As mentioned earlier, the yields of aggregate fractionation and values of soil stability index suggested a predominant influence of specific texture and clay content on the physical and structural properties of soils. However, all treatments in each site, despite the specific soil structural properties, showed a comparable macroaggregate dynamics throughout the experimental period, consisting first in a decrease and then in a recovery of soil stability. In fact, our results revealed a similar succession of aggregate yields and stability index, whose values varied annually around the average level of the respective undisturbed initial soils.
The effectiveness of clay particle sizes, as binding agent, is strongly related to soil physical-chemical conditions and imposed mechanical stress. Soil pH, ionic strength, and salt composition of soil solution, water holding capacity, freezing/ thawing and wetting/drying cycles, raindrop impact, and intensity and frequency of soil tillage, may temporarily overcome the glueing effect exerted by mineral colloids on soil aggregates. In cultivated soils with low SOM content, the lower resilience of clay particles may result in a slow response to the aggregation capacity of soil management (Spaccini et al. 2004), and a significant loss of structural stability for heavy textured soils. In fact, the susceptibility of cultivated soils to loose structural stability is a function of initial aggregation, that is greater for stable clayey than for fragile sandy soils (Spaccini et al. 2001).
Moreover, large soil aggregates, placed in the upper level of aggregate hierarchy, are usually characterized by large porosity, wide planes of weakness, and low tensile strength (Oades and Waters 1991). Therefore, macroaggregates are mostly affected by land use and soil disturbance, and undergo rapid turnover cycles and fast aggregation/disaggregation dynamics, especially for agricultural soils with low OC content (Plante and McGill 2002b). Since turnover time of macroaggregates ranges from 10 to 90 days (Plante et al. 2002; De Gryze et al. 2006), with significant fluctuation during growing seasons, more frequent samplings should better show the effect of management practices on soil aggregation (Daraghmeh et al. 2009). Thus, since the present experiment was mainly focused on the cumulative effects of soil management on SOC, the unique sampling date at the end of the growing season may have partly masked the differences among soil treatments.
Nevertheless, though results indicate a small effect of soil management as compared to intrinsic soil physical properties, significant differences on aggregate-size distribution and MWD were found among various treatments in the three sites. A similar aggregation process was revealed by fractionating TRA, MIN, and GMAN soils at any experimental site. This indicates that either a reduced soil disturbance by minimum tillage, or green manuring by soil incorporation of residues from leguminous crops, did not significantly modify the aggregate dynamics, neither in the sandy-loam soil of Torino, nor in the heavier textured soils of Piacenza and Napoli. On the contrary, a slight but significant improvement of soil aggregation and structural stability were found for both compost treatments (COM-1 and COM-2), in comparison to either TRA, MIN, or GMAN.
Contrasting results have been reported on the relation between soil aggregate stability and green manuring, when in combination with either reduced tillage or conventional tillage (Biederbeck et al. 1998; Podwojewski and Germain 2005). The main purpose of green manuring relies in the supply to soil of a slow-release source of organic nitrogen, in order to reduce or even replace mineral fertilization. The organic residues added with green manuring are easily decomposable and low in lignified tissues and hydrophobicity (Carvalho et al. 2009). These biolabile and hydrophilic characteristics are recognized to provide at best only a transient effect on soil stability (Piccolo and Mbagwu 1999).
Current findings on soil management methods based on no-tillage practices (NT) indicate an overall improvement of soil physical properties and soil stability, as compared to conventional tillage (CT) (Six et al. 2004). On the other hand, the main effects of reduced tillage methods on soil aggregation are limited to surface horizons, while small differences from conventional tillage are usually found below 10 cm with increasing plow depth (Liebig et al. 2004; Kasper et al. 2009). This emphasizes the importance of investigating the whole soil profile when studying the suitability of NT versus CT for aggregate stability and SOC sequestration (Plaza-Bonilla et al. 2010).
A large literature indicates that soil amendments with different compost materials provide an effective improvement in aggregate stability and related physical properties (porosity, infiltration rate, surface erosion, etc.), regardless of soil type and crop rotation (Weber et al. 2003, 2007; Sodhi et al. 2009). Although soil structure improvement is closely associated with an increase of organic carbon content, the quality and molecular composition of added organic matter play a basic role in the long-term stabilization of soil aggregates (Piccolo and Mbagwu 1999). Soil amendments with compost may promote two possible effects on soil physical quality (1) a transient or temporary soil aggregation through the stimulation of microbial activity and consequent production of stabilizing exopolysaccharides, and, (2) a long-term stabilization due to addition to soil of an effective amount of hydrophobic humified materials (Albiach et al. 2001; Roman et al. 2003; Bipfubusa et al. 2008). In fact, depending on the type and quality of compost, its amendment to soils may produce either short term, poor, or even negative effects on soil aggregate stability (de Leon-Gonzalez et al. 2000; Kohler et al. 2008).
As reminded above, the transient effect observed on soil aggregates of COM-1 field plots in the Napoli site in the first year (Table 4.5) may be related to an increased amount of microbial bio-products due to a priming effect on SOC. This explanation may be supported by the low OC amount found in bulk soil and large soil aggregates (Table 4.8). However, the results on aggregate-size distribution and stability index reported for COM-1 and COM-2 for all field sites after 3 years (Tables 4.3-4.5) indicate that the progressive incorporation of humified and hydro-phobic organic matter from compost allowed a slow but persistent improvement of soil aggregation. The largest effect was found for Torino, where the intrinsically low initial soil structural stability was more easily improved by stable compost (Piccolo et al. 2004), and favored a significant incorporation of fine aggregate sizes into larger stable aggregates.
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