on humic acids extracted from soil samples. DRIFT spectra were recorded with a Nicolet Impact 400 FT-IR Spectrophotometer (Madison, WI) equipped with an apparatus for diffuse reflectance (Spectra-Tech. Inc., Stamford, CT), according to Francioso et al. (2001) method. TG-DTA curves were carried out simultaneously using the TG-DTA92B (Setaram-France) device. Sperimental conditions: heating rate 10°C min-1 from 30 to 750 °C, in air atmosphere and calcinated kaolinite was used as reference material.

Results and discussion

TOC, TEC, HA+FA, NH, the degree of humification [DH% = TEC/(HA+FA) x 100] and the humification rate [HR% = TOC/(HA+FA) x 100] are shown in figures 1-2. The TOC has shown differences not statistically significant in the 0-20 cm horizon, whereas the difference in the row vs. inter-row 20-40 cm horizon was significant (Fig. 1). The phenomenon can be reasonably due to the irrigation that induces a higher microbial activity that increases the amount of TEC as well. On the contrary, a highest content of humic substances (HA+FA) was found in the 0-20 cm of the inter-raw with perennial grass vs. row tilled soil: the absence of tillage increases the accumulation of humified compounds. The values of humification parameters DH and HR were higher in the 0-20 cm horizon of the inter-row with perennial grass vs. the row (Fig. 2): similar differences were observed at 20-40 cm depth. The role of perennial grass, among the others agronomic properties, is to increase the TOC and the humification level, as well shown by the increase of the DH and HR in the top layer (0-20 cm). From a quantitative point of view, it can be calculated that a concentration of 1 g kg-1 of soil humic C corresponds to 2.5 tons ha-1, assuming a soil depth of 20 cm and density of 1.25 kg dm-3.

Figure 1: Total (TOC), extracted (TEC), humified (HA+FA) and non humified C (NH) of soil samples taken at 0-20 and 20-40 cm depth (row andin inter-row space). / Figure 2: Degree (DH) and humification rate (HR) of soil samples taken at 0-20 and 20-40 cm depth(row and in the inter-row space).
Figure 3: DRIFT spectra of the humic fraction of samples taken from the layer 20-40 cm along the row and in the inter-row space. / Figure 4: TG-DTA curves of the humic fraction of samples taken from the layer 20-40 cm along the row and in the inter-row space.

DRIFT spectra of humic fraction (HA) of soil taken from the top layer at depth 0-20 cm along the row and inter-row space did not show significant structural modifications due to treatment (data here not shown). Instead some small structural variation can be observed in the humic fraction from the samples taken from the layer at depth 20-40 cm (Fig. 3). The main modification might be assigned to different amount in carbohydrates content (Francioso et al., 2001). These results were supported by TG-DTA analysis (Fig. 4) as revealed by higher exothermic reaction at around 300 °C in inter-row sample (20-40 cm) than that found in row samples. This peak was mainly produced by the combustion of carboxylic groups and carbohydrates suggesting the formation of recent organic carbon (root exudates, microbial cells). The second peak at around 450°C was typical of high resistant temperature components such as aromatic structures (Francioso et al., 2005b).

Conclusions

After 16 years of cultivation the distribution of TOC in a peach orchard tilled-irrigated on the row and perennial grassed on the inter-row space did not show statistically significant differences in the 0-20 cm horizon, whereas the difference in the row vs. inter-row 20-40 cm horizon was significant. The highest content of humic C was found in the 0-20 cm of the inter-row with perennial grass vs. row tilled soil suggesting that the absence of tillage increases the accumulation of humified carbon. Moreover the presence of slight structural modifications in the humic fraction from the layer 20-40 cm along the inter-row space may suggest the influence of the activity of the roots.

Acknowledgments

Activity carried out within the biennal project “Qualità e Mercato della Frutta Prodotta con Sistemi Biologici e Sostenibili: Innovazioni Tecniche, Rintracciabilità – Quality and Market of the Fruits Produced in Organic and Sustainable Systems: Technical Innovation, Traceability” (acronym: BIOFRUIT II) funded by AlmaMaterStudiorumUniversity of Bologna (2004).

References

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Francioso O., Sànchez-Cortés S., Corrado G., Gioacchini P., Ciavatta C. (2005a). Characterization of soil organic carbon in long-term amendment trials. Spectr. Letters 38:283-291.

Francioso O., Montecchio D., Gioacchini P., Ciavatta C. (2005b). Thermal analysis (TG-DTA) and isotopic characterization (13C-15N) humic acids from different origins. Applied Geochemistry, 20:537-544.

Gioacchini P., Montecchio D., Francioso O., Giordani G., Toderi G., Ciavatta C. (2006). Carbon dynamics of humic substances in a long-term field experiment. Humic Substances – Linking Structure to Functions. In XIII International Meeting of International Humic Substances Society, Universität Karlsruhe (TH), Karlsruhe, Germany, July 30 - August 4, 2006. 157-160.

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Wedin D. A., Tieszen L. L., Dewey B., Pastor j. (1995). Carbon isotope dynamics during grass decomposition and soil organic matter formation. Ecology 76(5):1383-1392.

[1]Department of Agro-Environmental Science & Technology, viale Fanin n. 40, 40127 Bologna, Italy, E-Mail , Internet

[2]As Above

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