Effects of the Biomimetic Catalyst

Humic substances comprise the major part of stable organic matter in environmental compartments and their formation and decomposition processes regulate global carbon cycling. An increase in the conformational stability of humus may be achieved by increasing the intermolecular covalent bonds among heterogeneous humic molecules through a photo-oxidative coupling mediated by a biomi-metic (enzyme-like) catalyst, such as synthetic water-soluble metal-porphyrins (Piccolo et al. 2005). It was found that soil amendments with the biomimetic catalyst affected the molecular structure of SOM and decreased its biotic degradation, thereby significantly decreasing CO2 emission from soil (Gelsomino et al. 2010; Piccolo et al. 2011). However, since new molecules added to soil, though apparently harmless and eco-compatible, may deeply alter the behavior of microbial populations through complex and unexpected interaction (biotic and/or abiotic), we studied the impact of the biomimetic catalyst on the dynamics of microbial soil populations in the different experimental fields of the MESCOSAGR project.

6.7.1 Microbial Counts

The CAT treatment showed hardly any long-term effects on cultivable microbial populations, as evaluated in both bulk soil and rhizo soil. In fact, no significant difference was found between CAT plots and their control (No-CAT) in any of the experimental sites for the first 2 years of treatment (Figs. 6.6a, b and 6.7a, b).

Fig. 6.6 (continued)
Metal Sequestration

Fig. 6.6 Effect of synthetic metal-porphyrins addition on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in bulk-soils of Piacenza (Pc), Napoli (Na), and Torino (To). Asterisk indicates significant atp < 0.05 within site and years. NO-CAT: control, CAT: soil treated with biomimetic catalyst (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in bulk-soils of Piacenza (Pc), Napoli (Na), and Torino (To). Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst

Fig. 6.6 Effect of synthetic metal-porphyrins addition on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in bulk-soils of Piacenza (Pc), Napoli (Na), and Torino (To). Asterisk indicates significant atp < 0.05 within site and years. NO-CAT: control, CAT: soil treated with biomimetic catalyst (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in bulk-soils of Piacenza (Pc), Napoli (Na), and Torino (To). Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst

The only exception was in the Piacenza bulk soil that showed an increase in actinomycete populations during the whole experimental period (Fig. 6.6a). This long-term effect was probably due to the firm adsorption of the added metal-porphyrin on the large amount of clay particles present in this soil, thereby resulting in greater catalytic activity upon soil biotic and/or abiotic components.

In the last experimental year (2008), the CAT effect on the bulk-soil was the same in the three different field sites, since it significantly affected the microbial groups directly involved in OM mineralization. In fact, the number of total heterotrophic aerobic bacteria, fungi, actinomycetes, aerobic, and anaerobic cellulolytic bacteria was significantly larger in CAT than in No-CAT by an extent of about 1 Log CFU g-1 cycle (Fig. 6.6a, b).

Conversely, it appears that CAT negatively affected the OM mineralization communities in rhizo soils, although the effect on maize rhizo soil (Piacenza) was different from that on wheat rhizo soil (Napoli and Torino). In particular, CAT did not influence the cellulolytic bacteria in the wheat rhizo soils of both Napoli and Torino with respect to No-CAT for all experimental years (Fig. 6.7a, b). By contrast, CAT significantly affected microbial communities in maize rhizo soil.

Rhizo-soil

Rhizo-soil

Pc 06 Pc 07 Pc 08 Na 06 Na 07 Na 08 To 06 To 07 To 08

Pc 06 Pc 07 Pc 08 Na 06 Na 07 Na 08 To 06 To 07 To 08 ^ NO-CAT m CAT

Fig. 6.7 (continued)

Pc 06 Pc 07 Pc 08 Na 06 Na 07 Na 08 To 06 To 07 To 08 ^ NO-CAT m CAT

Fig. 6.7 (continued)

Rhizo-soil

Pc 06 Pc 07 Pc OS Na 06 Na 07 Na 08 To 06 To 07 To 08

Fig. 6.7 Effect of synthetic metal-porphyrins addition on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in maize rhizosphere of Piacenza agronomic station (Pc), and in wheat rhizosphere of Napoli (Na) and Torino (To) agronomic stations. Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in maize rhizosphere of Piacenza agronomic station (Pc), and in wheat rhizosphere of Napoli (Na) and Torino (To) agronomic stations. Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst

Fig. 6.7 Effect of synthetic metal-porphyrins addition on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in maize rhizosphere of Piacenza agronomic station (Pc), and in wheat rhizosphere of Napoli (Na) and Torino (To) agronomic stations. Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in maize rhizosphere of Piacenza agronomic station (Pc), and in wheat rhizosphere of Napoli (Na) and Torino (To) agronomic stations. Asterisk indicates significant at p < 0.05 within site and years. NO-CAT: control, CAT: biomimetic catalyst

In fact, a decrease in the number of all cultivable microorganisms in Piacenza was found after three experimentation years (Fig. 6.7a, b). However, this effect was more extensive in fungi populations which decreased from 6.06 ± 0.09 for No-CAT to 5.03 ± 0.08 Log CFU g-1 for CAT (Fig. 6.7a).

The different effect of CAT on microbial communities in rhizospheres of maize and wheat may be due to different root systems and root activities. Maize plants have more expanded root systems than wheat, and, thus, explore a greater volume of soil and possibly induce a larger root exudation in the rhizosphere, to promote microbial growth. It is known that when root exudates serve as sole source of C and energy for soil microbes, root exudation is 2-2.6 times greater than in the case of aseptically grown plants (Vancura et al. 1977; Prikryl and Vancura 1980). Thus, the lower content of soil microbes in CAT suggests that a reduction in root exudate stimulation has occurred, possibly because another source of C and energy was made available to microbes by the CAT treatment. In fact, the enhanced growth of microbial cells following root exudate stimulation is attributed to carbohydrates in exudates (van Overbeek and van Elsas 1995). Therefore in the rhizosphere, the activity of the catalyst on SOM may lead to release of novel organic molecules. The consumption of such molecules by microbes may have depressed the increase of the root exudates/microbial growth cycle observed in No-CAT rhizo soils. Gelsomino et al. (2010) added the biomimetic catalyst on a microcosm soil with and without maize plants and measured CO2 respiration (see also Chap. 10). They found that while respiration was reduced in catalyst-treated bare soils, there was an enhanced respiration when maize plants were present. Although these results are intriguing, they are difficult to compare since our findings refer to maize rhizo-soil microbial communities and were obtained from field experiments rather than in microcosms.

Nevertheless, the significant decrease observed for maize rhizo soil under CAT treatment suggests that maize plants appear as more suitable indicators than wheat plants in highlighting the effect of the biomimetic catalyst. Furthermore, our findings indicate that the biomimetic catalysts added to soil did not appear to be harmful to cultivable microbial communities, since no lethal effect was recorded.

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