Torino Experimental Site

After 1 year of maize cultivation, TOC values in bulk soils (Table 4.6) did not reveal significant differences among treatments, except for MIN, that showed the lowest OC content with a decrease of about 1 g OC kg—1 with respect to TRA. The addition of exogenous organic matter (GMAN, COM-1, and COM-2) did not produce any increase with respect to TRA. This implies that the fresh organic matter stimulated a microbial mineralization that may have counteracted the OC addition to soil.

With the exception of COM-2, the OC distribution in soil aggregate sizes generally showed a progressive larger OC absolute content with increasing aggregate size, while the relative (%) OC amount was mainly related to the aggregate mass distribution. The OC increase in larger soil aggregates is in line with the theoretical organization of aggregate hierarchy, by which macroaggregates become richer in OC due to the progressive mutual association of fine particles in larger aggregates (Puget et al. 1995; Six et al. 2004). The large OC found in the 0.50-0.25 mm aggregate size in COM-2 may be explained with a compost-induced greater microbial activity, whose bio-products are preferentially adsorbed on the large surface areas of fine aggregate sizes (Zech and Guggenberger 1996). This finding is in line with those for GMAN and COM-1, which also showed, with respect to TRA and MIN, the lowest relative (%) OC in large aggregates (4.75-1 mm), and greater absolute and relative OC content in small aggregate sizes (0.50-0.25 and <0.25 mm).

For wheat fields (Table 4.6), a greater OC content in bulk soil was found for plots treated with the water-soluble iron-porphyrin catalyst (13.2 g kg—1), as compared to control plots (12.4 g kg—1). However, aggregate-size fractionation provided a low OC recovery for both CAT and No-CAT treatments, with 91 and 93% of TOC recovered for bulk soil, respectively (Table 4.6). This result suggests a loss of soluble organic matter due to a weaker interaction between organic components and surface of mineral particles. Since these values were lower than those obtained for the same soil under maize, the reason cannot be attributed to differences in soil physical properties, but possibly to a different OC incorporation depending on both treatment and crop. The OC distribution in water-stable aggregates for CAT

Table 4.6 Torino 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 gkg-1

Aggregate

sizes

%

4.75-1.00

1.00-0.50

0.50-0.25

<0.25

gkg"1

%

gkg"1

%

gkg"1

%

gkg"1

%

gkg-1

Control soil

11.5

12.1

11.0

10.0

19.9

11.1

28.8

10.3

40.4

10.6

92.2

Maize

First year

TRA

11.6

13.6

19.2

11.9

29.1

10.4

25.3

9.6

26.4

11.0

94.8

MIN

10.7

10.9

14.3

10.2

31.0

10.0

27.2

9.8

27.5

10.1

94.4

GMAN

11.8

11.6

11.2

11.9

28.0

11.5

31.9

10.3

29.0

11.3

95.8

COM-1

11.8

13.1

10.2

12.2

27.7

11.8

32.9

10.2

29.4

11.5

97.5

COM-2

11.4

9.2

12.2

11.5

30.1

12.2

29.6

11.0

28.1

11.2

98.2

LSD

0.45

1.0

0.6

1.1

2.4

0.6

1.9

0.2

NS

1.0

Second year

TRA

11.5

13.4

10.9

10.9

26.1

10.4

38.9

9.9

24.1

10.7

93.0

MIN

11.5

17.2

14.2

14.4

28.8

10.0

33.0

10.0

24.2

11.3

98.2

GMAN

10.8

11.8

11.3

10.9

24.3

11.4

42.2

9.3

22.3

10.7

99.0

COM-1

12.1

11.4

9.0

10.2

20.3

13.8

43.7

11.7

26.9

12.1

100

COM-2

12.4

14.9

11.5

13.9

26.2

13.1

41.0

9.4

21.3

12.4

100

LSD

0.50

1.5

0.6

1.7

1.6

0.7

1.9

0.8

1.2

0.8

Third year

TRA

11.6

12.0

11.7

11.9

25.8

11.7

36.7

10.8

25.8

11.5

99.1

MIN

11.3

11.5

9.9

11.5

21.3

11.4

38.2

11.2

30.5

11.4

100

GMAN

12.0

12.2

10.1

11.5

19.8

11.4

38.7

10.7

31.3

11.3

94.2

COM-1

12.7

14.6

17.7

13.4

27.0

11.5

31.6

12.0

23.9

12.6

99.2

COM-2

13.2

15.0

23.1

15.1

31.8

12.0

27.1

10.7

18.0

13.2

100

LSD

0.40

0.9

0.7

0.9

1.3

0.4

1.4

0.7

1.0

0.7

First year

CAT 13.2a (0.11) 11.9(1.1) 18.8a (2.0) 11.5(1.0) 23.6(2.0) 13.4a (1.1) 28.6(3.0) 12.6(0.5) 29.1 (3.0) 11.0 (1.0) 83.3

No-CAT 12.4b (0.40) 11.7(0.9) 13.0b (1.9) 10.9(0.6) 27.5(3.0) 10.9b (0.6) 31.4(5.0) 12.6(0.7) 28.2 (4.8) 10.5 (0.9) 84.7

Second year

CAT 13.0a (0.4) 15.5a (0.9) 12.5(0.5) 12.8(0.6) 25.3a (0.2) 12.5(0.1) 40.0a (1.0) 8.9b (0.1) 22.3b (1.2) 11.8a (0.2) 90.8

No-CAT 11.8b (0.3) 12.4b (0.2) 12.4(1.0) 11.8(0.5) 23.6b (0.1) 11.7(1.0) 34.3b (1.2) 10.1a (0.5) 29.7a (1.7) 11.2b (0.3) 94.9 Third year

