Microbial Activity

Soil microbial activity can be estimated by measuring CO2 respired from soil, as a well-established parameter to monitor SOM decomposition (Anderson 1982). Soil respiration is highly variable and its natural fluctuation depends on substrate availability, moisture, and temperature (Alvarez et al. 1995; Brookes 1995). For valid comparisons among soils, respiration measurements must be conducted under controlled laboratory conditions (Anderson 1982). Here, the basal respiration of soil samples was estimated by gas chromatography (Fisons GC 8000 series) as CO2 evolution in standard conditions (4 h of incubation at 25°C, at dark), after adding 2 ml distilled water to 1 g of soil (Degens et al. 2000). The basal respiration was expressed in mg CO2 evolved per gram soil per unit of time. Therefore, the rate of OM mineralization and, hence, the potential capacity of soil to accumulate or dissipate carbon, is comprised in the coefficient of endogenous mineralization (CEM) that was calculated from soil respiration and SOM content, whereby CEM represents the CO2 evolved from soil per unit of organic C.

6.5.5 Statistical Analyses

To assess the differences among the project treatments as well as among years in each experimental site, data for cultivable microbial populations were analyzed by using XLSTAT-6.1 and applying standard analyses of variance (one-way and two-way ANOVA) at p < 0.05 level.

The three-way ANOVA followed by Holm-Sidak post hoc test for pair-wise comparison of means (at p < 0.05 level) was used to elaborate data of active fungal mycelium (AFM), microbial biomass and microbial respiration and to assess the differences among treatments and experimental sites, as well as those between bulk soil and rhizo soil. A two-way ANOVA was performed for CEM since only bulk-soil values for organic carbon were available due to missing measurements of SOM in the rhizosphere. Statistical analyses were performed by using Sigma-Stat-3.1 for Windows software package.

6.6 Effects of Compost Amendments

Soil managements, such as traditional and minimum tillage, induce reduction of SOM content and decrease of soil structural stability and, thus, have a great impact on functional processes of soil microbial communities. Application of materials rich in organic matter, such as compost, may be used to recover and/or improve soil structure and fertility. Amendments with compost can also strongly influence and modify the size, biodiversity, and activity of the microbial communities in soil (Albiach et al. 2000). Since compost is a source of nutrients which can be used by microorganisms, compost addition usually increases soil microbial biomass and global activity (Bailey and Lazarovits 2003).

6.6.1 Microbial Counts

The effect of compost amendment (COM-2) on the biomass of cultivable communities, as compared to traditional (TRA) and minimum (MIN) tillage, was studied in three different experimental sites (Napoli, Torino, and Piacenza) of the MESCOSAGR project during 3 years (2006, 2007, and 2008).

Microbial populations were significantly affected by agronomic practices. In fact, in all three sites the microbial populations were drastically reduced after

3 years of experimentation in both bulk soil and rhizo soil. This trend was most marked in field plots at Napoli, in which all enumerated microbial populations in COM-2 soils were 1 Log CFU g_1 smaller than those found in TRA and MIN soils. In particular, the COM-2 bulk soil showed a negative cumulated effect on total heterotrophic aerobic bacteria, fungi, actinomycetes, and aerobic and anaerobic cellulolytic bacteria due to repeated compost applications to soil. In fact, the

Bulk-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

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

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

Fig. 6.2 (continued)

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

Fig. 6.2 (continued)

Hi 111 III

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 □ TRA * COM2 m MIN

Fig. 6.2 Effect of management practices (TRA traditional amendment, MIN minimum tillage, COM-2 compost amendment) 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). Different letters indicate significant difference among treatments and years (ANOVA-Tukey test; p < 0.05) within site (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). Different letters indicate significant difference among treatments and years (ANOVA-Tukey test; p < 0.05) within site amount of microbial populations after the first compost application (2006) increased significantly more in COM-2 than in TRA and MIN (Fig. 6.2a, b), before declining in the following 2 years (2007-2008). A similar trend was observed in maize rhizo soil at Napoli (Fig. 6.3a, b), whereby a significant abrupt reduction of aerobic (from 8.07 ± 0.08 to 6.21 ± 0.16 Log CFU g-1) and anaerobic (from 6.72 ± 0.12 to 4.93 ± 0.05 Log CFU g-1) cellulolytic bacteria was observed in the third year of compost amendments (Fig. 6.3b).

At the Piacenza site, COM-2 plot was characterized by the lowest microbial values mainly in 2008 year (Fig. 6.2a, b), even though all treatments negatively influenced microbial populations in bulk soil for all 3 years of experimentation. The cumulative negative effect was clearly detectable in aerobic and anaerobic cellulo-lytic bacteria, which showed a decrease of 1-2 Log cycles in the third treatment

Soil Microb Key Effect Carbon

Fig. 6.3 (continued)

Pc 06 Pc 07 Pc 08 Na 06 Na 07 Na 08 To 06 To 07 To 08 □ TRA ■ COM2 IS MIN

Fig. 6.3 (continued)

year (Fig. 6.2b). The reduction of microbial populations, and particularly of cellu-lolytic bacteria, is interesting since the cellulose degrading enzymes of these populations are directly involved in key OM decomposition steps. In fact, the reduction of functional group of cellulolytic bacteria may result in an increase of organic matter stabilization due to compost addition. Even if this negative behavior

