Deep soil tillage is the crop management with most impact on soil properties (Fagnano and Quaglietta Chiaranda 2008). Soil tillage may have contrasting agronomic and environmental effects on soil structure and aeration of the tilled layer. The destruction of soil aggregates decreases macroporosity and infiltration rate, with enhanced risk of soil erosion and reduced oxygen diffusion rate. The consequent anaerobic conditions limit root growth and may increase denitrification and methanogenesis with enhanced greenhouse gas emissions (CH4 and N2O). Conversely, a macroporosity increase can be favorable in clayey soils (Pagliai et al. 2004). Increased aeration can stimulate SOM oxidation and nitrification (Reicosky et al. 1995), with positive effects on plant N uptake and growth, but negative effects on nitrate pollution of water table and CO2 emissions from soils.
Literature reports contrasting findings on the impact of soil tillage on crop yield and environmental quality, depending on soil (texture, structure, SOM content) and weather (rainfall distribution and intensity) conditions, as well as cropping systems (crop, rotation, and management of crop residues) (Lal 1989; Barberi 2006). Moldboard plowing is considered the soil tillage method most harmful to SOM maintenance, because of deep soil layer disturbance and dilution of fresh organic matter (i.e., crop residues, organic fertilizers) in the soil profile (Pagliai et al. 1998).
Increasing soil disturbance passing from no-tillage (zero-tillage), to minimum tillage, to deep plowing has been shown to enhance potential SOM oxidation and microbial respiration (Morris et al. 2004). Long-term experiments (Tebrugge and During 1999) demonstrated that reduced tillage increased aggregate stability and soil cover by crop residue, limiting surface sealing and erosion. Moreover, organic matter and nutrients increase in the top layer, while biological activities (i.e., earthworm) and water infiltration rates are enhanced. No tilled soils were also found to be more resistant to vehicle passage, thus reducing soil compaction.
In clayey soils, increase of SOM content following no-tillage was limited in the top layer (10 cm), while the negative impact in deeper layers (lower porosity and greater compaction) induces root growth reduction and maize yield losses (Mariotti et al. 1998). On the contrary, in sandy soils, reduced tillage increased soil porosity and crop yields of potato, wheat, burley, and oat (Ekeberg and Riley 1997).
In humid environments of Scotland, no-tillage caused yield losses due to lower permeability, waterlogging, and weed increase (Ball and Ritchie 1999). No-tillage determined a reduction of porosity and subsequent losses in wheat and maize yield in a silty soil (Cereti and Rossini 1995), while the porosity reduction did not affect root growth in deeper layers of sandy soils (Venezia et al. 1995).
Minimum tillage is believed to increase SOM content. However, it has been reported that its application can only determine a change of OM distribution in the soil profile, with larger content in the top layers and lower content in the deeper ones, without really altering its total content (Carone et al. 2000a; Triberti et al. 2000). Minimum tillage was found to reduce the yield of durum wheat and chickpea in some clayey soils of southern Italy (Mori et al. 2000). In general, reduced tillage may provide positive effects in dry environments, whereas it may bring very negative consequences on erosion and denitrification in regions with heavy rainy periods (i.e., in fall-winter period over Mediterranean clayey soils).
A lower impact of soil tillage is reported for spring-summer crops and in dry years (Marenghi et al. 2000), thus confirming that deep tillage is necessary in more rainy conditions and mostly in clayey soils (Carone et al. 2000b), while reduced tillage is more useful in dry conditions (Mori et al. 2000; Simanskaite et al. 2009).
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