Concluding Notes on Soil Organic Carbon

The field experiments of the MESCOSAGR project have provided sound indications that the innovative soil treatments, such as amendment with mature hydrophobic compost and in situ SOC photo-polymerization through biomimetic catalysis, sequestered carbon and stabilized SOM more than conventional soil management practices. In fact, the variability in SOC content throughout the experimentation period shown by both MIN and GMAN suggests that these treatments were not able to persistently stabilize OC in both bulk samples and soil particle sizes more than TRA. Moreover, NMR evaluation of soil HS extracted from MIN and GMAN excluded any significant and persistent variation in SOM chemical quality with respect to TRA.

Conversely, results obtained for COM-1, COM-2, and CAT plots for all experimental sites during the whole experimentation, suggest an overall positive effect of soil treatments with both humified compost and biomimetic catalyst on SOC accumulation and stabilization.

Notwithstanding the initial decrease of OC content due to a priming effect observed in the first year after compost addition to soils of Napoli and Torino, compost treatments progressively increased stable OC in bulk soils and soil aggregates in subsequent years. Such a persistent incorporation of hydrophobic components in compost-treated soils was also indicated by the molecular characteristics of HS extracted from different sites throughout the experimentation.

Soil treatment with biomimetic catalyst was found to positively affect both total SOC content and HS molecular characteristics. However, its effect was maintained throughout the whole experimental period only for Piacenza, whereas a decreasing effect was noted for Torino and Napoli in the third year. These contrasting results may be related to the different soil textural composition in the three experimental sites. The Torino sandy-loam soil and the still highly sandy Napoli soil may have not sufficiently adsorbed the water-soluble iron-porphyrin catalyst on soil particles, and it was partially lost by leaching. Conversely, an effective adsorption of biomimetic catalyst on mineral surfaces may have occurred in the heavy textured silty-clay soil of Piacenza, thereby allowing the catalyst to exert a prolonged photo-polymerization activity. However, the effectiveness of the catalyst when in close interaction with fine mineral particles in all three experimental sites was confirmed by the increased OC distribution in small soil aggregate sizes. In fact, as compared to No-CAT, absolute and relative OC content in CAT soils revealed a significant greater OC incorporation in microaggregates and smallest macroaggregates.

In the quest of summarizing the practical results of our field studies, Tables 4.14—4.16 show the ton ha-1 of TOC content in bulk soil for the various treatments in the three experimental sites. The values for MIN and GMAN confirm the short-term and even negative effect of both treatments on SOC accumulation.

Table 4.14 Torino experimental site, total organic carbon (ton ha in bulk soil under different treatments for three experimentation years

Treatments

First year

Second

year

Third

year

TOCa

Acb

TOC

Ac

Arb

TOC

Ac

Ar

Maize

Control

60.4

TRA

60.9

0.5

60.4

-

-

60.9

-

-

MIN

56.2

-4.2

60.4

0.5

4.7

59.3

-2.1

-1.6

GMAN

62.0

1.6

56.7

-3.1

-4.7

63.0

1.6

5.8

COM-1

62.0

1.6

63.5

3.7

2.1

66.7

5.3

2.6

COM-2

59.9

-0.5

65.1

5.2

5.8

69.3

7.9

3.7

Wheat

No-CAT

65.1

-

62.0

-

-

71.9

-

-

CAT

69.3

4.2

68.3

9.4

2.1

72.5

-9.4

-8.9

aTotal organic carbon = OC(gkg-1) x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) and to either initial or previous year TOC. E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) - (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

aTotal organic carbon = OC(gkg-1) x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) and to either initial or previous year TOC. E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) - (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

Table 4.15 Piacenza experimental site, total organic carbon (ton ha !) in bulk soil under different treatments for three experimentation years

Treatments

First year

Second

year

Third

year

TOCa

Acb

TOC

Ac

Arb

TOC

Ac

Ar

Maize

Control

78.3

TRA

76.0

-2.3

75.5

85.1

MIN

66.0

-12.3

74.2

-0.9

8.6

101.9

16.8

17.7

COM-2

68.7

-9.6

77.4

2.3

9.1

106.9

21.8

19.6

Wheat

No-CAT

74.2

55.5

77.4

CAT

76.0

1.8

61.0

24.1

3.6

88.3

-10.9

5.5

aTotal organic carbon = OC(gkg-!) x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) (Ac) and to either initial or previous year TOC (Ar). E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) — (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

aTotal organic carbon = OC(gkg-!) x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) (Ac) and to either initial or previous year TOC (Ar). E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) — (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

