Impact of Different Agricultural Practices on Soil Microbial Communities The Mescosagr Case Study

Within the National project MESCOSAGR, we investigated the impact on soil microorganisms of two innovative technologies applied to sequester carbon in agricultural soils. The hypothesis was that the structure and activity of microbial communities would be influenced by (1) addition of compost, a humified and hydrophobic material that protects the easily degradable organic fraction, and (2) in situ photo-oxidative polymerization of native SOM under the action of a biomimetic catalyst (CAT) (iron-porphyrin). The effect of these two technologies on soil microorganisms was compared with that exerted by traditional deep tillage and minimum tillage. The latter is an agronomic practice commonly used to reduce SOM depletion and limit CO2 emission from soil into the atmosphere (see Chap. 3).

Two types of approaches were pursued to estimate soil microbiological parameters (1) one aimed to characterize the composition of soil microbial communities involved into carbon cycle, (2) another one based on a holistic view that considers the microbial biomass as a whole, without distinguishing its individual components. The first approach is based on the Surface Spread Plate Count Method and Pour Plate Method, which use selective media to identify the major microbial groups involved in the different steps of organic matter decomposition, i.e., total aerobic heterotrophic bacteria, cellulolytic bacteria, fungi, and actinomycetes. This approach allows determining the size and composition of microbial communities and has been used to assess changes in the soil biota in response to land management, thereby providing an indicator of soil biological status (Harris and Birch 1992). The second approach is based on the determination of microbial community characteristics which include abundance and activity (1) abundance of fungal cells is measured by fluorescence microscopy; (2) soil microbial biomass is determined as microbial C by the SIR (Substrate Induced Respiration) method; (3) microbial respiration per unit of organic C becomes a coefficient of endogenous SOM mineralization. The combination of these two experimental approaches is expected to provide most information regarding the effects of soil managements on microbial communities.

6.5 Soil Sampling and Microbiological Analyses

The study comprised three different sites (Napoli, Torino, and Piacenza) under different soil and climate conditions. All microbial parameters were assessed in both the bulk soil and the rhizo soil. Soil samples were collected from the experimental plots under either maize or wheat (see Chaps. 3 and 7 for details on the field trials) during three consecutive years (2006-2007-2008). Bulk-soil samples were collected from the 0-15 cm soil layer after maize harvest (September) and before wheat sowing (November). Rhizo-soil samples were collected during stem elongation (April-May for wheat rhizosphere, and July for maize rhizosphere). Bulk-soil samples were a mix of three subsamples collected in three different locations for each treatment plot. Rhizosphere samples consisted of soil adhering to total roots of three crop plants collected from each treatment plot. The roots were shaken vigorously to separate the rhizo soil. All samples were collected in triplicate, brought to laboratories, stored in polyethylene bags at 4°C for no more than 24-48 h before soil microbiological analyses were conducted.

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