AFM Microbial Biomass and Activity

CAT significantly affected soil respiration and CEM (Table 6.2). Moreover, significant differences between bulk soil and rhizo soil were found for all microbial parameters (Table 6.2). The differences between sites were significant for Cmic in

2007 and 2008, as well as for respiration and CEM in 2008. Moreover, a significant interaction soil x site was observed for AFM and Cmic (Table 6.2).

As for bulk soils, AFM in Napoli was larger in CAT than in No-CAT, though the differences were not significant (Fig. 6.8), while in Torino AFM was first significantly lower in CAT than in No-CAT in 2007 and, then, significantly larger in 2008 (Fig. 6.8). In the case of rhizo soils AFM found in CAT treatments was always lower than in No-CAT, but the difference was significant only for Torino in 2007 (Fig. 6.8).

Microbial biomass was not found significantly different between CAT and No-CAT in either bulk or rhizo soils throughout the experimental period for either Napoli or Torino (Fig. 6.8).

Respiration showed a similar increasing trend from No-CAT to CAT treatments in both Napoli and Torino and for either bulk soil or rhizo soil. However, the increase was significant only for Napoli bulk soil in 2007 and Napoli rhizo soil in

Table 6.2 Levels of significance (p values from ANOVA) for effects of a biomimetic catalyst (iron-porphyrin) addition to soil on microbial biomass and activity in bulk soil and rhizo soil at Napoli and Torino sites, and differences between years

Respiration

2007 (Three-way) Treatments (CAT/NO- 1 CAT)

Soil (Bulk-Rhizo) 1

Site (Napoli-Torino)

1

0.422

0.015

0.893

Treatments x soil

1

0.166

0.125

0.138

Treatments x site

1

0.159

0.857

0.710

Soil x site

1

0.931

<0.001

0.365

Treatments x soil x site

1

0.571

0.133

0.522

2008 (Three-way)

Treatments (CAT/NO-

1

0.275

0.874

0.016

CAT)

Soil (Bulk-Rhizo)

1

<0.001

<0.001

<0.001

Site (Napoli-Torino)

1

0.154

0.003

<0.001

Treatments x soil

1

0.140

0.668

0.854

Treatments x site

1

0.816

0.487

0.049

Soil x site

1

0.002

<0.001

0.988

Treatments x soil x site

1

0.864

0.575

0.575

2007-2008 (Three-way)

Years (2007-2008)

1

<0.001

<0.001

<0.001

Site (Napoli-Torino)

1

0.280

0.030

0.033

Soil (Bulk-Rhizo)

1

<0.001

<0.001

<0.001

Years x site

1

0.099

<0.001

0.050

Years x soil

1

<0.001

0.003

0.680

Site x soil

1

<0.001

0.025

0.505

Years x site x soil

1

<0.001

<0.001

0.512

2007 (Two-way) Treatments

(CAT/NO-CAT) Site

(Napoli-Torino) Treatments site

2008 (Two-way) Treatments

(CAT/NO-CAT) Site

(Napoli-Torino) Treatments Site 1

0.010

0.165 0.850

2007-2008

(Napoli-Torino) Years site

0.008

0.082

dF degree of freedom, AFM active fungal mycelium, Cmic microbial carbon, CEM coefficient of endogenous mineralization, CAT conventional tillage with addition of biomimetic catalyst, NO-CAT conventional tillage without catalyst. Values in bold are statistically significant

The coefficient of endogenous mineralization (CEM) in Napoli was significantly larger in CAT than in No-CAT for both 2007 and 2008 years, while this was true for Torino only in 2007 (Fig. 6.5). Regardless of treatment, CEM for Napoli soils was greater than for Torino (Fig. 6.5).

When comparing wheat soil and maize soil (No-CAT/TRA), Cmic and respiration showed significantly lower values in wheat soil (p = 0.045 and p = 0.029, respectively). Water content in bulk soils was very similar for Napoli and Torino in both years (Fig. 6.8). Rhizo-soil water content was lower in Napoli than in Torino.