Table 4.16 Napoli experimental site, total organic carbon (ton ha in bulk soil under different treatments for three experimentation years

Treatments

First year

Second

year

Third

year

TOCa

Acb

TOC

Ac

Arb

TOC

Ac

Ar

Maize

Control

51.5

TRA

43.6

-7.8

44.6

-

-

46.6

-

-

MIN

48.0

-3.4

50.5

4.9

1.5

49.5

1.0

-2.9

GMAN

51.5

0.0

49.0

3.4

-3.4

49.0

0.5

-2.0

COM-1

46.6

-4.9

49.0

3.4

1.5

51.5

2.9

0.5

COM-2

54.9

3.4

55.4

9.8

-0.5

61.3

12.7

3.9

Wheat

No-CAT

53.9

49.0

-

-

55.9

-

-

CAT

51.0

-2.9

54.4

10.3

8.3

56.8

-5.9

-9.3

aTotal organic carbon = OC(gkg—^ x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) and to either initial or previous year TOC. E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) - (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

aTotal organic carbon = OC(gkg—^ x bulk density (1.5) x 10,000 (m2) x plow depth (0.35 m) bA = yearly difference in TOC for each treatment in respect to TRA (or No-CAT) and to either initial or previous year TOC. E.g.: Ac (cumulative) for MIN 2nd year = (MIN 2nd year-TRA 2nd year) - (TRA 2nd year-TRA 1st year); Ar (relative) for MIN 2nd year = (MIN 2nd year-MIN 1st year) — (TRA 2nd year-TRA 1st year)

Table 4.17 Net total organic carbon (ton ha-1) in bulk soil under different treatments for three experimentation years, as obtained by subtracting from measured SOC either 2.7 or 5.4 ton ha-1 of OC added with the COM-1 and COM-2 treatments, respectively

Treatments

First year nAca

Second year

Third year

nAc

nArb

nAc

nAr

Torino

COM-1

-1.6

0.4

-1.1

3.1

0.4

COM-2

-6.5

-0.7

-0.2

3.0

-1.2

Piacenza

COM-2

-12.7

-3.6

3.2

16.4

24.2

Napoli

COM-1

0.2

1.7

-0.25

2.2

-0.2

COM-2

5.9

5.4

-4.9

9.3

0.5

anAc (net cumulative variation) = (TOC COM-1-TOC TRA) - 2.7; (TOC COM-2-TOC TRA) - 5.4

bnAr (net relative variation) = (TOC COM-12nd year-TOC COM-11st year) - 2.7; (TOC COM-22nd year-TOC COM-2lst year) - 5.4

anAc (net cumulative variation) = (TOC COM-1-TOC TRA) - 2.7; (TOC COM-2-TOC TRA) - 5.4

bnAr (net relative variation) = (TOC COM-12nd year-TOC COM-11st year) - 2.7; (TOC COM-22nd year-TOC COM-2lst year) - 5.4

On the contrary, both cumulative (Ac) and relative (Ar) increments of TOC for compost and catalyst treatments indicate their relevant potential in favoring the long term sequestration and stabilization of organic carbon in soil.

After 3 years, the hydrophobic protection mechanism implied in the compost amendments to soils provided an amount of fixed SOC ranging from 3 to 22 ton ha-1 more than TRA, depending on experimental site.

The potential of mature compost treatment to sequester OC in soil is further revealed by the net relative increase of TOC when calculated by subtracting from soil carbon the compost OC annually added to soils (Table 4.17). Except for an initial SOC decrease at Torino and Piacenza, net SOC increments were generally observed. This shows that the hydrophobic protection mechanism exerted by compost progressively protected, against biotic and abiotic oxidation, both the indigenous SOM and the seasonal OC inputs (microbial biomass, plant roots, and crop residues). The net carbon sequestration obtained over the 3 years by hydrophobic protection of compost resulted thus in much greater than 0.5 ton C ha-1 year-1 achievable by reduced or zero tillage methods (Freibauer et al. 2004).

Though more variable, the OC fixed in the first and second years by the mechanism of in situ catalyzed SOM photopolymerization varied from 4 to 24 ton ha-1. Also these values overcome by far the sequestration potential of simple tillage management practices.

Both innovative methods adopted in the MESCOSAGR project appear then liable to become important to turn agricultural soils into OC sinks and contribute to alleviate the greenhouse effect and the global changes.

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