Rhizo-soil ra 9 t-

Rhizo-soil ra 9 t-

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

Fig. 6.3 Effect of management practices (TRA traditional amendment, MIN minimum tillage, COM-2 compost amendment) on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in rhizo soils of Piacenza (Pc), Napoli (Na), and Torino (To). Different letters indicate significant difference among treatments and years (ANOVA-Tukey test; p < 0.05) within site (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in rhizo soils of Piacenza (Pc), Napoli (Na), and Torino (To). Different letters indicate significant differences among treatments and years (ANOVA-Tukey test; p < 0.05) within site

Fig. 6.3 Effect of management practices (TRA traditional amendment, MIN minimum tillage, COM-2 compost amendment) on (a) total aerobic bacteria, fungi, and actinomycetes (mean of Log CFU g-1 of soil ± SE) in rhizo soils of Piacenza (Pc), Napoli (Na), and Torino (To). Different letters indicate significant difference among treatments and years (ANOVA-Tukey test; p < 0.05) within site (b) aerobic and anaerobic cellulolytic bacteria (mean of Log CFU g-1 of soil ± SE) in rhizo soils of Piacenza (Pc), Napoli (Na), and Torino (To). Different letters indicate significant differences among treatments and years (ANOVA-Tukey test; p < 0.05) within site was detected also in the rhizo soil of Napoli, no significant differences among soil management treatments were observed in Piacenza for aerobic and anaerobic cellulolytic bacteria in maize rhizo soil all through the 3 years (Fig. 6.3b). Therefore, the detrimental effect of compost may have been reduced by root exudation in the rhizosphere of maize cropping, as an additional organic carbon source stimulating the microbial growth. The size and composition of rhizosphere microflora is mostly plant-dependent, a phenomenon known as the "rhizosphere effect" (Burr and Caesar 1984) and is attributed to emission of root exudates. Composition of exudates was shown to depend on plant species (Wieland et al. 2001; Singh et al. 2007), as well as on the plant development stage (Jaeger et al. 1999; Yang and Crowley 2000; Feng et al 2003), environmental conditions, and management practices (Paterson and Sim 1999, 2000).

Results different from those of Napoli and Piacenza were obtained from Torino, where compost application did not show the same cumulative effect on soil microbial communities in either bulk or rhizo soil. In fact, bulk soil from COM-2 did not reveal significant difference among the three experimental years in microbial densities of heterotrophic aerobic bacteria, actinomycetes, and aerobic cellulolytic bacteria. By contrast, in 2008, anaerobic cellulolytic bacteria and fungi slightly increased or decreased, respectively (Fig. 6.2a, b). However, the medium used in this study for fungal population mainly selects for a physiological type of fungi characterized by rapid germination of spores and high rate of mycelial growth. Such fungi, which are pioneer colonizers, are able to use ephemeral substrates readily. Their rapid growth results in a sudden spike of activity followed by a rapid decline, since they are unable to degrade abundant substrates such as the resistant ligno-cellulosic structures present in the green compost of this study.

Overall, microbial populations detected in Torino rhizo soil at the third year of experimentation (2008) showed a significant decrease of 1-2 Log CFU g-1 for all soil treatments.

The different effect of compost amendment on cultivable microbial biomass at the three field sites should be ascribed to different soil texture and climatic conditions, which are the main determinants of structure and activity of microbial communities. Moreover, the compost used in the experiments was a green waste compost (for chemical composition of the compost see Chaps. 3 and 4). Green waste compost contains both readily decomposable (cellulose) and more recalcitrant (lignin) fractions from plant litter (Standing and Killham 2007). In a short-term experiment, Perez-Piqueres et al. (2006) evaluated the impact of organic amendments on soil microbial characteristics by using green waste and spent mushroom composts. They found that the microflora in two different soils was influenced by the type of compost. Green waste compost did not modify the densities of cultivable bacteria and fungi in either soil, while the spent mushroom compost significantly increased bacterial and fungal densities in both the clayey and sandy-silty-clay soil, respectively.

Therefore, the cumulated negative effect recorded in Napoli and Piacenza sites (silty-clay-loam soils) may be due to an interaction of compost with the abundant clay particles, which might protect organic matter physically and/or chemically. The mechanism by which organic matter is adsorbed on clay determines its bioaccessibility and the ability of microorganisms to use OM as substrate and to produce extracellular enzymes. Moreover, the presence of chaotropic and antichaotropic ions can influence the nutritional status of microhabitats (Stotzky 1997). By contrast, in Torino bulk soil with a low content of clay (sandy-loam soil), compost amendment led to a significant microflora stimulation, as compared to traditional and minimum tillage.

The largest number of cultivable microorganisms found only in the first experimental year in the bulk soil of COM-2 at all experimental sites should be attributed to the introduction of new community members with the compost rather than to a stimulation of the indigenous community. In fact, both in Napoli and Piacenza, the negative effects on microorganisms were generally observed at the end of the third experimental year.

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