CAT 13.8(0.5) 14.6(0.4) 13.0(0.1) 11.8b (1.0) 26.9a (0.6) 14.0a (0.1) 36.5a (0.9) 14.9a (1.1) 23.6b (0.6) 12.9(0.4) 93.5

No-CAT 13.7(0.3) 15.9(1.2) 11.9(1.1) 14.7a (0.7) 23.2b (0.1) 11.1b (0.5) 31.9b (0.8) 10.0b (0.7) 33.0a (0.2) 12.7(0.2) 92.7

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 at 0.05 probability level (n = 4)

revealed a generally larger OC content in greater aggregate size fractions than for No-CAT, though a significant difference from No-CAT was found only for the 0.50-0.25 mm aggregate size. This microaggregate size-fraction is composed of fine particles (silt and clay) (Six et al. 2004) mutually associated in organo-mineral complexes due to interactions exerted by adsorbed aliphatic and aromatic components (Baldock and Skjemstad 2000; Spaccini et al. 2002). Thus, it is likely that the catalyst added in the CAT treatment has favored the covalent coupling of aromatic and phenolic SOM components, thus further stabilizing the organic constituents adsorbed on this fine soil fraction.

After two experimentation years, both COM treatments determined an increase of TOC in bulk soils, whereas a decrease was observed for GMAN (Table 4.6). Although GMAN may represent an important supply for plant N requirements, the fresh organic matter added to soil with this treatment may not produce a stable incorporation of organic material (Scholes et al. 1997; Puget and Drinkwater 2001; Spaccini et al. 2004). Conversely, with respect TRA, about 0.6 and 1.0 g kg-1 of OC were additionally incorporated in COM-1 and COM-2 bulk soils, respectively. Based on plowed soil depth (0.35 m) and average soil bulk density of 1.4 (Table 4.1), the OC fixed corresponded to about 0.6 and 1.1 g kg-1 for COM-1 and COM-2, respectively. Therefore, at the end of the second year, the COM plots retained around 100 and 90% of OC added with compost. This means that, by comparing the OC content in MIN and COM-1 between first and second experimentation year (Table 4.6), the SOC fraction in COM-1 must have inherited an aliquot of organic matter incorporated in the first year. The MIN treatment was instead able to significantly fix OC during the second crop cycle, thus recovering the difference from TRA observed in the first year.

The OC distribution in water-stable aggregates after the second year showed that the relative OC content was still related to the physical fractionation yield (Table 4.6). An increase of OC content in large aggregate sizes was shown by TRA, MIN, and COM-2, whereas a greater OC concentration in smaller aggregate sizes was found in GMAN (0.50-0.25 mm) and in COM-1 (0.50-0.25 and <0.25 mm), thus suggesting an accumulation of microbially derived organic components in the latter treatments. The low OC recovery after aggregate fractionation of TRA (92.8% of OC in bulk soil) may imply a contribution of coarse debris from crop residues to TOC of bulk soil. The incorporation of fresh plant-derived organic material was suggested by the large OC concentration (17.4 g kg-1) found in MIN macroaggregates (4.75-1.00 mm), thus indicating that little soil disturbance in MIN may have favored the inclusion of particulate organic matter in the building up soil aggregates (Angers et al 1995; Puget et al. 1995; Angers and Giroux 1996; Six et al. 2000).

Despite the low OC recovery in the aggregate fractionation of wheat soils after two experimentation years (Table 4.6), an OC fixation was evident in CAT due to amendment with biomimetic catalyst. Both absolute and relative OC contents in CAT water-stable aggregates showed a preferential OC incorporation in larger size-fractions.

After three experimentation years, the TOC content showed that both COM-1 and COM-2 were able to increase OC with respect to other maize treatments. In comparisons to the first year, when no difference was observed among different treatments, COM-1 and COM-2 revealed, at final experimental time, an additional OC content of about 0.9 and 1.8 g kg-1, respectively, corresponding to the 87% of the total averaged organic carbon added with compost in the second and third year of soil treatment.

Also GMAN showed a 1.2 g kg-1 increase of TOC content in the third year, as compared to the second year. For an average OC content of about 55% in leguminous plants, this additional amount in soil corresponded to about 13 ton ha-1 of green manure material, and implied that the totality of leguminous plants added to soil as GMAN should have been retained. However, leguminous crops are reckoned to rapidly mineralize, once incorporated in soil (Fernandes et al. 1997; Spaccini et al. 2004), and hardly contribute to stable SOM (Scholes et al. 1997; Puget and Drinkwater 2001). Hence, the larger TOC found in GMAN should be mainly accounted to maize crop residues left on soil.

The OC content in soil aggregates of COM plots after 3 years indicates the capacity of this treatment to increase SOM (Table 4.6). All field treatments under maize revealed an increased OC in large aggregate sizes. However, the greatest absolute and relative OC content was found in the >0.50 mm aggregate class for both COM treatments. This confirms the effectiveness of hydrophobic humified matter in improving soil aggregation and stability, as already suggested by mass yield of soil fractionation (Table 4.2). Conversely, the lower OC recovery in soil aggregate size fractions for GMAN further indicates the poor OC stabilization conferred by this treatment that prevalently provides easily biolabile organic matter in soil.

No significant difference was found between CAT and No-CAT after 3 years for TOC content in bulk soils (Table 4.6). However, the OC distribution in separated aggregate sizes clearly indicates that SOM stabilization was induced by the biomi-metic catalyst treatment. In fact, the 0.50-0.25 and <0.25 mm aggregate size fractions revealed a significant OC increase due to the catalyst, reaching 14.0 and 14.9 g kg-1 for CAT, and only 11.1 and 10.0 g kg-1 for No-CAT, respectively. This further suggests that the catalyzed photo-polymerization of organic constituents of SOM had occurred in situ and its products were associated with the finest soil particle fractions.

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