Napoli Experimental Site

After the first experimentation year, the OC content in Napoli bulk soil decreased in TRA, MIN, and COM-1 to 8.9,9.8, and 9.5 g kg—1 values, respectively, as compared to the 10.5 g kg—1 level of control soil (Table 4.8). A maintenance of the initial OC content was shown by GMAN (10.5 g OC kg—1), while additional organic carbon (11.3 g OC kg—1) was incorporated in the soil by the COM-2 treatment. This result seems to indicate that also in the Napoli site a priming effect induced by COM-1 promoted the mineralization of previously incorporated OM.

The findings observed for bulk soils were confirmed by the OC content found in water-stable aggregates (Table 4.8). The soil fractionation of TRA, MIN, and GMAN followed the typical hierarchical distribution of aggregates with decreasing

Table 4.8 Napoli experimental site, amount (g kg and relative distribution (%) of organic carbon in bulk samples and water-stable aggregate sizes (mm) in soil under different treatments for 3 years of experimentation

Treatments

Bulk

Aggregate

sizes

Sum of fractions

4.75-1.00

1.00-0.50

0.50-0.25

<0.25

gkg-1

gkg-1

%

gkg-1

%

gkg-1

%

gkg-1

%

gkg-1

%

Control soil

10.5

11.6

43.3

10.5

30.7

9.3

12.7

8.9

13.3

10.5

100

Maize

First year TRA

8.9

9.4

37.5

7.8

30.7

8.2

16.5

7.2

15.2

8.2

92.0

MIN

9.8

10.4

34.1

9.9

36.0

9.0

17.0

8.8

12.8

9.8

100

GMAN

10.5

11.4

32.2

10.3

41.0

8.9

14.8

8.2

12.0

10.1

96.2

COM-1

9.5

8.4

49.8

9.6

27.6

11.4

11.3

11.1

11.3

9.4

98.9

COM-2

11.2

11.4

37.3

11.3

31.0

9.3

10.2

13.3

21.4

11.4

100

LSD

0.5

1.5

1.7

0.5

1.9

1.4

1.8

1.9

2.0

0.40

Second year TRA

9.1

9.5

27.7

8.6

38.6

9.8

22.0

7.8

11.7

8.9

97.8

MIN

10.3

11.6

36.8

10.5

42.0

9.2

13.9

7.8

7.2

10.3

100

GMAN

10.0

10.1

35.3

9.4

39.2

8.2

15.4

7.7

10.0

9.2

92.0

COM-1

10.0

10.7

41.3

9.2

34.9

9.6

14.7

8.6

9.2

9.8

98.0

COM-2

11.3

12.6

36.9

11.3

37.1

9.7

15.8

8.9

10.2

11.1

98.3

LSD

0.5

0.6

1.1

0.5

1.0

0.6

1.8

0.8

1.3

0.60

Third year TRA

9.5

10.0

44.4

7.8

30.4

8.5

15.6

7.2

11.1

8.7

91.6

MIN

10.1

10.1

44.0

9.2

31.3

9.2

15.8

7.5

10.8

9.4

93.1

GMAN

10.0

9.4

44.9

7.9

25.8

12.0

16.6

13.0

10.2

9.6

96.0

COM-1

10.5

10.1

41.9

11.8

33.9

10.9

16.6

7.2

11.1

10.2

97.1

COM-2

12.5

12.2

42.5

11.8

31.2

13.1

15.6

13.9

12.2

12.4

99.2

LSD

0.5

0.5

1.7

0.9

1.5

0.4

NS

1.2

0.5

0.60

Wheat

First year CAT

10.4 (0.1)

10.9 (0.2)

54.6a (1.1)

9.6b (0.3)

24.3b (0.2)

10.3 (0.7)

11.1b (0.8)

9.7 (0.3)

9.9 (0.3)

10.2 (0.3)

98.1

No-CAT 11.0(0.5) 10.9(0.4) 49.8b (1.5) 10.5a (0.4) 27.7a (0.4) 10.3(0.5) 12.9a (0.5) 8.7(0.8) 9.7(0.2) 10.6(0.5) 96.4

Second year

CAT 11.1(0.8) 11.6(0.9) 33.0b (0.4) 10.8a (0.4) 39.0(0.2) 10.6a (0.4) 16.3a (0.2) 11.2a (1.0) 11.5a (0.1) 11.0a (0.2) 99.1

No-CAT 10.0(0.2) 10.7(0.7) 39.4a (0.3) 10.0b (0.1) 40.3(0.9) 9.0b (0.3) 11.9b (1.0) 8.4b (0.8) 8.3b (0.1) 9.9b (0.1) 99.0

