ReviewA review of the effects of tillage systems on some soil physical properties, water content, nitrate availability and crops yield in the Argentine Pampas
Introduction
The Pampas is a vast plain of around 50 Mha, which runs from 30° to 40°S, 57° to 68°W in Argentina (Alvarez and Lavado, 1998). The relief is flat or slightly rolling and its natural vegetation consist of grasslands in which graminaceous vegetation species are dominant. Mean annual rainfall ranges from 200 mm in the west to 1200 mm in the east and the mean annual temperature ranges from 14 °C in the south to 21 °C in the north. Predominant soils are Mollisols formed on loess like materials of eolian origin, which present a wide range of variation in solum depth, texture, organic matter content and fertility (Alvarez and Lavado, 1998,). Because of its extension and yield potential the region is considered as one of the most suitable areas for grain crop production in the world (Satorre and Slafer, 1999).
Cultivation began in the Pampas during the 19th Century in a central humid portion of the region, with soils of high fertility, and widespread in recent decades to the south and the semiarid west (Soriano, 1991, Hall et al., 1992). Agriculture was introduced using low external input farming systems based on cattle grazing and harvest crop rotations (Viglizzo et al., 2001). The cropped area increased gradually due to economic reasons and, partially, as the consequence of an increase in rainfall during the last few decades (Viglizzo et al., 1995). At present the classical pasture-crop rotation is being replacing by continuous cultivation, especially under humid scenarios. Agriculture is performed on well drained soils, both in the semiarid and humid portions of the region, with nearly 50% of the whole pampean area devoted to soybean (Glycine max), corn (Zea mays) and wheat (Triticum aestivum), the main crops (Hall et al., 1992).
Soil losses by intense erosion under conventional tillage systems that employed the mouldboard plow (Alvarez et al., 1995), and a net loss of nutrients (Bernardos et al., 2001) and soil organic carbon (Alvarez, 2001, Hevia et al., 2003, Quiroga et al., 1996a) due to negative balances occurred in some pampean areas, but despite soil degradation crops yield increased (SAGPyA, 2008). Genetic improvement lead to potential and attainable yields increases mainly of wheat (Calderini et al., 1999) and corn (Maddonni et al., 2000), possible counteracting partially soil degradation. Limited tillage systems combined with fertilization, and specially no-till, were introduced as management practices in the 1990s and were adopted at an exponential rate by farmers since them (AACREA, 2008, Fertilizar, 2008, SAGPyA, 2008). Nowadays between 60 and 80% of the planted area of main pampean crops is conducted under no-till. This evolution may be ascribing mainly to economic reasons. The phenomenon leads to public institutions studying tillage system effects on soils and crops. Many field experiments were performed along the Pampas and also many local and some international papers published but they had not been integrated in a study that resumes the changes produced on soils by the adoption of limited tillage and their possible effects on productivity. Only tillage system impact on soil organic carbon had been assessed in the region, detecting that increases of around 3 t C ha−1 may be expected on average when soils are cropped under no-till in relation to plowed soils (Steinbach and Alvarez, 2005). Local reports of grain yield response to tillage systems are contradictory between different experiments (Buschiazzo et al., 1998, Díaz-Zorita et al., 2002), and tillage induced changes on soil physical properties and fertility too (Buschiazzo et al., 1996, Panigatti et al., 1998). In spite of this, concern about soil erosion and fertility lead to researchers and extension services to recommend the use of conservation tillage systems in the Pampas.
A widespread idea in some production circles of Argentina is that productivity increases when no-till practices are adopted. Conversely, others suppose soil compaction under this tillage system and possible constraints to crops growth. Our objective was to integrate results from field experiments where tillage systems effects on soil physical properties, water content, fertility and yields of the main pampean crops were reported. A metanalysis of this data was performed to detect possible impacts of tillage system on soil productivity.
Section snippets
Materials and methods
Results from 35 different field experiments in which the effect of tillage system were tested on soil physical properties, water content, nitrogen availability or crops yield were extracted from published reports, with one exception in which data were available from the researcher on charge of the experiment (Table 1, Table 2, Table 3). The experiments were widespread over the entire Pampean Region (Fig. 1), with a wide range of variation of climate, soil type and rotation. Selection of data
Results
Soil bulk density was significantly higher under no-till than in plow tillage, but no changes were detected between plow and reduced tillage (Fig. 2A). Bulk density increases in no-till were reported only for soils where bulk density was lower that 1.3 g ml−1 (Fig. 2B). Above this threshold, no-till had no impact on soil density. Increases of bulk density may be as high as 0.15 g ml−1 but averaged 0.05 g ml−1 for the whole data set and were not related to time since initiation of the experiment, soil
Discussion
Contradictory results had been obtained when tillage systems effects on soil bulk density had been reviewed. Some studies found that bulk density increased under no-till in relation to plow tillage (Tebrügge and Düring, 1999) or reduced tillage (Mc Vay et al., 2006), and also that different limited tillage managements may induced soil densification of the upper soil layers when compared to intense tillage methods (Rasmussen, 1999). Conversely, in other reviews, inconsistent effects of tillage
Acknowledgement
This study was granted by the University of Buenos Aires (UBACyT G004).
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