Carbon Tracers in the Mescosagr Project 571 Site Description and Experimental Design

Field experiments were conducted during the 2006, 2007 and 2008 seasons growing at the Agronomic Institute, for industrial crops (CRA-Ort) of Battipaglia (Salerno, Italy). The set-up consisted of a randomized complete block design with four replications, and four treatments (Table 5.7) with a total of 16 plots of 5 x 8 m.

The adopted crop was sorghum [Sorghum bicolor (L.) x S. sudanense (Piper) Stapf.; cv: BMR333], planted by hand in rows with a density of 20 plant/m2. The plots were weeded by hand twice a year before incorporating compost (0-15 cm) or

Table 5.7 Description of soil treatments

Treatments

Fertilization

Dose N (kg ha-1)

Irrigation

Sorghum

TRA

Urea

130

Yes

Yes

CPT1

Compost

130

Yes

Yes

CPT2

Compost

260

Yes

Yes

0-N

NO

0

Yes

Yes

adding nitrogen fertilizer (TRA plot), and sowing. Sorghum was harvested each year in September.

5.7.2 Soil Sampling and Sample Treatment

In order to study the plot spatial variation of soil C stable isotope (d13C), 36 soil samples (0-15, 15-30 and 30-60 cm depth) were collected, before starting the field trial, from 4 transects spaced 10 m apart with 3 sampling points along each transect (7 m between sampling points).

For each treatment, soil samples were also collected from 0-15, 15-30, 30-60, 60-90, 90-120,120-150,150-180 and 180-210 cm depths either before planting or after harvesting in the 2007, 2008 and 2009 growing seasons. All samples were air-dried and ground to a fine power. To avoid the possible misleading influence of inorganic carbon during determination of the isotopic signature of organic C, all carbonates were removed prior to the isotopic analysis (Harris et al. 2001). Subsamples of (1.5 mg) soil were placed in open Ag-foil capsules (4 by 6 mm). Silver capsules were required because Sn capsules disintegrate when exposed to HCl vapour. The capsules were placed in wells of a microtitre plate, sufficient water was added to each capsules (4 p.L) to moisten the soil near to field capacity. The microtitre plate was then placed inside a vacuum desiccator (5 L) with a beaker (150 ml) filled with 100 ml of 12-M HCl. Samples were exposed to HCl vapour for 6 h and were then oven-dried at 60°C and analysed for d13C signature. Duplicate soil subsamples (1.5 mg) were analysed for d13C using a Finnigan Delta-Plus isotope ratio mass spectrometer coupled to a Carlo Erba NC2500 elemental analyser. A Carlo Erba NC2500 elemental analyser was also used to analyse total and organic carbon of soil.

5.7.3 Plant Tissue Treatment and Analysis

Plant samples (leaves, stems and roots) were oven-dried at 70°C, ground to a fine powder with a ball mill and weighed in tin capsules. The samples (3.2-mg leaves, 6-mg stems and 8-mg roots) were then burnt in an elemental analyser (Carlo Erba Instruments NCS 2500) equipped with an auto-sampler carousel and controlled by

EAGER 2000 software. The d13C values were measured using a Finnigan DeltaPlus isotope ratio mass spectrometer coupled with autoanalyser and using CO2 as a reference gas.

The isotope analyses were expressed as d13C values d13C = „ Rsample , x 1; 000;

R standard 1

where R = 13C/12C is for either the sample or the reference standard. The d13C values were referred to PeeDee Belemnite (PBD) as standard. The standard deviation of duplicate samples for d13C was ±0.3.

The fraction of soil C derived from the new sorghum residue input (fnew) was calculated with the isotopic mixing model (Leavitt et al. 1994) as d13C _ d13C

where d13Csampie and d13Coid are the isotopic signatures of SOC after and before the sorghum experiment, respectively. d13Cnew represents the d13C value of new input C from C4-sorghum residues. In this calculation, we assumed that no isotopic discrimination occurred during the microbial decomposition of SOC and sorghum residues.

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