Third year

CAT 11.6(0.2) 12.6a (0.5) 47.4a (1.0) 10.2b (0.1) 30.3b (0.1) 10.1a (0.6) 14.0(1.2) 9.0(0.4) 8.3(0.9) 11.0a (0.3) 94.8

No-CAT 11.4(0.3) 10.8b (0.3) 41.6b (0.2) 10.6a (0.2) 36.1a (0.5) 9.1b (0.1) 13.5(0.2) 8.7(0.1) 8.8(0.1) 10.3b (0.3) 90.2

LSD least significant difference for p 0.05 (n = 4), NS not significant. Numbers in brackets for wheat plots represent standard deviation (n = 4). Different small letters in columns indicate significant difference

OC towards smaller aggregates. Conversely, a significantly larger OC content was found in the 0.50-0.25 mm size fractions for COM-1 and in microaggregates (<0.25 mm) for both COM-1 and COM-2. As already reminded, this result may be attributed to adsorption of microbially processed organic components on fine soil particles and their association in microaggregates. While COM-2 appeared to better incorporate organic carbon in water-stable aggregates, the low OC recovery from fractionation of the TRA soil (92%) suggests that its OC content was due mainly to hardly transformed and, thus, coarse incorporated plant residues.

For soils under wheat, no consistent differences in OC content were found between CAT and No-CAT for either bulk soils or water-stable aggregates (Table 4.8), thus showing that the biomimetic catalyst did not stabilize SOM components in the first year.

Some differences in SOM dynamics among treatments were shown in Napoli after the second experimental year (Table 4.8). An increase of about 0.5 g kg-1 in bulk soil OC, as compared to the first year, was noted for MIN and COM-1. The same OC level found in TRA (9.1 g kg-1) and COM-2 (11.3 g kg-1) indicates that both OC deposition by crop in subsoil and OM added with compost were able to maintain the SOC content of the first year. Conversely, the plant residues mixed in soil with the GMAN treatment were not similarly effective and TOC content in this bulk soil decreased in this treatment.

A stable OM incorporation in aggregate size fractions was suggested after the first year in MIN, COM-1, and COM-2 by the large OC recovery in sum of fractions of these treatments. The OC stabilization was particularly evident for both COM rates which showed a significantly greater OC content in <0.25 mm microaggregates (Table 4.8). On the other hand, the low OC recovery in GMAN sum of fractions (92.4%), with respect to the OC of bulk soil, further suggests a weaker interaction between soil aggregates and organic matter added by green manuring.

The treatment with the biomimetic catalyst resulted more effective in the second year of wheat cultivation (Table 4.8). TOC content in CAT bulk soil increased about 1.0 g kg-1 with respect to the first year, while that of No-CAT was reduced by 1.5 g kg-1. The effectiveness of SOM stabilization promoted by catalyst addition was further indicated by the OC distribution in water-stable aggregates. An increased absolute OC content was found in <1.0 mm aggregate sizes in CAT soil. However, the relative % OC distribution in the 4.75-1.00 mm size fraction was also significantly larger than for No-CAT. These results are in line with those found for wheat plots in Torino and Piacenza, for which OM was preferentially associated with the finest soil particles due to the catalyzed coupling of humus aromatic compounds.

The superior OM fixation in soils treated with compost and biomimetic catalyst, with respect to conventional management practices, was revealed by the OC content in bulk soils and aggregate size fractions of the third year (Table 4.8). Bulk soils under COM-1 and COM-2 showed an OC content larger than that of second year by 0.5 and 1.2 g kg-1, respectively, and even 1.0 and 2.5 g kg-1 greater than for TRA of the same third year. As for COM-1, the final OC value in bulk soil (10.5 g kg-1) was the same as that of the initial undisturbed soil. However, although the priming effect noted in all soils with this treatment caused SOM losses, the OC incorporated in the Napoli site by COM-1 over control corresponded to 0.5 g kg-1 year-1. Based on an average 1.4 soil bulk density and 0.35 m plow depth, this amount represents about 80% of the total OC added with COM-1 in the 2 years of experimentation. This consideration, along with the observed large OC fixed by COM-2, confirms that mature compost amendment in the conditions of the Napoli site appears as a reliable method for carbon sequestration in soil.

The same can be argued for the CAT treatment in the third year that was further capable of significantly enhancing OC content in the largest macroaggregate size fraction and firmly sequester carbon, as shown by the OC fractionation losses which were lower than for No-CAT. Finally, the stable OC content for MIN and GMAN soils throughout the experimantion period and its significantly larger values than for TRA indicate that these soil management practices do also effectively sequestered carbon, though to a significantly lesser extent than COM and CAT treatments.

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