Bulk-soil

Bulk-soil

30 20 10

Rhizo-soil

Na 07 Na 08 To 07 To 08

eh NO-CAT EZ CAT

Fig. 6.8 Active fungal mycelium, microbial C, microbial respiration and water content (mean ± SE) of soil sampled from Napoli (Na) and Torino (To) experimental sites. CAT: biomimetic catalyst, NO-CAT: control. Figures on the left report values for bulk-soil; figures on the right report values for rhizo soil. Different letters indicate significant differences between treatments (ANOVA-Holm-Sidak test; p < 0.05) within site and years a b b a a b

CAT addition increased water content in bulk soil with respect to No-CAT, whereas it had an opposite effect in rhizo soil (Fig. 6.8).

CAT increased AFM, and microbial biomass in bulk soils, but had an opposite effect in rhizo soil, well in agreement with plate-count results for total aerobic bacteria, cellulolytic bacteria, fungi, and actinomycetes. CAT increased respiration in both bulk and rhizo soils, thus suggesting, in line with CEM values, that the in situ photo-polymerization of SOM unexpectedly favors instead of limiting CO2

emissions. This result is consistent with the larger CO2 fluxes measured in the field from CAT soils compared to No-CAT, although the emissions include root respiration (see Chap. 9). Moreover, our findings are in line with the cited microcosm experiment (Gelsomino et al. 2010) that revealed that the addition of iron-porphyrin significantly reduced CO2 efflux from the unplanted soil, whereas CO2 emission was stimulated when maize plants were present. Gelsomino et al. (2010) hypothesized that the coarser root system induced by iron-porphyrin favored enhanced destruction of soil macroaggregates, thus exposing physically protected SOM to microbial decomposition. However, they were not able to quantify the contribution to CO2 emission from soil of autotrophic respiration (maize roots) and heterotrophic respiration (rhizosphere microorganisms).

Our data refer to the effect of CAT treatment on soils under wheat and they do not take into account root respiration. Moreover, we found that respiration increased in both the rhizo and bulk soils. Therefore, at least for bulk soils, the explanation proposed by Gelsomino et al. (2010) should be definitely excluded. However, there were contrasting responses of microbial communities to CAT for either bulk or rhizo soils, and it is likely that root systems inhibit growth of the microbial community. Despite the observed evidence of CO2 being released as much or more in CAT than in No-CAT, the catalyst-assisted in situ photo-polymerization of SOM has been shown to sequester organic C throughout the experimentation period in all sites (see Chap. 4).

These contrasting results cannot be yet totally explained since the mechanism underlying the interactions among the catalyst, microbial community, and root systems is complex. However, a possible reason for such an opposite behavior may be the fact that substrates for oxidative photo-polymerization are the phenolic or oxidized aromatic moieties of SOM, which produce the free radicals, whose coupling increases covalent bonds among humic molecules. These aromatic photo-polymerized components of SOM certainly become more biologically stable in soil, thus possibly explaining the reduction of AFM in some cases. Consequently, the carbon-chain alkyl compounds of SOM may result more easily accessible to microbial degradation due to alteration of humic conformations following separation of the photo-polymerized aromatic moieties.

It is also interesting to note that water content in rhizo soils is lower in CAT that in No-CAT, while the opposite is true for bulk soils. This may be due to the fact that the interaction of root systems with the catalyst induces an alteration of the surrounding soil structure, thus limiting the water retention capacity. Such alteration may also influence the size of the microbial community.

When comparing results for wheat and maize soils, it is evident that microbial biomass and activity are larger under maize. Given that wheat and maize grow in different seasons, climatic conditions could at least in part explain such differences (Mahmood et al. 2005). However, it is important to recall that different plant species produce different rhizosphere effects (Vancura et al. 1977; Cheng et al. 2003). There was a weak rhizosphere effect on fungal communities at both sites under either maize or wheat. In contrast a positive rhizosphere effect on Cmic was observed under maize at Torino, where, an increase of microbial biomass was accompanied by an increase in respiration.

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